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Bulletin 266 March, 1925 ^2
QJunnrrttrut A^rtrultural ^£xpnmmt ^tattu«
'^^ta Haopn, (Cottn^rttrut
The Improvement of Naturally Cross-
Pollinated Plants by Selection in
I. THE PRODUCTION OF INBRED STRAINS
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
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
E. H. Jenkins, Ph.D., Director Emeritus.
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.
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-
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.
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.
Donald F. Jones, S.D., Geneticist in Charge.
P. C. Mangelsdorf, M.S., Graduate Assistant.
M. F. Morgan, M.S.. Investigator
George C. Scarseth, B.S., Graduate Assistant.
-, In Churge.
N. T. Nelson, Ph.D.. Plant Physiologist.
The Wilson H. Lee Co.
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
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-
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-
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
CONNECTICUT EXPERIMENT STATION
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
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-
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
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.
CONNECTICUT EXPERIMENT STATION
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
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
Yield and Height of Four Inbred Learning Strains of Corn Self-Fertilized
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
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.
Per cent, of Plants Showing Smut Infestation in Fonr Inbred Learning
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
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.
CONNECTICUT EXPERIMENT STATION
41 -« 41 4^ 41
41 41 41 41 -H
lO I^ 00 CD t^
THE EFFECT OF INBREEDING UPON CORN
CONNECTICUT EXPERIMENT STATION
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
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
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.
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
CONNECTICUT EXPERIMENT STATION
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
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
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
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
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
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
CONNECTICUT EXPERIMENT STATION
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
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
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
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-
CONNECTICUT EXPERIMENT STATION
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
CONNECTICUT EXPERIMENT STATION
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
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
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-
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
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
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
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.
CONNECTICUT EXPERIMENT STATION
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
Two additional changes have occurred in other inbred material
THE VALUE OF INBREEDING
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
CONNECTICUT EXPERIMENT STATION
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
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
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
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
CONNECTICUT EXPERIMENT STATION
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
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
CONNECTICUT EXPERIMENT STATION
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 :
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.
CONNECTICUT EXPERIMENT STATION
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
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
CONNECTICUT EXPERIMENT STATION
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
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
CONNECTICUT EXPERIMENT STATION
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
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
Figure 45. A chlorophyll-deficient dwarf compared to a normal
plant in the same family.
CONNECTICU*r EXPERIMENT STATION
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
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
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
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
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.
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
/ 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
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^ ,^ .^ '/% \
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
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
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
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
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
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
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
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
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
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
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
s Leaming 49.5
CONNECTICUT EXPERIMENT STATION
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
CoeflEicients of association between early and later generations of self-
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
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
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
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
The relative position of the same self-fertiUzed Unes at the
and at the end of the period of selection.
No. of Strains
Height of plant in inches
in the first and fourth
Per cent, of moldy ears
in the first and fourth
Number of tillers per plant
in the first and fifth
Per cent, of plants with
smut in the second and
Yield of grain in bu. per
acre in the first and
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
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
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
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
Figure 61. Graph showing two lines which differed in the amount of
tillering and which changed positions during the five generations of self-
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
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
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.
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
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squash. Vermont A. E. S. Bull. 222.
East, E. M., 1908. Inbreeding in corn. Report Conn. A. E. S. 1907-
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.
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:
Jenkins, M. T., 1923. A new method of self -pollinating corn. Jour. Her.,
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.
King, H. D., 1918. Studies on inbreeding, I-III. Jour. Exper. Zool. 26.
Lindstrom, E. W., 1920. Chlorophyll factors of maize. Jour. Her.
1923. Heritable characters of maize XIII. Endo-
sperm defects: sweet defective and flint defective. Jour. Her., 14:
Mangelsdorf, P. C, 1923. The inheritance of defective seeds in maize-
Jour. Her., 14: 119-125.
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