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S 

Bulletin 266 March, 1925 ^2 

QJunnrrttrut A^rtrultural ^£xpnmmt ^tattu« 

'^^ta Haopn, (Cottn^rttrut 



The Improvement of Naturally Cross- 
Pollinated Plants by Selection in 
Self-Fertilized Lines 

I. THE PRODUCTION OF INBRED STRAINS 
OF CORN 



D. F. Jones 
P. C. Mangelsdorf 



The Bulletins of this Station are mailed free to citizens of Connecticut 
who apply for them, and to other applicants as far as the editions permit. 



CONNECTICUT AGRICULTURAL EXPERIMENT STATION 

OFFICERS AND SIAFF 
March 1925. 



BOARD OF CONTROL. 
His Excellency, John H. Trumbull, ex-officio, President. 

Charles R. Treat, Vice President Orange 

George A. Hopson, Secretary Mount Carmel 

Wm. L. Slate, Jr., Director and Treasure r New Haven 

Joseph W. Alsop Avon 

Elijah Rogers Southington 

Edward C. Schneider Middletown 

Francis F. Lincoln Cheshire 

STAFF. 
E. H. Jenkins, Ph.D., Director Emeritus. 



Administration. 



Chemistry. 

Analytical Laboratory. 



Wm. L. Slate, Jr., B.Sc, Director and Treasurer. 
Miss L. M. Brautlecht, BooRkeeper and Librarian. 
Miss J. V. Berger, Stenographer and Bookkeeper. 
Miss Mary E. Bradley, Secretary. 
William Veitch, In charge of Buildings and Grounds. 

E, M. Bailey, Ph.D., Chemist in Charge. 

R. E. Andrew, M.A. 1 

C. E. Shepard I 

Owen L. Nolan [ Assistant Chemists. 

Harry J. Fisher, A.B. 

W. T. Mathis J 

Frank C. Sheldon, Laboratory Assistant. 

V. L. Churchill, Sampling Agent. 

Miss Mabel Bacon, Stenographer. 



Biochemical 
Laboratory. 

Botany. 



Entomology*. 



T. B. Osborne, Ph.D., Sc.D., Chemist in Charge. 



G. P. Clinton, Sc.D., Botanist in Charge. 

E. M. Stoddard, B.S., Pomologist. 

Miss Florence A. McCormick-, Ph.D., Pathologist. 

Willis R. Hunt, M.S., Graduate Assistant. 

G. E. Graham, General Assistant. 

Mrs. W. W. Kelsey, Secretary. 

W. E. Britton, Ph.D., Entomologist in Charge; State Ento- 
mologist. 
B. H. Walden, B.Agr. 
M. P. Zappe, B.S. 
Philip Garman, Ph.D. 

Roger B. Friend, B.S., Graduate Assistant. 
John T. Ashworth, Deputy in Charge of Gipsy Moth Work, 
R. C. Botsford, Deputy in Charge of Mosquito Elimination. 
Miss Gladys M. Finley, Stenographer. 



Assistant Entomologists. 



Forestry. 



Walter O. Filley, Forester in Charge. 
A. E. Moss, M.F., Assistant Forester. 
H. W. HiCOCK, M.F., Assistant Forester. 
Miss Pauline A. Merchant, Stenographer. 



Plant Breeding. 



Donald F. Jones, S.D., Geneticist in Charge. 
P. C. Mangelsdorf, M.S., Graduate Assistant. 



Soil Research. 



M. F. Morgan, M.S.. Investigator 

George C. Scarseth, B.S., Graduate Assistant. 



Tobacco Sub-station 
at Windsor 



-, In Churge. 



N. T. Nelson, Ph.D.. Plant Physiologist. 



The Wilson H. Lee Co. 



CONTENTS. 

Page 

The effect of inbreeding upon corn 353 

Result of crossing 361 

An interpretation of hybrid vigor 364 

The transitory nature of hybrid vigor 369 

Inbreeding after crossing 371 

The attainment of complete homozygosity 374 

Mutations in corn 375 

The value of inbreeding 377 

Possibility of obtaining vigorous inbred strains 380 

Selection in self-fertilized lines 3S2 

Method of pollination 385 

Selection of ears for planting 385 

Elimination of self-fertilized lines 386 

The production of abnormalities 390 

The approach to uniformity and constanc}^ 399 

Differences in the selected lines 401 

Susceptibility to disease 404 

Criterions of selection 410 

Classification of selected lines 411 

Correlation between the first and last generations 412 

Limiting factors 415 

Conclusion 417 



SUMMARY. 

The results of previous investigations on inbreeding corn are 
reviewed to show the development of the method of selection in 
self -fertilized lines. 

Four varieties of corn have been self -fertilized and selected for 
five generations. Eighty-six lines were started and twenty of 
these were lost or discarded. 

The method of procedure was to grow three progenies in each 
line and self -pollinate five of the most desirable appearing plants 
in the best progeny each year. 

A large number of clear-cut recessive abnormalities appeared 
during the course of the inbreeding. In all except one case these 
were eliminated by the fifth generation. 

No significant difference in yield was found between segregating 
and non-segregating progenies in lines showing recessive abnormal- 
ities in the previous generation. Also lines having recessive 
abnormalities at the start showed no greater reduction in yield 
during the five generations than lines that were free from them 
throughout the experiment. 

All lines showed a marked reduction in yield and a slowing down 
of the rate of growth. Although great differences were shown, no 
lines were as productive as the original variety. No appreciable 
correlation was found between the characters of the seed ear, 
weight of seed, size of seedling, or the appearance of the plants at 
pollinating time and the production of grain in the same genera- 
tion. 

Some correlation in certain characters was found between the 
first and last generations, particularly in height of plant and in 
per cent, of moldy ears. Less association was shown in amount 
of tillering and in smut infection, while in productiveness practically 
no relation was found, showing that good and poor yielding strains 
may come from productive or unproductive plants at the start. 



THE IMPROVEMENT OF NATURALLY CROSS-POLLI- 
NATED PLANTS BY SELECTION IN SELF-FER- 
TILIZED LINES. 

I. The Production of Inbred Strains of Corn. 

D. F. JONES and p. c. mangelsdorf 

The improvement of naturally self -fertilized plants, particularly 
the small grains, has gone steadily forward following the develop- 
ment of effective methods of procedure. In contrast to the older 
methods of mass selection based upon appearances, stands the 
system of individual plant selections chosen on the basis of the 
performance of their progeny, as worked out by Louis de Vilmorin 
in 1856 and later appHed by Hjalmer Nilsson in 1891 at Svalof in 
Sweden and by W. H. Hays at the Minnesota Agricultural Experi- 
ment Station in 1892. Although the early methods of applying 
the progeny performance test involved much unnecessary^ effort, 
the principle was sound and its extensive application has resulted 
in a large number of valuable new varieties of important crop 
plants, notably wheat and cotton. The theoretical soundness of 
this procedure, first applied in an empirical way, was later fully 
established by the re-discovery and demonstration of Mendel's 
Law, which postulates that a large part of inherited variability is 
due to the recombination of stable units. This led directly to 
Johannsen's genotype conception of organisms which appear 
alike but breed differently and those which are themselves diverse 
but give similar offspring. 

The improvement of naturally cross-fertilized plants, reproduced 
by seeds, is in no such satisfactory situation. The variation 
brought about by Mendelian recombination makes it very difficult 
to have any adequate control over the heredity when inter- 
pollination is continually going on. Moreover, intensive selection 
for particular characters often results in decreasing the niimber 
of hybrid combinations and this, like all other forms of inbreeding, 
brings about a reduction in vigor. Any advantage which might 
come about from the concentration of desirable germplasm is 
offset by the loss of growth due to consanguinity. 

Com, a monoecious plant and wind pollinated, is almost com- 
pletely cross-fertilized in every generation. This mode of pollina- 
tion has brought about a condition in which a continuation of the 
same degree of germinal heterogeniety is necessary to maintain 
full vigor. The experimental results of inbreeding and crossing 
and their theoretical interpretation show clearly why the methods 
aimed at the improvement of com in the past have been largely 
fruitless. Formerly the selection practiced with this plant was 
largely based upon the appearance of the mature ear. Investiga- 
tion has shown that com has now been brought to such a high 
plane of development that the correlation between the appearance 

(349) 



350 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



of the seed and the productiveness of the crop grown from that 
seed is very low; so low in fact that it is often possible to get as 
good results from planting the poorest looking ears to be found in 
a field as from the choicest specimens. This is due to the fact 
that hybrid combinations of hereditary factors which make 
possible high production can not be transmitted intact and there- 
fore the offspring of any exceptional individual can not all be 
equally productive. 

An early appreciation of this situation following the application 
of experimental methods to the study of com breeding led to the 
ear-to-row system in which selection was based on the performance 




Figure 16. The seed from these large and small ears yielded the same. 
Their difference in size is due, not to heredity, but to the place where the 
plants that produced them happened to grow, one lot in a good, the other 
in a poor situation. This shows the complete lack of correlation in this 
case between the appearance of the seed ears and their performance. 

of the progeny instead of the appearance of the seed parents. 
Although the progenies differed markedly in yield those above the 
average failed to maintain their high production in later genera- 
tions. 

In 1908 G. H. ShuU outlined a method of com breeding radically 
different from any previously followed. In this he called attention 
to the large number of germinally different types which exist in 
every field of com and suggested that these cotild be separated out 



INTRODUCTION 351 

by inbreeding. Although vigor was lost by this process this was 
to be regained by crossing inbred strains and utilizing only the first 
following generation in which hybrid vigor is at its maximum. 
East also advocated the same method and reached the same con- 
clusions as to the importance of hybrid vigor, as the result of 
independent observations on the effects of inbreeding and hybrid- 
ization. The crossing of different varieties of com had been 
advocated long before this by Beal at the Michigan Agricultural 
Experiment Station, and Morrow, Gardner and McCluer at 
Illinois. Two important contributions to methods for com im- 
provement were made by Shull and East. One was making clear 
the complex germinal constitution of a variety in a cross-fertilized 
plant such as com and the way in which the composition of any 
particular individual is masked by hybrid vigor. The other was 
in showing that the maximiim degree of hybrid vigor could be 
secured by first reducing the plants to homozygosity and then 
crossing, thereby bringing about the greatest number of hybrid 
combinations of hereditary units. Both East and Shull con- 
sidered hybrid vigor as a physiological stimulus resulting from the 
condition of hybridity itself, differing from the specific action of 
individual hereditary factors. For this reason they stressed the 
importance of securing the maximum effect of hybrid vigor. The 
more important service of inbreeding in automatically eliminating 
abnormalities and serious weaknesses and in making possible the 
detection and isolation of the potentially most valuable germ- 
plasm was not fully appreciated at first by those who attempted to 
apply this method to com improvement. For that reason the full 
utilization of the pure line principle was delayed until hybrid vigor 
was shown to be merely the expression of dominant hereditary 
factors. This brought out clearh^ and forcefully the great value of 
inbreeding as a means of obtaining the finest hereditary material 
existing in a cross-fertilized plant like com by controlling the 
inheritance through the pollen parent as well as through the seed 
parent, and fixing this in such a way that it would not be lost. 
Following up this line of attack a method of corn improvement was 
outlined in 1920 under the general title of "Selection in Self- 
fertilized Lines.".* It is here proposed to review the results of 
inbreeding and crossing which have led to the development of this 
method and show how inbreeding can best be applied to the im- 
provement of com and other naturally cross-fertilized plants. 
As the application of this method is still in progress the plan is to 
publish the results in a series under the general heading of "The 
Improvement of Naturally Cross-PoUinated Plants by Selection 
in Self -fertilized Lines." The first of this series, submitted in the 
following pages, deals only with the detection and isolation of 
desirable hereditary qualities in com, that is, the production of 
inbred strains which possess either in visible expression or in 

*Jour. Agronomy, 12:77-100. 



352 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



potential power those valued characters that make for increased 
production. Later publications are planned to deal with the test- 
ing and utilization of inbred strains of com and the application of 
the same principle and method to other cross-fertilized plants. 




Figure 17. Two inbred strains from the same variety that have been 
grown side by side for eighteen years. The difference in abiHty to stand 
erect is inherited. 



THE EFFECT OF INBREEDING UPON CORN 



353 



The Effect of Inbreeding Upon Corn. 

All of the main types of com such as dent, flint, sweet, pop and 
flour corn have been inbred by self-fertilization for several succes- 
sive generations. The results have been the same in general for 
all types. Particular attention has been given to several strains 
resulting from a variety of Learning grown originally in central 
Illinois. Inbreeding was started by Dr. E. M. East in 1905. Four 
lines descending from three individual plants at the start have been 
continued to the present time under the direction of Dr. H. K. 
Hayes and later by the writers, and in 1923 they had been inbred 
by seventeen successive self-fertilizations. The results obtained 
have been reported from time to time. Particular reference is 
made to "Inbreeding in Com" and the "Distinction between 
Development and Heredity in Inbreeding" by East, published in 
the report of the Connecticut Agricultural Station and in the 
American Naturalist, and "Heterozygosis in Evolution and in 
Plant Breeding" by East and Hayes in a Bureau of Plant Industry 
Bulletin. Later results are given in a bulletin of the Connecticut 
Agricultural Station under the title of "The Effects of Inbreeding 
and Crossbreeding on Development" and the "Attainment of 
Homozygosity in Inbred Strains of Maize" in Genetics by the 
senior writer. As the method of selection in self-fertilized lines 



Table I. 

Yield and Height of Four Inbred Learning Strains of Corn Self-Fertilized 
Seventeen Generations. 





Strain A 


Strain B 


Strain C 


Strain D 


No. of 


Yield 


Height 


Yield 


Height 


Yield 


Height 


Yield 


Height 


Gen. 


Bu. 




Bu. 




Bu. 




Bu. 




Selfed 


per Acre 


Inches 


per Acre 


Inches 


per Acre 


Inches 


per Acre 


Inches 





74.7 


117.3 


74.7 


117.3 


74.7 


117.3 


74.7 


117.3 


1 


42.3 




60.9 




60.9 




59.1 




2 


51.7 




59.3 




59.3 




95.2 




3 


35.4 




46.0 




59.7 




57.9 




4 


47.7 




63.2 




68.1 




80.0 




5 


26.0 


76.5 


25.4 


81.1 


41.3 


96.5 


27.7 


86 '.7 


6 


38.9 
















7 


45.4 


85.0 


39.4 








41.8 




8 


21.6 




47.2 


83.5 


58.5 


88 ".6 


78.8 


96.0 


9 


30.6 


78 '.7 


24.8. 








25.5 




10 


31.8 


82.4 


32.7 


84.9 


19.2 


86.9 


32.8 


97.7 


11 


35.1 


79.7 


42.3 


78.6 


37.6 


83.8 


46.2 


103.7 


12 


24.5 


77.0 


27.2 


80.3 


20.4 


85.2 


49.6 


100.4 


13 


26.9 


85.5 


29.0 


83.7 


25.1 


80.6 


25.8 


85.3 


14 


23.6 


87.3 


38.3 


86.9 


36.3 


87.8 


35.2 


94.0 


15 


21.1 


85.4 


33.4 


89.9 


30.0 


98.2 


33.6 


99.6 


16 


17.6 


76.1 


24.6 


89.1 


25.3 


94.6 


29.8 


97.7 


17 






27.8 


91.7 


16.9 


88.9 


19.8 


88.4 



354 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

has been the direct outgrowth of these investigations as to the 
effects of inbreeding, a brief restime of the results obtained to date 
will be given here. 

The method of inbreeding followed in the earlier experiments 
was to self -pollinate a ntmiber of plants at random and use one 
of these as the progenitor for the following generation. Such a 
family descending from a single self -fertilized plant in each genera- 
tion is called a line or strain. The yield of grain and height of plant 
of four hnes from Learning during seventeen successive self- 
fertilized generations compared to the non-inbred variety are given 
in Table I. The four lines A, B, C, D, were derived at the start 
from three different plants. One of these was separated in the third 
generation into two lines, B and C. These have been continued 
separately since. Other lines were started from the same variety 
but have since been lost on account of failure to secure self -polli- 
nated seed. In some cases this loss has been accidental, but for 
the most part these strains were maintained previous to their 
extinction with great difficulty and showed a much greater reduc- 
tion in growth and vigor than the other strains which survived. 

Although there is wide variation in yield of grain and height of 
plant from year to year the general direction is downward. After 
the ninth generation size and productiveness have remained on 
about the same level. The original variety yielded at the rate of 
eighty-eight bushels per acre the year it was first self-fertilized. 
In 1916 seed of the same variety was obtained from the original 
source and grown in comparison with these strains, then in the 
ninth or tenth generation. On account of its change to a new 
location under conditions to which it was not as well adapted as the 
inbred strains, which had been grown there for many years, no 
strict comparison can be made. In spite of their possible ad- 
vantage the inbred strains were only from one-half to one-third as 
productive and were also noticeably reduced in height. 

This decrease in yield which results from a reduction in size of all 
parts of the plant and a lessening of the growth rate has so far 
been the universal result of inbreeding com as far as known to the 
writers. Several hundred self-fertilized strains have been grown 
long enough to bring this out clearly. Accompanying the lessening 
of productiveness and growth vigor there has been a reduction in 
variability. From a variety that showed the usual variation in 
height, color of silks, glimies and leaf sheaths, number of ears, 
position of the ear and other details in all parts of the plants there 
resulted in the four self -fertilized lines a marked uniformity among 
all of the plants within each line. This similarity in type became 
noticeable in the earlier generations of inbreeding, and after seven 
or eight successive self-fertilizations every plant in any one line 
was as much like every other plant in that line as any two plants 
in a naturally self-fertilized species, such as wheat or tobacco, 
from seed from the same individual. In other words, the vari- 



THE EFFECT OF INBREEDING UPON CORN 



355 



ability that resulted from the recombination of hereditary factors 
was in time eliminated. 




Figure 18. Two inbred strains from the same variety of flint corn, one 
with many tillers and the other without any. 



356 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

Where the original variety had some plants with colored silks 
and others with uncolored, some of the lines now have all their 
plants with red silks while in others all the silks are green. In 
some lines the foliage on all the plants is a bright glossy green, in 
others a dull bluish green. All the plants of one of the lines re- 
main green and stand firmly erect throughout the season while in 
other lines the foliage turns yellow towards the end of the growing 
season and in still another the plants frequently go down on 
account of a weak root system. Differences in susceptibility to 
smut are shown in these four strains as brought out in table II. 
In every detail of structure of the plant, including tassel and ears, 
all the individuals of one line are remarkably alike and noticeably 
different from the other lines. Some of these differences are shown 
in the accompanying illustrations, figures 17, 18 and 19. The uni- 
formity within the line and the differences between the several 
lines are brought out statistically in tables III to VI, which show 
the height of plant, length of ear, n amber of nodes and rows of 
grain on the ear for the original variety and the four strains derived 
from this variety. 

Table II. 

Per cent, of Plants Showing Smut Infestation in Fonr Inbred Learning 
Strains. 

Strain 1917 1918 1919 1920 1921 1922 1923 Ave 

A .3 .7 1.9 14.3 15.2 3.0 .0 5.1 

B 9.8 25.9 8.6 32.8 50.0 27.3 69.0 31.9 

C .5 9.1 4.1 6.0 13.8 17.5 52.7 14.8 

D .0 1.0 1.4 25.0 4.1 2.2 .7 4.9 

During the early generations of self-fertilization various forms of 
abnormalities appeared. The most frequent of these are seedlings 
wholly or partially lacking in chloroph3dl, various types of striped 
plants, golden plants, dwarfs, plants with ears showing many 
poorly developed and aborted seeds, and others with sterile tassels 
and ears. These are a few of the more strikingly aberrant types. 
Some of these are able to produce seed and when self-fertilized 
come true to their abnormal condition. Others are wholly in- 
capable of reproduction and are eliminated, but the inbred strains 
in which they appear may continue to produce them regularly as 
part of their offspring in the following generations. After several 
generations these abnormalities are usually no longer produced 
and the remaining plants are all normal in type although reduced 
in size and in rapidity of growth. Many of the abnormal forms 
which appear in large numbers in the inbred families are occasion- 
ally seen in fields of com which have never been artificially self- 
fertilized. Obviously, inbreeding is not responsible for their 
creation. They are recessive in mode of inheritance; that is, when 
crossed with other plants the following generation is all normal 
but the abnormality reappears in the subsequent generations. 



THE EFFECT OF INBREEDING UPON CORN 



357 




K^^^^Hii^ 



Figure 19. Differences in height of two inbred strains from the same 
variety self-fertilized four generations and selected for vigor and produc- 
tiveness but not for height. 



358 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



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THE EFFECT OF INBREEDING UPON CORN 



359 





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o 


Q 




'u 
> 


c 

CO 


+-> 







360 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



In ordinary fields of com they are generally kept out of sight by 
continual crossing with normal types which are dominant. Plants 
carrying such factors for abnormality, when self -fertilized, produce 
them in approximately one-fourth of their progeny. Some of the 
normal plants in the same progeny carry the abnormality and some 
do not. Sooner or later, progenitors are used which do not carry 
any of these striking abnormalities, after which they cease to 
appear. 

The rate at which reduction in growth takes place and the final 
size and productiveness of the several lines, after the reduction 
comes to an end, vary in different lines. Of the four Learning 
strains the D line has regularly been taller and larger and has 
yielded more than the others. The rate of reduction has been 
nearly alike in all of the four lines although A was reduced in yield 
somewhat more quickly than any of the others. The attainment 




Figure 20. Comparative production of a variety of Learning corn, 
two inbred strains derived from this variety, and their first generation 
hybrid. Grown in adjoining rows, they yielded 96, 32, 20 and 115 bushels 
per acre respectively. 



of uniformity may also proceed at a different rate, depending upon 
the degree of heterozygosity of the plant chosen as progenitor. 
Some strains remain variable for many generations while others 
become uniform in nearly every feature after a few generations of 
self-fertilization. 

From the foregoing facts it is obvious that inbreeding is a process 
of sorting out. From a mixture of many genetically different 
individuals all varying in hereditary composition and in heterozy- 
gosity any number of homozygous lines can be ultimately obtained, 
each differing to a greater or less degree from every other. A 
naturally cross-fertilized species is thus changed into an artifically 
self -fertilized species. In uniformity and constancy these artific- 
ially inbred plants are quite comparable to naturally self -fertilized 
species, with the important difference that in com they are mark- 
edly reduced in size and vigor. 



result of crossing 
Result of Crossing. 



361 



The vigor which is lost by inbreeding is at once restored when 
two self-fertiHzed Hnes descending from different plants at the start 




Figure 21. Two inbred strains and their first generation hybrid show- 
ing differences in time of flowering. 

are crossed. This is shown in figure 20. Here the ears produced 
by the original non-inbred variety are shown in comparison with 
the ears produced by two Hnes self -fertilized 12 generations and the 



362 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



first generation hybrid between these two Hnes. An equal number 
of plants of the four lots were grown in adjoining rows and yielded 
96, 32, 20 and 115 bushels per acre respectively. A comparison of 
a large number of first generation crosses between inbred strains 
derived from the same variety showed that the yield of the hybrids 
was increased 180 per cent., height of plant 27, length of ear 29, 
number of nodes 6, and rows of grain on the ear 5 per cent, above 
the average of their inbred parents.* From this it is seen that 
size characters such as height of plant and length of ear are affected 
more noticeably by hybrid vigor than the number of parts, such 
as nodes and rows of grain on the ear, while yield, which stmis up 




Figure 22. Representative ears of three inbred strains of dent corn 
and two first generation hybrids resulting from the crossing of the two 
adjoining types, harvested at the same time to show the difference in 

maturity. 

the entire growing capacity of the plant, is increased more than 
anything else. In other words hybrid vigor has much the same 
effect as favorable environmental factors. Fertile soil, good 
season and careful cultivation influence the growth of the com 
plant. Under these conditions corn grows taller, the ears are 
larger and the production of grain is much greater than under 
the less favorable conditions, while the number of nodes or the 
rows of grain on the ear are not so much changed. 

*"The effects of inbreeding and crossbreeding upon development." 
Connecticut Agric. Exper. Station Bull. 207. 



RESULT OF CROSSING 



363 



Another noticeable effect of crossing inbred strains of com is 
that of hastening the time of flowering and maturing. Figure 
21 shows two inbred strains in which the tassels are just beginning 
to appear. No silks are out. The first generation hybrid of 
these two strains in the center is shedding pollen from nearly all 
of the tassels and the silks are well out on many of the plants. 
Representative ears of three inbred strains and first generation 
hybrid ears resulting from the cross of the two adjacent strains are 
pictured in figure 22. All were picked at the same time and show 
the greater maturity of the hybrid ears. 

All of the combinations of inbred strains have shown increased 




Figure 23. A first generation hybrid showing the uniformity in height 
and in tassel type. The two inbred parental strains are in the adjoining 
rows at the left. 



growth and yield whether the parental strains come from the same 
original variety or from different varieties. Some combinations 
have yielded more than others. A few have been better than 
others in many respects. Crosses between strains from different 
varieties have not been conspicuously better than crosses within 
the variety although no extensive test of this point has been made. 
Furthermore, no reliable comparison of the yield of the hybrids 
with the original variety can be made because this variety is not 
well adapted to the local conditions in which the self-fertilized 



364 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

lines have been grown for many years. Kiesselbach reports the 
average yield of seven first generation hybrids tested two years 
as 52 bushels per acre in comparison with 42 bushels for the original 
variety. This is an increase of 24 per cent. The highest yielding 
hybrid produced 59 bushels or an increase of 40 per cent. 

The most noticeable and important feature of the first generation 
hybrids between fixed inbred strains is the even gro^vth, similarity 
in size and structural details and uniform production of all plants 
where the growing conditions are equal. This is shown for height 
of plant and tassel type in figure 23. Barring accident every plant 
is like every other plant. They grow to the same height. All ears 
are borne usually at the same node. The tassels and silks appear 
at the same time and the plants all ripen within a few days of each 
other. The fact that every plant produces a good ear is a most 
important factor in making crosses between strains so productive. 
In ability to yield from every plant and in uniformity of ripening, 
these first generation com hybrids are equal to any naturally self- 
fertilized crop such as wheat and tobacco or any vegetatively 
propagated plant as potatoes and sugar cane. Since com is very 
susceptible to damage by unfavorable weather at pollinating time, 
the uniformity in flowering may be undesirable particularly in those 
regions where hot dry weather is a frequent occurrence at this 
critical time. For that reason some other method of utilizing 
inbred strains may prove to be more practicable. This will be 
considered more fully in later publications. It is sufficient here to 
point out that in these first generation hybrids we have a new 
kind of corn which in many important respects is radically different 
from the mixtures of hybrids of varying degrees of heterozygosity 
now constituting an ordinary field of com. 

An Interpretation of Hybrid Vigor. 

The observations of gardeners and animal husbandmen have 
led to a general conviction that crossing somewhat different but 
related plants or animals usually results in a greater gro^\'th. 
Many instances of this phenomenon of hybrid vigor, in which the 
offspring excel both parents have been noted in the higher plants 
and in mammals, birds, insects and some of the lower forms of 
animals. Largei size or more rapid growth usually results when 
the parents are visibly different in some respects but are sufficiently 
related to produce fertile offspring. Many notable cases of hybrid 
vigoi' also occur in wider crosses where the offspring are partially 
or wholly sterile. This is well illustrated by the mule, which is 
sterile. A similar wide cross in plants is the combination of the 
radish and cabbage in which the hybrid makes a luxuriant gro^^^h 
but sets no seed. Some species crosses show no increased vigor 
but on the other hand may be extremely weak. East and Hayes 
have given several illustrations of tobacco hybrids which are 
barely able to live and make only a weak growth. Many crosses 



AN INTERPRETATION OF HYBRID VIGOR 



365 



of different species in animals and plants do not develop normally. 
Hybrid weakness as well as hybrid vigor must be taken into con- 
sideration although this is not be to expected in crosses that are 
fertile. 

Afier the limits of physiological compatibility are reached 
cross-fertilization cannot be accomplished. A series can therefore 
be arranged as follows: (1) Crosses between organisms which are 
so nearly alike in germinal constitution that no increased growth 



.<^>N. 



/ .^^i^i^-^ 




Figure 24. Crossed corn showing vigorous growth. 



results. (2) Crosses between germinally diverse but closely related 
organisms that grow to a larger size and at a more rapid rate and 
are fully fertile. (3) Sterile crosses between more distantly 
related organisms which are extremely vigorous. (4) Sterile crosses 
which are weak and often abnormal. (5) Crosses which cannot 
be made on account of the germinal difference in the forms united. 
H^^brid vigor in domestic animals and cultivated plants most 
frequently results when breeds or varieties of different type are 
brought together. Thus it is a common practice to cross the 



366 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



bacon and lard types of hogs or the mutton and wool breeds of 
sheep to secure some of the advantages of both parental races. 
Dent and flint varieties of com when crossed usually give greater 
increases in yield than crosses within either type. In these diverse 
crosses many of the desirable features of both parental races are 
brought together. How this works is well illustrated in the cross 
of a "golden" type of com which is deficient in chlorophyll with 
a "dwarf" as shown in figure 26. The plants resulting from this 
cross are tall, normally green and quite vigorous and productive. 
In this particular case one parent contributes normal stature and 
the other normal chlorophyll. Both these characters are 
dominant over the recessive condition so that all the hybrid plants 




Figure 25. It is the uniform production of a good ear on every plan- 
that makes the first generation hybrids between inbred strains so product 
five. 



are alike in their tall stature and green color. Another case is 
shown in figure 27 of two dwarfs which are genetically different 
and which, when crossed, give a tall, vigorous hybrid. One of the 
dwarfs lacks something essential to normal height and all the 
plants are alike as long as they are not out-crossed. The other 
dwarf is lacking in some other essential factor present in normal 
When these two small plants are combined each type 



com. 



supplies what the other lacks so that the result is normal stature 
in all the hybrid plants the first year after crossing. These illus- 
trations of the result of crossing are extreme cases which show how 
conspicuous abnormalities are suppressed by crossing so that the 
hybrid offspring are able to make a greater growth than either 
parent. The same situation in principle exists in all crosses from 



AN INTERPRETATION OF HYBRID VIGOR 



367 



which hybrid vigor ensues. Different organisms possess different 
hereditary quahties. When brought together there is always a 
tendency for the hereditary factors which make for greater growth 
vigor to dominate the factors for lesser growth. The bringing 




Figure 26. The result of crossing a golden, liguleless type, on the left, 
with a green dwarf on the right. The hybrid, in the center, has tall 
stature, normal foliage and green chloropliyll due to dominant factors 
contributed by each parent. 

together of the best of both parents in this way gives the hybrid 
offspring a temporarv^ advantage over either parent in the first 
generation following the cross. Recessive weaknesses are con- 



368 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



tinually occurrinj^ as mutations as shown by the many controlled 
observations on the fruit fly and other forms of life. In cross- 
fertihzed organisms, and particularly in domesticated animals^ 
and plants, crossing keeps these covered over and out of sight by 
combining them with normal factors. Many of these recessive 
weaknesses are not distinct and visible characters as are the 




Figure 27. Two genetically different dwarf types give tall plants when 
crossed, due to the fact that the normal growth factor which each lacks 
is supplied by the other. 

chlorophyll deficiency or dwarfness in com but nevertheless they 
weaken the organism in some way. When such crossbred races 
are inbred, the heterozygous combinations are reduced and the 
resulting individuals which are homozygous to a greater and 



THE TRANSITORY NATURE OF HYBRID VIGOR 369 

greater degree, as the inbreeding is continued, show the recessive 
weaknesses and are either unable to reproduce themselves or are 
reduced in size and rate of growth to a point below that of the 
original stock. The inbred individuals each receive some of the 
hereditary factors for vigorous growth. Some receive more than 
others as a chance allotment and are therefore better able to sur- 
vive the inbreeding process. Others are so weakened that they 
perish. On account of the way in which the hereditary mechanism 
operates it is extremely improbable that any one individual will 
receive all the more favorable growth factors, and in actual practice 
inbred strains of com are all reduced by inbreeding. It is theo- 
retically possible to obtain individuals which possess an unusually 
large share of the more favorable growth factors or even all of them 
and for that reason show no reduction from inbreeding. Darwin 
obtained self-fertilized races of Iponiea and Mimulus which were 
more vigorous than the naturally cross-fertiHzed variety at the 
start. Cummings reports self -fertilized strains of squash that are 
as productive as the original variety and much more uniform in 
type. King has obtained inbred rats after long-continued brother 
and sister mating that are fully as vigorous as the material with 
which she started. The fact that no such result has been ob- 
tained with com shows how dependent this plant has become 
upon cross-fertilization to maintain production. 

The Transitory Nature of Hybrid Vigor. 

The increased growi;h resulting from crossing is quickly lost in 
the following generations when the h^-brid individuals are bred 
among themselves or again inbred. In other words, hybrid vigor 
is a temporan,' manifestation which ordinarily cannot be fixed and 
made permanent in sexually reproduced offspring. The reason for 
this is readily appreciated when the illustrations previously given 
are followed into the later generations. The cross of the golden 
and dwarf com gives all normal tall green plants in the first 
hybrid generation. Seed from these h^^brid plants, either selfed 
or inter-crossed, always gives in the next generation aU the possible 
combinations of characters that went into the cross. In this 
particular case the golden plants also lacked the ligule which is 
the small extension of the leaf sheath surrounding the stalk above 
the leaf blade. Liguleless plants hold their leaves in a characteris- 
tically upright position close to the stalk. In the second generation 
of this cross of liguleless golden by dwarf, eight different kinds of 
plants are produced. These are shown in figure 28. Due to the re- 
combination of Mendelian units, this generation is extremely 
variable, and while some of the tall, green, liguled plants may be as 
vigorous and productive as the first crossed plants this generation 
as a whole averages much less productive. By further inbreeding, 
eight distinct pure-breeding combinations of these three characters 



370 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



can be obtained and within each type still further minor differences 
could be established. Crossing any two of these types gives 
increased growth and restores the normal condition provided the 
factors for normal growth are all present in one or the other type. 
In the same way the vigorous and productive crosses between 
inbred strains of com fall off in size and yield in the second genera- 
tion and are much more variable. This always results whether 
the first crossed plants are self -fertilized or are inter-crossed among 
themselves. If the inbred strains are uniform and fixed in their 
type the first generation hybrid plants are germinally all alike so 




Figure 28. The second generation offspring from the crossing of golden 
Uguleless by dwarf. Eight different combinations of these three characters 
are obtained by Mendehan segregation and recombination. 



that it is easily understood why self-fertilization and inter-crossing 
give the same result. To test this out two inbred strains were 
crossed after 14 generations of self-fertilization. A number of the 
hybrid plants were self -fertilized and an equal number were inter- 
pollinated. The seed of these two lots was planted in alternate 
rows, replicated three times. The self -fertilized plants averaged 
76. 2 ±.57 inches in height in comparison with the intercrossed 
plants which averaged 73. 8 ±.70. In production of grain they 
stood respectively 22. 2 ±1.2 and 22.0 ±2.4 bushels per acre. In 
neither case are the differences significant. 



inbreeding after crossing 371 

Inbreeding after Crossing. 

When the second generation plants are allowed to intercross 
naturally no further reduction in vigor is expected. Variability 
and yield should remain at the same level thereafter until 
natural or artificial selection eliminates certain strains. But when 
the second generation plants are self -fertilized there is a further 
reduction in size, and if the inbreeding is continued the decline in 
size and vigor and in variability proceeds in approximately the 
same way as when the parental strains were first inbred. This is 
shown in figures 29, 30 and 31. 

In this demonstration of inbreeding after crossing, two inbred 
strains, self-fertilized for eight generations, were crossed and the 
first generation plants again self -fertilized. In the second genera- 
tion a single plant was again chosen as the progenitor and polli- 
nated in the same way, and this was continued for eight successive 




Figure 29. The result of inbreeding after crossing. Two inbred strains 
at the left, their first generation hybrid adjoining, followed by seven suc- 
cessive generations self-fertilized. 

generations. Seed was saved from each year's selfing up to the fifth 
generation. Since com seed will not retain its germination satis- 
factorily for more than six years, single plants were again self- 
fertilized the fifth year in each generation and this seed was used 
from then on. All eight inbred generations were growm in 1923 
along with the two parental strains as shounti in the accompanying 
illustrations. This demonstration has been gro\\Ti each year 
since the original cross was made and the yields obtained in the 
different years are given in table VII. Production has varied 
rather widely from season to season and from generation to genera- 
tion. This is due in part to the character of the individual plants 
chosen for progenitors. A ver}^ noticeable drop takes place from 
the first to the second generation amotuiting to over 30 per cent, 
as an average of the six years. Kiesselbach tested the first and 
second generations of eight hybrid combinations of different strains 
during two seasons and obtained an average of 52.2 and 27.8 
bushels per acre respectively for the two generations, to be com- 



372 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

pared with 41.7 bushels for the original com from which the 
inbred strains were obtained. He secured his seed for the second 
generation by pollinating several first generation plants with 
composite pollen from 15 sib plants. The reduction from the first 
to the second generation of nearly 50 per cent, is even greater than 
in our case where the plants were self -fertilized. Kiesselbach also 
grew a third generation from seed of interpollinated plants. The 
comparative yields obtained for the first, second and third genera- 
tions were 51.5, 29.4, and 25.6 bushels per acre. The reduction 
from the second to the third as would be expected from this mode 
of pollination is small compared with the drop from the first to 
the second. Continued inter-pollination should cause no further 
decrease in yield unless particularly unfavorable strains are 
isolated. 

The average height of these successive self -fertilized genera- 
tions compared with the first generation hybrid and the parental 
strains is shown graphically in figure 32. There is a continued 

Table VII. 

The production of grain in bushels per acre, of two inbred strains of 
corn and their hybrid and the Fi to the Fs generations successively self- 
fertilized. 



Year 










Generati 


ons 










Grown 


Pa 


Pb 


Fi 


F2 


F, 


F4 


Fi 


Fs 


F7 


Fs 


1917 


22 


6 


65 


56 














1918 


27 


24 


121 


128 


is' 












1920 


16 


28 


128 


48 


35 


'29' 


io' 








1921 


20 


13- 


73 


55 


49 


33 


15 


'23" 






1922 


20 


26 


160 


83 


74 


68 


49 


36 


'2.3' 




1923 


13 


21 


61 


45 


41 


47 


16 


23 


26 


27" 


Ave. 


20 


20 


101 


69 


43 


44 


23 


27 


25 


27 



reduction in each generation, but the decrease is much less during 
the last three generations than in the first four. From the first to 
the fifth generation there is a decline of 27.2 inches in stature and 
from the fifth to the eighth 8.6 inches. The rate of growth as 
measured by the daily gain in height is also steadily reduced as 
shown in figure 33, the decline being greater during the first stage 
of inbreeding than in the last. The dift'erences between the last 
two generations in all measurable characters, including yield, 
height, length of ear and rate of growth, are so small that it seems 
evident that the reduction in size and vigor is rapidly approaching 
an end. The last two generations are so similar in appearance 
that they cannot be distinguished in the field. In tassel type, 
foliage character, position of the ear on the stalk, and in the size 
and conformation of the ears these two generations are practically 
identical. 

The reduction in variability from the first to the eighth genera- 
tion was very noticeable in the field. One of the parent strains 
has green silks, the other red. The first generation hybrid plants 



INBREEDING AFTER CROSSING 



373 



all had red silks. The second, third and fourth generations segre- 
gated for this color while the remaining generations were all 
uniformly colored. Height of plant, position of the ear on the 
stalk, form of tassel and all structural details were noticeably uni- 
form in the parents and the first hybrid generation. The plants 
in the generations from the second to the fifth were quite 
variable but later became more and more uniform until in the 
last two generations they showed as little variation as either of 
the parental strains. 

The inbred strain which resulted from this second period of self- 
fertilization differs from both parental strains. In tassel, ear, and 
character of the foliage it is quite unlike either but is noticeably 
susceptible to smut like one of the parents. In other words, 




Figure 30. Inbreeding after crossing 
generations shown in figure 29. 



Representative plants from the 



Mendelian recombination has taken place so that the details of 
structure are altered. Apparently this inbred strain has about 
the same nimiber of favorable growth factors, and for that reason 
it is no better or no worse than the parental stocks that went into 
the vigorous and productive hybrid from which the new strain was 
derived a' few generations before. 

For all practical purposes the reducing effect of self-fertilization 
in this particular case has ceased at the sixth inbred generation. 
This closely parallels the course of events when the parental 
strains were first inbred. Theoretically the loss of vigor follows 
the rule of halving the remaining dift'erence in each generation. 
If we take an individual heterozygous for a single Mendelian pair 
of factors such as Aa we expect in the next generation fifty per 



374 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

cent, of the plants homozygous for this pair of factors and 
having the composition A A or aa; the other fifty per cent, will 
on the average still be heterozygous for this factor pair; i. e. Aa 
in composition. In choosing a single self -fertilized individual for 
the progenitor the chances are even that it will be homozygous or 
heterozygous. This holds for any number of factor pairs and 
since each pair when once alike must remain so thereafter in self- 
fertilization the niunber of mixed pairs is steadily reduced by 
half in each generation. Starting with an individual 100 per cent, 
heterozygous, the following generations would be on the average 
50, 25, 12.5, 6.25, 3.125, 1.5625, etc. 

Naturally the progeny of any heterozygous individual will vary 
greatly in composition. Some will be nearly or completely homo- 
zygous while others will be nearly or completely heterozygous 
with respect to all factor pairs. For that reason the result of any 
process of inbreeding depends entirely upon the composition of the 
individual plants which are chosen as progenitors. It is theoreti- 
cally possible to obtain individuals in each generation which are 
as heterozygous as their parents and others that are completely 
homozygous. For that reason inbreeding may cause no reduction 
in size, vigor or variability, or complete reduction may take place 
in a single generation. The chances that such a result will be 
obtained, however, are extremely remote. Actually the reduction 
follows the rule of halving the remaining difference very closely 
so that it is evident that a very large number of factors play a 
part in hybrid vigor. How many such factors there are, we have 
no way of estimating at the present. Many factors which bring 
about visible differences possibly have no effect upon vigor but 
apparently the number of them which are essential to normal 
development in com is exceedingly great. 

The Attainment of Complete Homozygosity. 

Whether complete fixity of type, absolute homozygosity, is 
possible of attainment by continuous self-fertilization has been 
•previously discussed. (Jones 1924.) The experimental results 
show that small germinal differences may remain after many 
generations of inbreeding. Two lines separated from one in the 
third generation and then continued separately for several genera- 
tions gave a marked increase in size when crossed, although not as' 
great as in the case of lines separated at the beginning, showing 
that two self-fertilizations had not produced much uniformity in 
germinal constitution. The four original Learning strains were 
continued as single lines up to the eighth generation. At that 
time they were all remarkably uniform and apparently fixed in 
their type. Then each line was separated into two lines which 
were continued separately thereafter for eight or more additional 
generations. At that time two of the paired lines had remained 
exactly alike. No visible differences in any respect could be seen. 



MUTATIONS IN CORN 375 

One of the paired lines differed only in color of the seeds, one being 
noticeably brighter in color in some seasons. As the growing 
conditions were alike for all plants this slight difference can not be 
accounted for in any other way than as an heritable difference. 
The other paired line differed noticeably in many respects. One 
of the members was taller, the leaves were broader and lighter 
colored and the ears were larger, the seeds broader and duller 
in color. 

Crossing these paired lines gave significant increases in all 
measurable characters in the one strain whose paired lines were 
visibly different. The other strains all showed slight but appar- 
ently significant increases in some characters. The two strains 
whose paired lines showed no visible differences were again tested 
after fourteen generations of self-fertilization in the following way. 
The two strains which were distinct from the beginning were cross- 
ed and gave the usual vigorous and uniform hybrid plants. A 




Figure 31. Inbreeding after crossing. The production of grain from 
the plants shown in figure 29. 

number of these were self-fertilized and an equal number were 
inter-pollinated by sib plants. A careful test failed to show any 
differences in size or productiveness in the plants grown from these 
two lots of seed. If the parental strains were not germinally alike 
within themselves, intercrossing the first generation hybrid plants 
would not cause such a decrease in heterozygosity as self-fertiliza- 
tion. The fact that no diff'erence was shown indicates that the 
parental strains were completely homozygous for all factors which 
influence gro^vth vigor. However, this test is not a ver\' delicate 
one and final proof awaits the crossing of the paired lines which 
have been separated in the seventeenth generation and will be 
carried along for several additional generations. 

Mutations in Corn. 

Complete homozygosity may be impossible to attain because of 
spontaneous variations, mutations, occurring from time to time. 



376 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



During the seventeen years in which the four inbred Learning 
strains have been under observation only two apparent germinal 
changes have been recorded. Until a fairly high degree of uniform- 
ity was reached, after six generations, various abnormalities 
occurred singly or in greater numbers in the rather small progenies 
that were grown. Presumably these were, at least in the great 
majority of cases, merely segregations from a heterozygous com- 
plex. But new characters appearing after uniformity is obtained 
which have not been noted previously have every indication of 
being mutations. Two such have been observed in different lines. 
One produced in the thirteenth generation a single self -pollinated 
ear segregating for defective seeds. All of the lines had been 
examined for the new character during three previous generations, 
without noting anj^thing of this kind, and since the character 



Figure 32. Graph showing the height of the two parental strains and 
the generations from the Fi to Fs. 

segregated as a single Mendelian recessive when out-crossed, there 
is every reason to assimie that a germinal change took place 
shortly before its appearance. Among approximately a thousand 
plants of another line, self -fertilized more than ten generations, 
which has always produced white cobs, four ears were found with 
light red cobs. The cobs of this strain are flattened and the 
plants are otherwise easily identified. The red cob plants were 
examined at harvest and noted to be typical for the strain in all 
respects except cob color. Neither of these changes could have 
been due to out-crossing. Stray pollen from any outside source 
immediately results in vigorous plants twice as large as the inbred 
plants ever grow and the crossed plants are completely changed in 
type. Since the mutant plants were in other respects typical 
plants of the strain and were no larger they could not have resulted 
from out-crossing. 

Two additional changes have occurred in other inbred material 



THE VALUE OF INBREEDING 



377 



such that they have every indication of being recent germinal 
alterations. One strain after five generations produced for the 
first time striped, variegated plants which bore no pollen or seed. 
They occured in later generations in about 25 per cent, of the off- 
spring from normal plants. Another strain after nine generations 
gave small narrow-leaved dwarf plants which were quite distinct 
from the normal plants. They produced a small amount of pollen 
and when out-crossed to normal plants they reappeared in later 
generations showing that the change was heritable. 

These four apparent mutations are all that have been noted in a 
large number of uniform strains which have been under obsen-ation 
for many years. Hayes and Brewbaker record the production of 
chlorophyll deficient seedlings in four lines out of 953 which had 



Figure 33. Graphs showing rate of growth (average daily gain in 
height) for the same generations as in the preceding ilhistrations. 

not shown such abnormalities previousl^^ In these cases the 
appearance of the abnormalities may have been due to delayed 
segregation, since the lines had not been reduced to uniformity and 
constancy. While it is evident that com does mutate, the fre- 
quency of these changes is so low that inbred strains, when once 
reduced to uniformity, are stable for all practical purposes. Some 
care will be needed to maintain self -fertilized lines true to type, and 
when recessive abnormalities appear those progenies which show 
them will have to be discarded. 



The Value of Inbreeding. 

This review of the effects of inbreeding and crossing upon com 
has been given in considerable detail because the facts learned from 



378 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



these investigations form the basis for the method of ini])rovement 
by selection in self-fertihzed Hnes. In the inbreeding experiments 
just described no selection of superior individuals to perpetuate the 
strain was made. The aim was to take normal plants at random 
and note the outcome. Nevertheless a great deal of natural 
selection has taken place. All abnormalities which interfere with 
or markedly reduce reproductive ability have been automatically 
eliminated. In this way many chlorophyll deficiencies, endosperm 
abnormalities and inherited sterility in tassels and ears, unfavor- 
able conditions almost always present in every cross-pollinated 



75 




Figure 34. A diagrammatic representation of the actual and theoretical 
results of inbreeding corn. The solid lines represent strains which have 
already been obtained, the dotted lines those which may be expected when 
corn is worked with more extensively. 



variety of corn, have been cleaned out. But this outcome of in- 
breeding, valuable as it may be, is less important than the control 
over the heredity made possible by hand pollination and the result- 
ing fixity of type. 

In common practice, selection with nearly all cross-fertilized 
plants has been based on the appearances of the plant or upon the 
performance of the progeny, and no adequate control of the 
heredity brought in from the pollen parent has been possible. As 
generally practised, corn breeding has been similar to a system of 
animal breeding in which selection is carried on only with the 
dams paying no attention whatever to the sires. The disastrous 



THE VALUE OF INBREEDING 



379 



result that such a system would have upon purebred live-stock 
can readily be appreciated. With all cross-fertilized plants it 
would be theoretically possible to follow the method now used in 
animal breeding. Certain desirable individuals could be chosen 
as seed parents and others as pollen parents. Pollination could be 
made by hand and the progenies compared on the basis of their 
performance. There is no doubt that this system followed up as 
carefully as it is in mating farm animals would give equal results. 
But such a method is wholly impracticable on account of the small 
value of the individual plant. The time spent on selecting the 




Figure 35. Self-pollinated ears grown on selected plants of Burweirs 
Yellow Flint, No. 40. Each ear is the starting point of a selected Hne. 
These are numbered 1 to 9, top row, and 10 to IS, bottom row, left to right. 

parents and on polHnating each generation would not be repaid 
by the possible gains. Furthermore, with com, selection is greatly 
handicapped due to the fact that the principal objective, pro- 
duction of grain, is not visible until after pollination. 

A new method of attack, which will make possible a control of 
the heredity transmitted thru the pollen as well as thru the egg, is 
needed for all naturally cross-fertilized plants. Since inbreeding 
is a sorting-out process, selection carried on dtrring the time the 
plants are being reduced to uniformitv and constancv makes 



380 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

it possible to look for desirable qualities with a certainty of being 
able to hold them, when once secured, that has never before been 
possible. From this viewpoint inbreeding is not so important as a 
method of gaining the maximum effect of hybrid vigor when the 
inbred strains are crossed as it is of separating out and making 
visible the very best hereditary qualities that may exist in a 
heterozygous stock. Strains when once reduced to fixity remain 
the same indefinitely, barring mutations. With due regard to 
seasonal variation, crosses between inbred strains give the same 
result whenever the same combination is made. The uniform 
production of the first generation hybrids between homozygous 
strains is an important feature. In this respect cross-fertilized 
plants are equal to self -fertilized plants in uniformity and fixity of 
type and have the added advantage of crossing to bring together 
and use in the first generation the desirable qualities within the 
species, which in a self -fertilized organism can be used only when 
recombined and fixed in a homozygous condition. It should there- 
fore be clearly understood that the crossing of inbred strains as 
such is without particular value and that the opportunity afforded 
to find and to fix the very best hereditary qualities possessed by a 
cross-bred race is the more important function of inbreeding. 
Crossing is merely a means of utilizing this good heredity by giving 
it maximum vigor. It is to be expected that many inbred strains 
will have only medium value and give no improvement over the 
original variety when crossed. The bulk of the germplasm in 
every population is mediocre. Of necessity only the exceptionally 
few will give outstanding results. For these reasons the outcome 
of selection in self-fertilized lines depends upon how extensively 
and skillfully it is applied. 

Possibility of Obtaining Vigorous Inbred Strains. 

Most of the inbred strains of corn so far produced have been 
reduced to about fifty per cent, or less of the production of the 
original cross-bred varieties. Some strains have failed to repro- 
duce after one generation of self-fertilization. Others have per- 
sisted in a weakened condition for several generations and then 
perished. Still other strains are able to survive, but are continued 
only with the greatest difficulty. The majority of the self- 
fertilized lines, when uniformity and fixity of type are reached, are 
about one-third as productive as at the start. A few are exception- 
ally good. They grow more vigorously and yield more than the 
rest and are equally uniform and fixed in their type. But even the 
best of these are still below the original variety in amount or 
quality of grain produced. On the basis of hybrid vigor being 
due to dominance of the more favorable factors it is theoretically 
possible to secure inbred strains that will show little or no reduc- 
tion in vigor, and a few may sometime be obtained that are even 



OBTAINING VIGOROUS INBRED STRAINS 



381 



m(3re vigorous and productive than the cross-bred variety. This 
is deduced from the fact that most heterozygous combinations of 
factors are less effective than the homozygous combinations of the 
same factors. Thus the cross of yellow and white corn gives a 
lighter color than pure yellow. The cross between a determinate 
gro\\i;h type of tobacco with an indeterminate growth type (Jones, 
1921) which involves a single factor, differs from either parent in 
size of plant and number of leaves. Dominance is seldom perfect 
and while there is little direct evidence in this respect for characters 




Figure .36. Self-pollinated ears grown on selected plants of Gold 
Nugget, No. 105. Each ear is the starting point of a selected line. These 
are numbered 2 to 10, top row, and 11 to 20 bottom row, left to right. 
(Ear 1 was shelled before photographing. It was similar to No. 2.) 



which directly affect vigor there is every reason to expect that a 
homoz^'gous combination of all the more favorable dominant 
growth factors will make possible a greater development than the 
heterozygous combinations of the same factors with weaker allelo- 
morphs. However, as just noted, certain results are obtained from 
heterozygous combinations that can not be obtained from either 
factor alone. If there are many of these that play a part in growth 
vigor, then heterozygosity may be indispensable to maximum 
development. Moreover, recombinations of large nimiber of 



382 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



factors are extremely difficult to obtain and since favorable and 
unfavorable growth factors are distributed indiscriminately 
throughout the hereditary mechanism the chances of securing 
self-fertilized strains of com which equal the cross-bred varieties 
are so exceedingly small that there is little hope of obtaining them. 
The most that can reasonable be expected are inbred strains which 
are appreciably better than any that have so far been produced. 
The results that have already been obtained from self-fertilizing 
corn, and the theoretical possibilities, some of which may be attain- 
ed in the future, are shown diagrammatically in figure 34. 




Figure 37. Self-pollinated ears grown on selected plants of Century 
Dent, No. 110. Each ear is the starting point of a selected line. These 
are numbered 1 to 9 top row and 10 to 18, bottom row, left to right. 

Selection in Self-Fertilized Lines. 

To demonstrate the value of inbreeding as a means of isolating 
good heredity a system of selection in self -fertilized lines was begun 
in 1918. Four varieties of com were chosen as material with which 
to work. These varieties have been grown in Connecticut for 
many years and are well adapted. In a variety test of long 
duration they have proven to be among the best in production of 
grain and in other qualities. The four varieties are as follows : 

Burwell's Yellow Fhnt, No. 30 and No. 40. An eight rowed 
yellow com of the Canada Flint type. The ears are medium in 
size, one or two on the stalk. The plants are medium in maturity. 



SELECTION IN SELF-FERTILIZED LINES 



383 



Gold Nugget, No. 105. An eight rowed yellow flint com with 
large ears, broad kernels and heavy cobs. The stalks are large 
with few suckers. The plants mature late in the season. 

Century Dent, No. 110. A light yellow dent corn with broad, 
smooth, shallow dented kernels. The ears are medium in size 
and have from 14 to 18 rows. The plants are medium in size and 
mature well in practically every season. 

Beardsley's Learning, No. 112. A yellow dent corn with taper- 
ing ears with 16 to 22 rows and small, shallow kernels. The stalks 
are large. This variety is later in maturing than Century Dent 
and is usually more productive. 




Figure 38. Self-pollinated ears grown on selected plants of Beardsley's 
Learning, No. 112. Each ear is the starting point of a selected line. These 
are nunAered 1 to 8, top row, and 9 to 16 bottom row. 



The plan of procedure was to self -fertilize a niunber of the best 
plants in each of these four varieties and to use each of these plants 
as the starting point of an inbred line. These lines were to be 
continued by self-pollination of the best plants in each generation 
until uniformity and constancy were reached. Accordingly from 
about 60 plants each of the four ^^arieties gro\\Ti from a general 
mixed lot of seed, 20 plants of each variety were selected at 
pollinating time and self-fertilized. These four lots of ears are 
shown in figures 35 to 38. Some of the seh'-pollinated plants 



384 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



failed to set seed but all of the ears that had enough seed to work 
with were planted. The original hand-pollinated ears were ranked 
according to their appearance in size, form of the ear and quality 
of the seed. Ear number one represents the best, number two 
the next best and so on down. The ear numbers became the 
numbers for the self -fertilized lines derived from them. Therefore, 
the number of the line shows how its original progenitor was classi- 
fied. It is of considerable interest to note to what extent good 
strains can be obtained from unpromising ears at the start. 

Each self -pollinated ear was planted in a row the following year 
and five plants of each were again selected at pollinating time as 
the most desirable and were self -fertilized. It was noted that the 
best appearing plants at the tirrie of pollination were not always 



••PLANTS oj : 
ORIGINAL VARIETY 



LINE A 
I 



LINE C 
i 



o o 




Figure 39. Diagram of a method of selection in self-fertilized lines. 
An individiial plant becomes the starting point of each inbred strain. 
Three progenies are grown but only one is selected to continue the line. 

the most productive at maturity. For this reason more plants 
were self -pollinated than there were progenies planted, thus allow- 
ing for some failures of pollination and also to permit of some 
selection among the hand pollinated ears. Also, in order to base 
selection upon progeny performance rather than upon the appear- 
ance of the seed ear, three progenies from each line were grown each 
year. At pollinating time the best appearing progeny was chosen 
and five plants were again self-fertilized, the other two progenies 
being discarded. This method of carrying on selection is shown 
diagrammatically in figure 39. 

About thirty plants were grown in each progeny. From three 
to five times this niimber of seeds was planted and the poorest 



SELECTION OF EARS FOR PLANTING 385 

seedlings pulled out after they were well started, leaving the tallest 
and most vigorous plants. An even stand was obtained in most 
cases. The end plants in each row were usually avoided in 
selecting the plants for hand-pollination as these are nearly always 
larger and better developed than the others on account of their 
better opportunity to grow. 

METHOD OF POLLINATION. 

The plants were pollinated by hand as shown in figures 40 and 41 . 
The general method used is as follows:* A three pound manila 
grocer's bag is placed over the ear shoot before the silks appear. 
The tassels are covered with an eight or ten pound bag as soon as 
they are above the upper leaves. When the silks are about three- 
fourths out, pollen is dusted over them and the tassel bag placed 
over the ear. Care is taken not to touch the silks or the inside of 
the tassel bags with the hands in order to "avoid contamination 
with foreign pollen. If the silks extend more than three or four 
inches beyond the tip of the ear the}^ are cut back with a knife 
sterilized in alcohol. After the first generation or two, out-crossed 
plants can be easily noted by their much greater size and darker 
green color so that contaminating pollen is not a cause for great 
concern. Effort is made to pollinate as rapidly as possible. Only 
one application of pollen is made. If sufficient seed does not result 
from this application the ears are not used. Some good plants 
are lost because all the pollen has been shed and has lost its 
viability before any silks appear. This tendency to protandry is 
accentuated in some inbred lines. Such strains could be main- 
tained by sib-crossing but since this method of inbreeding is much 
less effective than self-fertilization in bringing about homozygosity 
the latter system has been rigidly adhered to. In this way sterility 
and recessive abnormalities of all kinds are most quickly eliminated. 

SELECTION OF EARS FOR PLANTING. 

Each hand-pollinated plant is tagged with a printed form upon 
which notes as to the character of the plants in the field and the 
hand-pollinated ears when mature are entered as follows: 

Pedigree number Color and markings of foliage 

Field plot number Infection on plant 

Height to ear-bearing node Smut on ear 

Height to first branch on tassel Mold on ear 

Number of ears containing seed Number of rows of grain on ear, 

Number of leaves regularity of rows, and length of 

Number of tillers ' ear 

Posture, whether erect, leaning, Color and general character of seeds 

bent, broken or fallen Color and shape of cob. 



* A method of pollinating proposed by Jenkins and known as the 
"bottle method" was also tried. Under our conditions it did not prove 
as satisfactory as the procedure described here. 



386 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



At harvest these tags are transferred to the hand-pollinated 
ears. In choosing the three ears for planting in each line, from the 
five ears pollinated, the characters of the plants in the field as well 
as the size and appearance of the ears are taken into consideration, 
chief attention being given to ability to stand erect, color of foliage, 
freedom from smut and other infection on the plant and ear and 
absence of mold on>the ears. 

ELIMINATION OF SELF-FERTILIZED LINES. 

In all, 86 self-fertilized lines were started, distributed among 




Figure 40. Plant bagged for hand pollination. Small bags can be 
used over the ear shoot and the tassel bag placed on the ear when polli- 
nated. Wire clips are now used to hold the bags on the ear and tassel. 

the four varieties as follows: From Burwell's Yellow Flint 
number 40 there were 18 ears self -pollinated in 1918, ranked and 
numbered from 1 to 18 in order of their excellence as shown in 
figure 35. In addition to these there were 14 ears of the same 
variety which had been self -fertilized in 1914 for another purpose 
and not used. These were included among the Bur^vell strains 
with the variety ntunber 30 to distinguish them from the other 
strains which were ranked according to their appearance. The 
fact that these ears had been held five years before planting has 
interest in connection with the possible elimination of abnormal- 



ELIMINATION OF SELF-FERTILIZED LINES 



387 



ties due to the age of the seed, as will be noted later. From the 
Gold Nugget variety, number 105, twenty lines were started 
(figure 36); from Century Dent, number 110, eighteen lines 
(figure 37), and from Beardsley's Leaming, number 112, sixteen 
lines were started (figure 38). 

The once self-pollinated ears beginning these 86 lines were 
planted in 1919 and hand-pollinated ears were obtained from all 
lines except one in Gold Nugget and two in Century Dent. These 
failures to produce seed in all five pollinations in each line may 
have been due to delayed pollination and unfavorable weather 
conditions. But since good ears were obtained in the other lines 




Figure 41. Pollinating corn. Only one man is necessary' for this opera- 
tion. Care is taken not to touch the silks or the inside of tassel bag. If 
the silks are more than three inches long they are cut back to about one 
inch with a knife sterilized in alcohol. 



it is fair to assume that these lines were less vigorous or for some 
reason were not as able to reproduce under this method of polhna- 
tion. In the second generation two more lines were lost because 
no self -pollinated seed was obtained. In the third generation 
four lines were discontinued. In two of these no hand-pollinated 
ears were obtained, and the other two were so badly damaged by 
■mold that they were discarded. 

In the fourth generation eleven lines were eliminated. Nine 
-were discarded because they were so \'ery poor and unpromising 



388 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



that it was thought advisable not to carry them further. Some of 
these failed to produce any seed on any plants. All of the hand- 
pollinated ears of two lines proved to be out-crossed, due possibly 
to the fact that the bags covering the ears of the previous genera- 
tion were broken and allowed foreign pollen to enter. By the 
fifth generation practically all of the lines had- become uniform 
and stable. All that had survived up to this point gave promise 
of being able to continue indefinitely if sufficient effort was put 
forth and provided the season was not too unfavorable. During 
the course of the five-year selection period the following lines were 
eliminated for various reasons : 




Figure 42. Self-fertilized ears showino; defective or aborted seeds. 



In No. 30, line 2 was accidentally lost. 
In No. 40, lines 2, 5, 11, 12, 17, 18 were discarded. 
In No. 105, lines 1, 4, 5, 12, 19 were lost or discarded. 
In No. 110, lines 8, 12, 13, 14 were lost or discarded. 
In No. 112, lines 2, 5, 11, 13 were discarded. 

In all, 20 lines were not continued to the end of the fifth genera- 
tion. Three of these were accidentally lost thru no fault of their 
own. The others were too poor to be carried along. An examina- 
tion of the original ears from which these lines came (figures 35 to 
38) shows no marked relation between their poor behavior and 
their appearance when first pollinated. Dividing each lot of ears 
into two equal groups and not counting the three lines that were 



ELIMINATION OF SELF-FERTILIZED LINES 



389 



accidentally lost, we find that seven from the best appearing lines 
at the start were discarded and ten from the poorest. 

The original plan was to keep all lines that could be successfully 
propagated even though they became extremely poor. It was 
fully appreciated that inbred strains may themselves be very 




Figure 43. Seedlings lacking chlorophyll are common hereditary- 
variations in corn. 



undesirable and still have potentially great value when crossed 
with other strains. For this reason no lines were discarded unless 
the amount of seed produced was so small that enough plants to 
permit satisfactory measurements could not be grown. Many lines 
were continued which were extremely weak, unproductive and 
showed markedly undesirable characters. They were continued 



390 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



to compare them in crossing with other strains. The results of 
these comparisons will be reported in a later pubhcation. It should 
be emphasized here that the 20 lines, or 23 per cent, of the original 
number, which were lost or discarded, represent for the most part 
extremely poor and undesirable material that would probably be 
lost in any selection experiment. By growing a larger number of 
plants in order to give a greater opportunity for selection and by 
hand-pollinating a larger number of individuals it would probably 
have been possible to continue many of these lines and some 
might even have turned out to be good strains in the end. Whether 
it is worth while to work more intensively with a few lines or 
expend the same amount of time on a larger number of strains less 
intensively selected is one of the most important problems to be 
considered. 




Figure. 44 Various tj^pes of chlorophyll deficiencies found in inbred 
strains of corn. 



THE PRODUCTION OF ABNORMALITIES 

An examination of the original ears after the first self-fertilization 
(figures 35 to 38) showed eight that were segregating tor small, dull 
colored seeds that were clearly abnormal. These recessive seeds 
varied on different ears from almost entirely empty pericarps to 
seeds nearly normal in size but shriveled and opaque in appear- 
ance, as shown in figure 42. These aborted seeds, in most cases, 
failed to grow and those which did germinate, produced abnormal 
seedlings none of which reached maturity. The normal seeds 
from ears showing defectives when planted produced segregating 
ears on some of the plants in the following generation. In addition, 
five ears which were not clearly segregating in the first generation 
produced some ears with abnormal seeds in their second generation 
progenies. It has since been found that this defective seed condi- 
tion is due to a large number of lethal or semi-lethal factors which 
are hereditarily distinct. They are wideh' distributed in all kinds 
of corn. In cross-pollinated plants only a few of these abortive 



PRODUCTION OF ABNORMAfilTIES 



391 




Figure 45. A chlorophyll-deficient dwarf compared to a normal 
plant in the same family. 



392 



CONNECTICU*r EXPERIMENT STATION 



BULLETIN 266. 



seeds are seen on any ears and these are not conspicuous. It is 
quite possible that the plants carrying these factors in a hetero- 
zygous condition may be seriously weakened by them and for 
that reason the elimination of these lethal endosperm factors is 
probably important. 

When the first generation self -fertilized ears were grown, chloro- 
phyll-deficient seedlings appeared as Mendelian recessives in 
fifteen lines. Eight of these segregated for white seedlings, one 
yellow, three yellowish green and three light green. These abnor- 
mal seedlings were quite distinct and most of them died as soon 
as the food stored in the seeds was exhausted. Several distinct 
types of striped and variegated plants which represent various 
forms of chlorophyll deficiency were observed and are shown in 
figure 44. 

Other clear-cut abnormalities which appeared in the first genera- 
tion as recessive segregates were golden plants in four lines, 
various forms of dwarfs in three lines, sterile tassels which pro- 




Figure 46. Various types of dwarfs found in inbred strains of corn. 

duced no pollen in five lines. Barren plants without ears and 
which had the appearance of being simple Mendelian recessives 
were found in three lines but the inheritance of such steiility 
factors has not been definitely proven. 

In addition to these common abnormalities some new characters 
were found which had not been observed in other material. A few 
plants of one line bore ears with no silks and such plants were 
therefore entirely sterile in the pistillate parts as shown in figure 47. 
Good pollen was produced and when crossed on to normal plants 
the silkless ears reappeared in later generations. This character 
was not found until the second generation. It may have occurred 
the first year and been overlooked. Another strain produced 
square cobs and another had ears with many silks in place of one 
for each seed. This latter character failed to reappear in later 
generations and apparently was not inherited, or at least not as a 
simple recessive. IVIany other variations from normal occurred. 
They differed in degree of abnormality, some affecting the plants 
much more seriously than others. 



PRODUCTION OF ABNORMALITIES 



393 



In twelve lines no abnormalities were noted in the first two 
generations, but in the third or fourth generation, various types 
appeared, in the form of chlorophyll-deficient seedlings, striped 
and variegated plants, dwarfs, seedlings with tube leaves 
instead of normally flat, and plants with only the mid-ribs in place 
of normal leaves. In some of these cases recessive segregates may 
not have appeared in the first generations on account of elimination 
due to poor germination or they may have been thinned out with 




Figure 47, 
family. 



Silkless ears compared to normal specimens from the same 



the weaker seedlings. In some cases, however, there seems to be 
no question that they are due either to original mutations or to 
delayed segregation resulting from some complicated mode of 
inheritance. A good illustration can be given in the production 
of the narrow-leaved plants shown in figure 48. Such a striking 
variation as this could not be easily overlooked. All the selected 
lines were carefully examined for abnormalities throughout the 
season, beginning with the early seedling stage. Narrow-leafed 
plants were first observed in the third generation in lines 112-13 



394 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 




Figure 48. Plants with narrow leaves occurred in two inbred lines. 



PRODUCTION OF ABNORMALITIES 395 

and 112-14. All three progenies of 112-13 produced some abnormal 
plants; two, nine and eleven narrow-leafed individuals appearing 
in the different progenies in a total of about 25 plants in each. 
This line had been segregating previously for dwarfs, golden plants, 
yellowish seedlings and striped dwarfs. Line 112-14 produced one 
narrow-leafed plant in the third generation. Though only normal 
plants were self -pollinated in the third generation, all of the fourth 
generation plants in line 112-13 were abnormal, being short and 
with streaked and wrinkled leaves varying in width from a mere 
mid-rib to nearly full width. The plants were so poor that no 
self -pollinated ears were obtained and the line was lost. Line 
112-14 produced no narrow leaves in the fourth generation. All 
the plants were described as uniform, leafy but short in stature. 
In the fifth generation three progenies, all from ears borne on 
normal plants in the fourth generation, were grown. No plants 
were obtained from one and only a few in the other two. All 
of these had typical narrow leaves and were badly stunted. They 
made a feeble growth and produced no ears. 

Pollen from typical narrow-leafed plants of the third generation 
out-crossed on to normal plants failed to show any abnormal 
plants in either the first or the second generation. Five self- 
fertilized progenies of the third generation were grown and in 
about 30 plants one narrow-leafed plant was found. The inheri- 
tance of this abnormality is not understood. 

In the fourteen lines of Burwell's Flint which came from ears 
self -pollinated in 1914 and not planted until 1919 no abnormalities 
of any kind were noted in the first two generations. In the third 
and fourth a few chlorophyll-deficient seedlings, striped plants and 
tube leaves appeared. In contrast to this are the 18 lines of the 
same variety self-pollinated in 1918 and planted the following 
year which segregated the first generation for defective seeds, 
dwarf plants and chlorophyll-deficient seedlings in five lines. Five 
other lines of this lot were so poor they were discarded, while none 
of the 1914 lot were eliminated. Though the number of lines is 
too few to be conclusive it seems that the delay of five years in 
planting may have eliminated many abnormalities by the death of 
the seeds carrying them. A germination test of these ears, made 
in 1919, showed a viability ranging from 10 to 100 per cent. Eight 
of the 14 ears germinated 90 per cent, or less. None of the one 
year old self -pollinated ears of the same variety germinated less 
than 85 per cent, and only two were less than 95 per cent. There 
was clearly an elimination of- seeds in the five-year resting period 
and this could easily have been selective, the seeds carrying the 
recessive abnormalities being less viable. If this is proven to be 
the case, some method of destroying the less viable seeds such as 
exposure to high temperature, alternate germinating and dr}'-ing 
or similar harsh treatment may be an effective means of weeding 
out defective germplasm. 

Many of these recessive abnormalities after they once appeared. 



396 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 










^ 



^ J- 






J .-"if 








Figure 49. Representative plants of three flint lines; from 
top to bottom they are 40-4, 105-10, and 105-20. 



PRODUCTION OF ABNORMALITIES 397 

kept reappearing in the following generations, but were finally 
eliminated, in every case except one, by the fifth generation. One 
line which was vigorous and productive and quite uniform in the 
fifth generation has segregated for white seedlings in CA^ery genera- 
tion. Selection of progenies has usually been based upon produc- 
tiveness and general appearances of the plants without regard to 
whether they were segregating for abnormalities or not. 

Out of the original 86 lines only 32 lines or 37 per cent, showed 
no clear-cut recessive abnormalities during the five generations 
they were self -fertilized. As stated before, 13 lines or 15 pei cent, 
segregated for defective seeds, and 15 lines or 17 per cent, for 
chlorophyll-deficient seedlings. Many of the lines had several 
types of abnormality. In a lot of 575 self-fertilized ears from six 
varieties of white fiint com in another selection experiment there 
were found 19 ears or a little more than 3 per cent, segregating 
for defective seeds. Of these, 441 were grown and 40 lines or 9 
per cent, were found to be segregating for chlorophyll-deficient 
seedlings. Hutchison self -fertilized 2,110 ears from a large number 
of different varieties of corn common!}^ grown in various parts of 
the country and found 3 per cent, segregating for defective seeds 
and 36 per cent, for various seedling characters, of which the greater 
nxrmber were chlorophyll deficiencies. 

The widespread occurrence of these recessive abnormalities is 
fully established. In normally cross-pollinated plants they are 
comparatively rare in appearance since they are present as reces- 
sives in the heterozygous condition. To what extent, if any, they 
reduce growth in the heterozygous condition has not been estab- 
lished. Lindstrom (1920) suggests that in eliminating these 
recessive abnormalities many desirable factors with which they 
are linked may also be taken out. Since these recessives are 
presumably scattered throughout the chromosomes many other 
factors both good and poor will be taken out A^ath them. 

It has been argued that the recessive abnormalities tend to be 
eliminated by natural selection except in those cases where they 
happen to be closely linked with exceptionally favorable growth 
factors, in which case they would be preserv^ed, and in weeding 
them out the factors which promote growth woiild be lost with 
them. The only answer to such an argument is to see what the 
facts are. Twenty-five lines segregating for clear-cut abnormalities 
gave progenies in the following generation, some with and some 
without the recessives. The 25 progenies which still carried the 
recessives averaged 50.8 bushels per acre yield in comparison with 
50.4 bushels for the 25 progenies grown in the adjoining rows, and 
from which the abnormalities had been eliminated. An equally 
good stand was obtained in each case, as an excess of seed was 
planted and the recessive abnormalities thinned out. The differ- 
ence in yield in the two lots is not significant. If there are favorable 
gro\\^h factors in the segregating progenies which are not present 



398 CONNECTICUT EXPERIMENT STATION BULLETIN 266 








S^nipsisppp ' 




% 



/V^t; 






/ X „ ' "!/' 







Figure 50. Representative plants of three earl}^ dent lines; from 
top to bottom they are 110-4, 6, 10. 



UNIFORMITY AND CONSTANCY 399 

in the non-segregating progenies from the same grand-parental 
plant they have no more effect than to counterbalance an}^ weaken- 
ing influence that the recessive abnormalities may have in the 
heterozygous condition. 

Another comparison is made by finding the average per cent, 
reduction in yield of all segregating lines from the first generation 
to the fifth generation, by which time the abnormalities were 
eliminated. This reduction was found to be 57.1 per cent, com- 
pared to the reduction of 58.1 for all lines which were free from 
abnormalities at the start. If any favorable groA^ith factors were 
lost when the recessive characters were weeded out, their departure 
caused no greater reduction in yield than took place in the other 
material from which no abnormalities were removed. 

From this it seems evident that the chances are no greater for 
good factors to be eliminated than poor ones and with other things 
being equal it seems highly desirable to take out these clear-cut 
recessive abnormalities. In fact it is necessary, in most cases, to 
eliminate all lethal and semi-lethal factors, in order to bring the 
strains to uniformity. 

THE APPROACH TO UNIFORMITY AND CONSTANCY. 

As expected, the first and second generations were quite 
variable but in the third generation, after three successive self- 
fertilizations, a number of lines became fairly uniform in height 
of plant, color of foliage and in general characteristics. In the 
fourth generation the majority of the lines had become well fixed 
in their type, and after five generations all of the selected lines, 
with a few exceptions, were alike within themselves. This 
uniformity was apparent in the plants of each progeny and in the 
similarity among the several progenies of the same line. A few 
lines remained variable throughout the five generations. As a 
rule the lines that showed uniformity in the third generation de- 
clined somewhat in size and yield in the two subsequent generations. 
Practically all of the best strains can be picked in the fifth genera- 
tion. Many of them can be recognized in the fourth and a few 
in the third. However, it is necessary to have a record of their 
performance during two and preferably three seasons after uni- 
formity is reached in order to be sure that they are fixed in their 
type. Several strains that were considered to be ^^ery promising 
in the third generation declined so in vigor and productiveness 
in the two following generations that they were much inferior to 
strains that had, earlier, been far less promising. On the other 
hand a few of the most vigorous and productive lines in the fourth 
and fifth generations were not noted as being promising in the 
third. While it cannot be asserted positively that strains which 
are uniform and good in appearance during the fourth and fifth 
generations will maintain themselves without further reduction 
the evidence from the older inbreeding experiments indicates that 



400 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 



^-^l^ ,^ .^ '/% \ 




7.,1 \-%h 







Figure 51. Representative plants of three late 
dent lines; from top to bottom they are: 112-1, 4, 9. 



DIFFERENCES IN SELECTED LINES 401 

they can be expected to maintain their level of vigor without much 
loss. Therefore in carr^dng out a selection process of this kind 
the fourth and fifth generations are the most important in 
affording an opportunity to pick the best-appearing self -fertilized 
strains. 

The selection process was carried out with the aim of securing 
the most vigorous and productive inbred strains, uniform and 
fixed in their t3^pe so that their good qualities could be maintained 
indefinitely. For this purpose five generations of self-fertilization 
are necessary in most cases. 

Differences in the Selected Lines. 

In the fourth generation all of the selected lines had become 
strikingly differentiated. Differences in height, color of foliage, 
size and shape of ears made each line distinct from every other line. 
In the Burwell Flint lines differences in average height ranged from 
51 to 98 inches, in the Gold Nugget lines from 44 to 84, in the 
Century Dent from 44 to 76 and in the Beardsley's Learning lines 
from 54 to 100. Color of foliage varied from ver\^ dark bluish 
green, through all gradations in shade to light green and yellowish 
green. In some lines the leaves were streaked with alternate 
rows of light and dark tissue. Various forms of fine and coarse 
flecking and mottHng of the leaves were a regular feature of some 
strains while others were entirely free from this ph}'siological irreg- 
ularity of the chlorophyll. 

The flint strains were most noticeably different in number of 
tillers. A number produced no large tillers and some had only a 
very few inconspicuous shoots from the base of the plants. Others 
branched very freely, producing many large branches on every 
plant. Alany of these were as large as the main stalk and bore ears. 
Some strains regularly produced seeds in the tassels on nearly all 
plants while others never did this. 

The ability to stand erect throughout the season is one feature 
that has been carefully selected for in all lines. Marked differ- 
ences in this respect were sho'WTi, being greater in some seasons 
than in others. Certain lines regularly went down sometime 
during the latter part of the season while others stood stiffly erect 
up to maturity. Equally pronounced differences in time of 
flowering are sho^^^l by the lines derived from the same variety. 
Most of the lines matured satisfactorily every season while others 
were so late as to be barely able to ripen seed. The weakening 
effect of inbreeding delays maturity in all lines but in spite of this 
some were earlier in ripening than the variety from which they 
were derived. Along with these diff'erences in maturity were 
great dissimilarities in character of the grain. The seeds of some 
were hard, translucent and bright colored; others were soft, dull 
colored and in some lines regularly moldy. 



402 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 




Figure 52. Representative ears of four 
productive Burwell Flint lines; from top to 
bottom they are: 30-19, 40-1, 7, S. 



DIFFERENCES IN SELECTED LINES 



403 




Figure 53. Representative ears of four unpro- 
ductive Burwell Flint lines; from top to bottom 
they are: 30-5, 6, 40-15, 16. 



404 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

The features named are the more striking ones. Differences in 
structural details are brought out in the accompanying illustra- 
tions showing the plants and ears of some of the selected lines in the 
fourth generation (figures 49 to 57). In details of structure and 
arrangement of parts the lines are so distinct that they can usually 
be easily recognized in the field and after harvest. In a few 
features certain strains may be alike. Some strains have similar 
plants but differ decidedly in ear structure. In others the ears 
are somewhat similar but are borne on markedly different plants. 
For the most part the differences are far more obvious than the 
similarities. 

Susceptibility to Disease. 

The most common diseases with which com has to contend in 
Connecticut are smut (Ustilago) , leaf blight (Helminthosporium) , 
and various -root, stalk and ear roots (Diplodea, Gibberella and 
other forms of Fusarium). Marked differences in smut infection 
were shown. Two lines 105-14 and 110-17 showed no smut 
infection on any plant in any progeny during the five generations 
they were grown. Eleven strains had no more than one plant 
affected in any one year throughout the same period. The place 
on the plant where the smut balls appeared was usually quite 
characteristic, some strains having them on the basal nodes, others 
at the ear node, still others on the leaves or tassels. In some lines 
numerous light infections on the plant or ears were shown which 
apparent^ did not do any serious damage. Other strains had 
many plants badly injured and sometimes killed outright during 
mid-season. The most striking case of segregation of suscepti- 
bility to parasitism by the smut fungus occured in line 110-3. In 
the first generation four per cent, of the plants were smutted. In 
the second three progenies were grown having twelve plants in 
each. In one progeny none of the plants had any indication of 
smut infection. In another all of the plants were smutted and 
most of them were killed during the middle of the summer. In 
the third progeny 27 per cent, of the plants were attacked. The 
original seed of the two strikingly different progenies was planted 
again the following year with the result that out of 57 plants of the 
resistant progeny, only one plant o^ 1.7 per cent, was infected. 
The smutted lot had 14 plants infected out of 31 grown, or 45.2 
per cent. In the next generation no smutted plants were seen in 
the one line and 65.6 per cent, in the other. Marked differences 
were shown in the seeds of the two lines. Plants of the susceptible 
line were extremely weak but the seeds were normal in appearance. 
However the germination of these seeds was poor and in the fifth 
generation no plants were obtained. The resistant line produced 
more vigorous plants having a noticeably darker green color. 
All of the seeds produced on these plants were distinctly abnormal . 
When dry they were shriveled and discolored although not showing 



SUSCEPTIBILITY TO DISEASE 



405 




Figure 54. Representative ears of four Century 
Dent lines: from top to bottom they are: 110-3, 
4, 5, 10. 



406 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

any of the usual molds. Ears of this line are shown in figure 54. 
In spite of their unfavorable appearance some of the seeds germ- 
inate and the plants produced are about as good as the average 
inbred strain of the same variety. 

None of the smut-free lines were outstandingly good in other 
respects and some of the most vigorous and productive strains now 
regularly show a high percentage of smut infection. The smut -free 
or low-smut strains may have value in crossing with other strains 
which have good qualities but are lacking in smut resistance. 

The growing season of 1922 was unusually wet and the selected 
lines then in the fourth generation showed very pronounced 
differences in the amount and severity of infection of Helmintho- 
sporitun. This organism, which is seldom injurious to ordinary 
cross-pollinated corn, readily attacks many inbred plants and on 
some completely kills the leaves after seed formation begins. Leaf 
blighting due to this organism had been noted each year in some 
lines but in the wet season of 1922 it was particularly injurious. 
Seventeen of the eighty-six lines showed heavy infection. Some 
of them lost all their foliage prematurely and the ears were badly 
stunted, the grains being small and poorly developed. Some of 
the most vigorous and productive strains in former years were so 
injured in this way as to give them a very low rating. The follow- 
ing year was unusually dry. Very little damage from this cause 
was seen, but the effect of the drought on different strains was very 
striking. Some strains which had always before produced green 
luxirriant foliage had their leaves killed at the sides and tips by the 
dry heat and were unproductive for that reason. Most of the 
strains which had been badly injured by leaf infection in the wet 
season were beautifully green throughout the dry period of 1923 
and were among the best appearing and most promising of all the 
selected lines. These marked differences in different seasons makes 
it extremely difficult to judge the value of inbred strains and makes 
it necessary to test them during several years after they have 
become uniform and fixed in type. 

The investigations of Hoffer, Holbert and others have empha- 
sized the importance of the root, stalk and ear rot organisms 
attacking corn. The results of the earlier inbreeding experiments 
indicated that marked differences would be found among inbred 
plants to resist infection. Throughout the selection experiment 
great importance was placed on the ability of the plants to stand 
erect throughout the season and have the ears free from any in- 
dication of mold. Fallen plants or moldy ears were avoided when- 
ever possible. The most outstanding differences in ability to 
stand erect and in freedom from mold on the ears, were seen in the 
third and later generations. In 1922, a wet season, four lines 
(30-6, 105-20, 110-2, 110-15) had all the plants of all three progenies 
erect throughout the season. This same vear twelve lines (30-8, 
30-9, 105-3, 105-18, 110-1, 110-2, 110-6^ 110-7, 110-18, 112-6, 



SUSCEPTIBILITY TO DISEASE 



407 




Figure 55. Representative ears of four Gold 
Nugget flint lines; from top to bottom they are: 
105-3, 10,-17, 20. 



408 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 




Figure 56. Representative ears of four produc- 
tive Beardsley's Learning lines; from top to bottom 
they are: 112-1,4, 6, 9. 



SUSCEPTIBILITY TO DISEASE 



409 






1 = 


w 




/ 


^C!S< 


< • 


I*- , 






1? 




Figure 57. Representative ears of four unpro- 
ductive Beardslev's Learning lines; from top to 
bottom: 112-3, 10, 14, 15. 



410 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

112-7, 112-8) produced no moldy ears. Only one line 110-2 had all 
plants erect with ears free from mold. In the first generation this 
line had four per cent, of moldy ears and ten per cent, of fallen 
plants but no smut. In the second generation there were ten per 
cent, moldy ears, ten per cent, fallen plants and no plants showing 
smut infection. In the third, fourth and fifth generations there 
was no mold, smut or fallen plants on the three progenies grown 
each year. This strain is also productive for the variety, although 
surpassed in this respect by several other strains. The seeds are 
hard and bright but very pale yellow in color and almost white on 
top. 

In contrast to this is line 105-20 with all the plants erect in the 
second, third and foirrth generations but with 29, 17 and 44 per 
cent, of moldy ears in the same years. On the other hand, 40-8 
had all of the plants fallen in two progenies of the fourth generation 
and no moldy ears. To complete the combinations 112-11 had 87 
per cent, of the plants on the ground in 1922 and 67 per cent, of 
ears moldy. 

Criterions of Selection. 

At the beginning of the selection experiment the plan as previous- 
ly stated was to self -pollinate five plants in each line and to select 
three of the best self -fertilized ears for planting the following year. 
Even when these ears differed greatly in appearance no consistent 
differences were noted in the progenies grown from them. The 
coefficient of association between the appearance of the ear and 
yield of the different progenies within several lines is- — .18. This 
indicates that self -pollinating a large number of ears in order to 
make more extensive selection of desirable looking ears is of doubt- 
ful value. Of the three progenies grown only one was to be chosen 
to continue the line, the other two not being pollinated. It was 
soon noted, however, that there was very little relation between the 
appearance of the progenies at the time of bagging and their pro- 
duction of grain and the general appearance of their plants at 
harvest. The coefficient of association between the appearance 
of the plants at pollinating time and the yield of the different 
progenies within the several lines is — -.28. Seedlings were groum 
in the greenhouse and their weight and height after thirty days of 
growth were compared with the yield of the same progenies in the 
field. The third and fourth generations showed that those prog- 
enies that had the tallest seedlings yielded 1.6 bushels per acre 
more than the other progenies in the same lines. This difference is 
hardly enough to make a selection of the progenies on this basis 
worth while. 

Since there is no appreciable correlation between the characters 
of the seed- ear, weight of seed, size of the seedling, or the appear- 
ance of the plants at pollinating time and production of grain the 
only selection of progenies that can be made with any degree of 



CLASSIFICATION OF SELECTED LINES 



411 



effectiveness is at maturity. Here also yield is highly influenced 
by the amount of heterozygosity remaining. In some lines there 
are more homozygous combinations than in others and they are 
correspondingly less vigorous and productive although they may be 
potentially more desirable. For this reason final judgment must 
be left until the plants are reduced to uniformity and constancy. 
Hence it is interesting to note what resemblance the resulting 
inbred strains, when finally reduced to imiformity and fixity of 
type, have to the same strains in the first generations of inbreeding. 

Classification of Selected Lines. 

Taking into consideration all features of these selected lines as 
they grow in the field and after han^est in the fourth and fifth 
generations and giving most importance to the production of 
bright sound grain, the four outstanding good and poor strains in 
each variety are listed as follows, with their yields in bushels per 
acre in the fifth generation compared with that of the original 
variety grown the same ^^ear: 



Bur-well's Flint 51.2 



Good Lines 






Poor 


Lines 




Number 


Yield 




Number 




Yield 


30-5 


12.2 




30-10 




44.2 


30-19 


15.3 




30-18 




18.3 


40-4 


33.6 




40-3 




35.1 


40-8 


25.9 




40-16 




24.4 




Gold 


Nugget 54.0 






Good Lines 






Poor 


Lines 




Number 


Yield 




Number 




Yield 


105-11 


29.0 




105-3 




9.2 


105-15 


33.6 




105-8 




13.7 


105-17 


22.9 




105-13 




7.6 


105-20 


10.7 




105-16 




29.0 




Century Dent 


48.3 






Good Lines 






Poor 


Lines 




Number 


Yield 




Number 




Yield 


110-2 


15.3 




110-1 




28.9 


110-4 


16.8 




110-9 




4.6 


110-5 


10.7 




110-15 




1.5 


110-10 


19.8 




110-17 




12.2 




Beardsley' 


s Leaming 49.5 






Good Lines 






Poor 


Lines 




Number 


Yield 




Number 




Yield 


112-1 


42.7 




112-3 




10.7 


112-6 


27.5 




112-7 




21.4 


112-9 


33.6 




112-14 




.0 


112-12 


12 2 




. 112-16 




9.2 



412 



CONNECTICUT EXPERIMENT STATION 



BULLETIN 266. 



This is purely an arbitrary classification based upon the general 
appearance of the plants and ears. Some of the poor lines yielded 
more than the good lines but produced a very poor quality of grain. 
The original ears from which these lines descended (figures 35 to 38) 
show that there is no relation between the good and poor strains 
after uniformity was attained and the appearance of the seed ears 
from which they came. Low and high numbers are represented 
about equally in the good and poor strains. 

Correlation Betwee^t the First and Last Generations. 

In order to find out whether the elimination of the poor lines at 
the beginning of the inbreeding period is advisable, the correlation 

Table VIII, 

CoeflEicients of association between early and later generations of self- 
fertilized corn. 

Generations Compared 1-4 1-4 1-5 2-5 1-4 

Variety Height Mold Tillers Smut Yield 

Burwell's Flint 60 .89 .64 —.08 

Gold Nugget 35 .38 .38 .14 

Century Dent 80 .80 —.72 .72 .28 

Beardsley's Learning 95 .38 .50 .20 .50 

Ave. Flints 50 .65 .55 .10 .05 

Ave. Dents 89 .63 .17 .52 .38 

Average 71 .64 .27 .27 .19 

between the behavior of the plants in the first generation and the 
last generation has been worked out for the most important 
characters. In Table VIII are shown the coefficients of association 



^ 






1 


— 



I ■ 



I 



I 



i 



r 



FIRST FOURTH 

HEIGHT 



FIRST FOURTH 

MOLD 



FIRST FIFTH 

TILLERS 



SFCCND FIFTH 
SMUT 



FIRST FOURTH 

YIETLD 



Figure 58. Diagram representing the average of the upper and lower 
groups in the first generations and the average of the same lines in the 
last generations, based on the data in Table IX. 



between the first or second inbred generation and the fourth or 
fifth for height of plant, per cent, moldy ears, number of tillers, 
per cent smutted plants, and yield of grain. The fifth generation, 
grown in 1923, was so variable on account of the extremely dry 
season afTecting different parts of the field unevenly that the 
coefficients for height and yield are based on the first and fourth 
generations. There was very little smut infection in the first 



CORRELATION BETWEEN GENERATIONS 



413 



generation and practically no mold in the fifth so that the coefficient 
for per cent, smut is based upon the second and fifth generations 
and for per cent, mold upon the first and fourth. 

The figures show a fairly high association for height of plant and 
moldy ears. This means that by selecting the highest lines in the 
first generation the resulting inbred strains in the fourth generation 
would tend to be taller than the average. Similarly, by selecting 
lines at the start that were free from mold, inbred strains could 




GENERATIONS 

Figure 59. Graph showing the behavior of two lines with respect to 
height during four generations of self-fertihzation, selected for vigor and 
productiveness but not for height. From one to three progenies are 
grown in each generation. 



finally be attained that would on the average be freer from moldy 
ears than other strains which showed more mold at the start. 
This relation does not hold so well for the other characters ; number 
of tillers and per cent. smut. For these the coefficients are low 
and in two of the varieties a negative correlation is shown. This 
means that lines without tillers and showing low smut infection 
may be obtained from plants at the start which have tillers and are 
susceptible to smut infection. 



414 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

Another method of bringing out the relation between the several 
lines at the start and at the end of the selection period is to separate 
all the lines of each variety into the upper and lower halves, with 
respect to the characters studied, in the first generation and then 
compare the average of these two groups with the averages of the 
same lines after being inbred for four or five generations. This has 
been done in Table IX, making the separation within each variety 
into equal sized groups in the first generation. Thus the basis for 
separating the groups is the median instead of the mean. The 
results are stmimed up graphically in Figure 58. It will be seen 
that the relative position of the upper and lower halves remains 
nearly the same at the end of the period of selection as at the 





Table IX. 












The relative position of the same self-fertiUzed Unes at the 
and at the end of the period of selection. 


; beginning 


Characters measured 


Groups 


No. of Strains 
First Last 


Average 
First Last 


Relative 
First Last 


Height of plant in inches 
in the first and fourth 
generations 


High 
Low 


36 
36 


94 
96 


81 
70 


70 
61 


100 

87 


100 

87 


Per cent, of moldy ears 
in the first and fourth 
generations 


High 
Low 


36 
35 


88 
95 


IS 
5 


15 

8 


100 
29 


100 
57 


Number of tillers per plant 
in the first and fifth 
generations 


High 
Low 


32 
33 


91 

94 


.9 
.3 


1.2 

.8 


100 
36 


100 
67 


Per cent, of plants with 
smut in the second and 
fifth generations 


High 
Low 


32 
33 


90 
93 


14 
1 


12 

7 


100 
9 


100 
59 


Yield of grain in bu. per 
acre in the first and 
fourth generations 


High 
Low 


31 
31 


84 
82 


81 
52 


44 
41 


100 
64 


100 
93 



beginning for such characters as height of plant, number of tillers, 
per cent, of moldy ears and smutted plants although the difi^erences 
are generally less at the close than at the start. This tendency to 
change during the period of inbreeding is most marked for 3deld of 
grain. In this respect the high and low groups are ver}^ nearly 
ahke at the end of the selection period in spite of the fact that all 
along attention has been given to productiveness. These results 
indicate that it is unwise to eliminate the unproductive strains in 
the first generations, as from them lines may be obtained that are as 
productive as those from high yielders at the start. Other char- 
acters can apparently be somewhat more surely selected for at the 
beginning of the inbreeding period. If such characters as freedom 
from mold and smut are of chief importance it might be advisable 
to eliminate those lines which show much mold and smut in the first 
inbred y:enerations. 



LIMITING FACTORS 415 

The general tendency for some of the lines to hold the same 
relative position throughout the process of selection is illustrated 
by the height of plant of two lines shown graphically in figure 59. 
In the first inbred generation the two lines averaged 69 and 77 
inches in height. In the second generation two progenies over- 
lapped but from then on they were clearly distinct, the difference 
in height increasing until the end of the selection period. The 
same result is shown in the average number of tillers per plant of 
two other lines as brought out in figure 60. Differing at the start 
the two lines remained distinctly different in all their progenies 
throughout the period of inbreeding. In marked contrast to this 
is the result shown graphically in figure 61. Two fines differing 
noticeably in their nimiber of tillers changed positions so that in 




3 
GENERATIONS 

Figure 60. Graph showing the behavior of two lines with respect to 
tillering during five generations of self-fertilization, selected for vigor and 
productiveness but not for the number of tillers. The relative position 
of these two lines remained the same. 

the end the few tiller strain at the start averaged more tillers on all 
progenies than did the many tiller strain. Similarly two strains 
which were alike in this respect at the start became extremely 
different as uniformity and constancy was reached, as shown in 
figure 62. 

Limiting Factors. 

In planning and carrying out a selection program the best 
procedure will depend upon the number of plants which can be 
grown and the number of hand pollinations which can be made in a 
season. Where the facilities available for artificial pollination is 
the limiting factor, and this is usuall}' the case, the best procedure 



416 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

is to self-pollinate just enough plants to continue as many lines 
as possible until a reasonable degree of homozygosity is reached. 
If the amount of land available to grow the plants is the limiting 
factor it would be better to pollinate a larger number of plants 
within each line, although extensive selection within a progeny 
has been shown to have little value, as the better individuals are 
almost certain to be more heterozygous, making it difhcult to arrive 
at their true value. More attention should be paid to increasing 
the number of progenies within the more desirable appearing Hnes, 
basing selection on their behavior throughout the season and their 
uniformity and productiveness at maturity. 

The method now being used at this station is to grow three 
progenies in each line and to pollinate two plants in each progeny. 
On the basis of the general appearance of the plants in the field and 



CrNCRATIOINS 

Figure 61. Graph showing two lines which differed in the amount of 
tillering and which changed positions during the five generations of self- 
fertilization. 

their productiveness at maturity the best and second best progenies 
are noted where there is an appreciable difference. Two ears from 
the best progeny and one from the second best are used for planting 
the following year. If no differences are shown, one ear from each 
of the three is planted. This procedure is based upon the results 
in the five-year selection experiment described above in which no 
reliable criterions of selection were found which could be used 
before the time of pollination. It is still provisional and will be 
modified as future experience justifies. It is possible that better 
results can be obtained by paying still less attention to selection 
during the reduction period than the method outlined. By expend- 
ing the same amount of time and effort on more lines, growing only 
one progeny in each generation and pollinating only enough plants 
to insure the perpetuation of the strain until uniformity and 
constancy are reached, more diverse material would be available 



CONCLUSION 417 

from which to select the best inbred strains. In this procedure 
there would be the possibility, and even probability, of missing 
altogether valuable material which might exist in some lines. 
However, since it has been shown that many of the lines change 
greatly during the reduction process, selection during this period 
will always be somewhat ineffective. From a theoretical stand- 
point the best method is the one which will produce the largest 
number of fixed strains from which to choose the ones best suited 
to the purpose for which they are to be used. 

In this connection one further point should be mentioned. When- 
ever any particularly outstandingly good strain has been obtained 
there is the possibility that still better material may exist in that 
strain in the earlier generations. This would indicate that it 




GENERATIONS 

Figure 62. Graph showing two lines which showed the same amount 
of tillering at the start but differed widely at the end. 

might be well worth while to go back to the earlier generations and 
grow as much of this material as possible from the remaining seed 
in order to obtain the very best gemiplasm available in this strain. 
In fact, this procedure has already been followed with several of the 
more promising lines and it has been possible to isolate new strains 
which are distinctly superior in some respects to the old ones. 

Conclusion. 

The one fact that stands out from the results secured in this 
selection experiment is that there is no single criterion by which 
high-yielding strains can be obtained. During the process of 
inbreeding, with the resulting segregation and recombination and 
the automatic elimination of heterozygous combinations of factors, 
selection for particular characters is somewhat effective. By 



418 CONNECTICUT EXPERIMENT STATION BULLETIN 266. 

choosing tall plants as progenitors in each generation tall strains 
can be produced. By selecting plants free from tillers, strains with , 
few tillers can be obtained. Similarly, freedom from disease in- 
fection, as far as resistance is inherited, can be expected by selecting 
during the reduction period only those plants which show no 
infection in fields where infection is present. Even with these 
characters the association is far from complete. But productive- 
ness, yield of grain, which sums up the plant's entire energies shows 
no such simple relation. High yielding strains may come, and 
have come, from plants which are poor producers. Promising 
strains during the first generations may be very unproductive or 
undesirable in some respect when finally reduced to uniformity and 
constancy. This emphasizes the fact that effective selection must 
be based upon the performance of the plants after homozygosity 
is attained. 

LITERATURE CITED. 

Cummings, M. B., and Stone, W. C, 1921. Yield and quality in Hubbard 

squash. Vermont A. E. S. Bull. 222. 
East, E. M., 1908. Inbreeding in corn. Report Conn. A. E. S. 1907- 

1908. 

1909. The distinction between development and heredity 

in inbreeding. Amer. Nat. 43: 173-181. 
East, E. M., and Hayes, H. K., 1912. Heterozygosis in evolution and in 

plant breeding. U. S. Dept. Agr., Bur. P. I. Bull. 243. 
Eyster, W. H. 1924. A primitive sporophyte in maize. Amer. Jour. Bot. 

11:7-14. 
Hayes, H. K.,and Brewbaker, H. E. 1924. Frequency of mutations for 

chlorophyll-deficient seedlings in maize. Jour. Her., 15: 497-502. 
Hoffer, G. N. and Holbert, J. R. 1918. Selection of disease-free seed corn. 

Ind. A. E. S. Bull. 224. 
Hutchison, C. B., 1922. Heritable variations in maize. Jour. Agron., 14: 

73-78. 
Jenkins, M. T., 1923. A new method of self -pollinating corn. Jour. Her., 

14: 41-44. 
Jones, D. F. 1918. The effects of inbreeding and crossbreeding upon 

development. Conn. A. E. S. Bull. 207. 

1920. Heritable characters of maize, IV. A lethal factor- 
defective seeds. Jour. Her. 11: 160-167. 

1921. The indeterminate growth factor in tobacco and its 
effect upon development. Genetics, 6: 433-444. 

1924. The attainment of homozygosity in inbred strains of 
maize. Genetics, 9: 405-418. 

Kiesselbach, T. A., 1922. Corn investigations. Neb. A. E. S. Res. 
Bull. 20. 

King, H. D., 1918. Studies on inbreeding, I-III. Jour. Exper. Zool. 26. 

Lindstrom, E. W., 1920. Chlorophyll factors of maize. Jour. Her. 
11:269-277. 

1923. Heritable characters of maize XIII. Endo- 
sperm defects: sweet defective and flint defective. Jour. Her., 14: 
127-135. 

Mangelsdorf, P. C, 1923. The inheritance of defective seeds in maize- 
Jour. Her., 14: 119-125. 



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