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Genetics and Breeding in

the Improvement of

the Soybean

C. M. WOODWORTH

UNIVERSITY OF ILLINOIS
AGRICULTURAL EXPERIMENT STATION

BULLETIN 384

Urbana, Illinois November, 1932

Publications in the Bulletin series report the results of investigations
made by or sponsored by the Experiment Station

CONTENTS

PAGE

INTRODUCTION 297

DESCRIPTION OF THE PLANT 298

METHOD OF REPRODUCTION 300

Extent of Natural Crossing in Soybeans 300

Method of Making Crosses Artificially 302

INHERITANCE IN SOYBEANS 305

SEED CHARACTERS 305

Cotyledon Color 305

Coat Color 310

Hilum Color 312

Abnormal Hilum 314

Defective Seed Coats 316

Mottling of Seed Coat 317

Bloom on Seed Coat '. 322

PLANT CHARACTERS 323

Flower Color 323

Stem Color 324

Pubescence Color 324

Glabrousness 324

Leaf Shape 326

Number of Leaflets 326

Height of Plant ' 328

Maturity 328

Sterility 329

Variegation 329

Fasciation 331

Pod Color 331

Pod-Bearing Habit 333

LIST OF GENES IN SOYBEANS 333

LINKED CHARACTERS IN SOYBEANS 334

VARIATION IN SOYBEANS 336

SELECTION AS A METHOD OF BREEDING 344

COMPOSITION OF VARIETIES 344

NATURAL PURIFYING METHOD OF SELECTION 346

PEDIGREE SELECTION METHOD • 347

MASS SELECTION METHOD 349

CROSS-FERTILIZATION OR HYBRIDIZATION AS A METHOD

OF BREEDING 352

BREEDING FOR SPECIAL FEATURES 357

OIL AND PROTEIN CONTENT 357

QUALITY OF OIL 371

RESISTANCE TO DISEASE 373

Bacterial Diseases 373

Fungous Diseases 374

Mosaic Disease 375

YIELD OF SEED 377

Number of Nodes per Plant 380

Number of Pods per Node 381

Number of Seeds per Pod 385

Percentage of Abortive Seed 386

Size of Seed 390

Correlation Between Yield Components and Yield 391

Correlation Between Yield Components Themselves 398

SUMMARY 399

LITERATURE CITED.. .402

Genetics and Breeding in the Improve-
ment of the Soybean

By C. M. WOODWORTH, Chief in Plant Genetics

INTRODUCTION

A NATIVE of the orient, the soybean has proved a valuable ad-
f=\ dition to American crop plants. Since its introduction into the
-^- -^-United States it has spread over much of the territory east of
the Mississippi river, becoming well established in the crop schemes
of this area. It has many characteristics to recommend it to American
agriculture. Being a legume it has the ability to utilize the free nitro-
gen of the air thru the symbiotic relationship with bacteria living on its
roots, and is therefore of value from the soil fertility standpoint. The
plant itself is high in nutritive value, the seeds especially so, making it
particularly valuable as a livestock feed. The crop also has high com-
mercial value, the seed furnishing oil and other products useful in
industry.

Hundreds of varieties of soybeans have been brought into the
United States thru the plant introduction service of the U. S. Depart-
ment of Agriculture. Practically all as introduced were badly mixed
and had to be purified before they could be multiplied for trial in
different parts of the country. In this work single plant selection
has been resorted to, for the most part, rather than the elimination of
rogues or off-type plants. As a result of the work done by the U. S.
Department of Agriculture and subsequent work by various state ex-
periment stations, numerous types have been developed that differ in
maturity, habit of growth, seed color, and special adaptation to varying
soil and climatic conditions.

The work of developing this versatile plant to its full possibilities,
however, is only in its beginning. The problem is complicated, and
will take long and patient study. In this bulletin an attempt has been
made to bring together the essential information on soybean genetics
available at the present time; to discuss the principles of breeding that
are applicable to soybean improvement; and to review the results of
investigations made at this and other institutions with a view to im-
proving the crop in certain special features.

297

298 BULLETIN No. 384 [November,

DESCRIPTION OF THE PLANT

The soybean belongs to the genus Soja, family Leguminosae. This
genus contains about fifteen species that are native of Asia, Africa,
and Australia. Only one species, Soja max, is of any economic im-
portance. The wild soybean, S. ussurriensis, as grown at Urbana is
very fine-stemmed, prostrate in habit of growth, and very late in ma-
turing. It is probably the type from which our common varieties have
descended.

The soybean plant is an erect, bushy, rather leafy annual and at-
tains a height of several feet under favorable conditions. If given suffi-
cient space in which to grow, it branches profusely from the lower
nodes of the main stem, but if the plants are crowded, very little
branching occurs. In general, the soybean may be said to be indeter-
minate as to habit of growth. Types are known, however, that differ
greatly from the typical indeterminate habit, resembling plants show-
ing the determinate type in that there is a terminal inflorescence. In
some indeterminate varieties the stems are so fine and small at the tip
that they show a tendency to twine about one another.

Like the clovers, the soybean plant has trifoliate leaves; that is,
each compound leaf is made up of three leaflets. Occasionally com-
pound leaves with four and five leaflets have been observed. The first
leaves to appear above the cotyledons in the seedling are simple, and
these are opposite, one on each side of the stem. The compound leaves
originate at the nodes and are alternate. The shape of the leaflet is
ovate-lanceolate (Piper and Morse35*) but a few types are known in
which the leaflets are narrowly lanceolate or almost linear. The leaves,
as well as the stems, are covered with numerous fine hairs, the pubes-
cence, except of course in glabrous varieties. As the time of maturity
approaches, the leaves of yellow cotyledon varieties turn yellow and
finally drop off, but in green cotyledon varieties the leaves do not turn
yellow — they remain green until they finally drop off, leaving the stems
bare except for the pods.

The flowers are borne in axillary racemes or peduncles at the nodes.
Considerable variation exists in the average number of flowers per
raceme in different varieties. Flowers appear first at the base of the
main stem, then progressively toward the tip, and this order of bloom-
ing is followed on the branches also. The period of flowering in the
soybean is usually not over three weeks. Hence the pods come to
maturity at almost the same time. This is of considerable importance
from the standpoints of harvesting and subsequent handling of the crop.

"These numbers refer to literature citations on pages 402 to 404.

1932] SOYBEAN BREEDING 299

Soybean pods are small, containing one to four seeds. The pro-
portion of 1-, 2-, 3-, and 4-seeded pods varies greatly in different varie-
ties as well as in different plants of the same variety. Soybean pods are
straight or slightly curved like a scythe. In size they show a relation to
the size of the seeds they contain. Varieties with large seeds bear large
pods, and those with small seeds, small pods. In all but the glabrous
varieties the pods are covered with numerous fine hairs, the pubes-
cence. Soybean pods are two-valved. The shattering or bursting apart
of the valves or halves of the pods is an objectionable feature because
the seeds are scattered over the ground and lost. The tendency to
shatter is, to some extent, a varietal characteristic. Pods of some varie-
ties shatter badly, others only slightly.

The seed of the soybean is pea-like, containing an embryo, two
cotyledons, two seed coats, but no endosperm. Practically all the food
material in the seed is contained in the cotyledons. The seeds of differ-
ent varieties vary in shape from round to elliptical and from small to
large in size. The hilum, or seed scar, is the place of attachment of
the seed to the pod. Usually the seed separates from the pod cleanly,
leaving the hilum with a smooth, straight surface, but in the Soysota
variety, as noted by Owen,32* the seed separates from the pod in such
a way as to leave the hilum with a characteristically rough surface.
The seeds are attached to one side of the pod only. This is readily
seen when the two halves of the pod are broken apart. Also in multi-
seeded pods the seeds are attached first to one side of the pod, then to
the other side, and so on, giving a zigzag arrangement.

Soybean seeds with two embryos have been observed by Owen.29*
These occurred in a Chinese variety to the extent of .44 percent (based
on a sample of 5,000 seeds). There apparently exists a tendency for
the abnormality to be inherited in this variety. An occasional double
embryo seed was also observed by Owen in the Mandarin and Manchu
varieties. The two embryos inclosed within the same seed coat were
not identical, as pointed out by Owen, since in progenies segregating
for cotyledon color two cotyledons were of one color and two of the
other color. Hence it appeared likely that two ovules were fertilized
and developed independently within the same seed coat.

As stated above, the soybean seed has two cotyledons. In tests at
the Illinois Station a three-cotyledonous seedling (Fig. 1) occurred in
the Virginia variety. This seedling was propagated in the field and
seed was saved. When tested in the greenhouse, six seedlings (3.3
percent) out of a total of 182 showed a third cotyledon. In two of
these, two of the three cotyledons were not completely divided. In ad-

300 BULLETIN No. 384 [November,

dition one seedling had four cotyledons which were not completely di-
vided. Since the percentage of such types is higher than has been ob-
served before at this Station in this or any other variety, it would
appear that there is a tendency for this abnormality to be inherited.

METHOD OF REPRODUCTION

The soybean is normally a self-fertilized plant. The flowers are
perfect, producing both pollen grains and ovules. As pollination occurs
when the flower opens or a little before, there is very little chance for
pollen from other flowers on the same plant or on different plants

FIG. 1. — A SOYBEAN PLANT WITH THREE COTYLEDONS

Such plants are rare. The variation is probably inherited. It will be noticed
that this plant also has three primary simple leaves, while plants with two
cotyledons have only two such leaves.

to enter and effect fertilization. Nevertheless a small amount of
natural crossing does occur, as will be shown later. Since the soybean
plant is almost exclusively self-fertilized, it is pure for its hereditary
characters unless of course it is a hybrid or the offspring of a hybrid.
The principles of breeding that apply to self- fertilized crops, such as
wheat, oats, and barley, apply as well to the soybean. .

EXTENT OF NATURAL CROSSING IN SOYBEANS

There is considerable evidence for the occurrence of natural cross-
ing in the soybean. Piper and Morse34* found in a bulk lot of seed

193Z] SOYBEAN BREEDING 301

certain oddly colored seeds some of which produced plants whose
progeny showed segregation in various seed and plant characters.
These authors did not determine the amount of natural crossing but
believed it to be "small, perhaps not one individual in two hundred."
Woodhouse and Taylor54* grew seventy-five or more plots, each de-
rived from a single plant, and found one of these to be a hybrid. They
concluded that "natural crosses do not occur on the plains of India to
such an extent as that noted by Piper and Morse in America." Hayes
and Jones16* selected single plants from a mixed variety and found
that the progeny of each bred quite true to the parental type. They
suggested natural crossing as one way in which a variety may become
mixed but stated that, "No clear cases of natural crosses are known to
the writers to occur in soy beans but it is not unlikely that crossing
does sometimes take place."

During the course of certain genetic studies on soybeans, Wood-
worth60* observed a few plants whose progeny segregated for various
plant characters. In an attempt to get at the amount of crossing more
accurately, two experiments were conducted. In the first, white-flow-
ered plants were planted between purple-flowered plants and seed saved
only from the plants bearing white flowers. When these were tested
the next year, none of them were found to be hybrid. In the second
experiment cotyledon color was used as the criterion of hybridity in
place of flower color. Plants of a green-cotyledon variety were planted
in rows in such a way as to be entirely surrounded by plants of a
yellow-cotyledon variety. Thus ample opportunity was afforded for
natural crossing to occur. As green cotyledons are recessive to yellow in
inheritance, and as cotyledon color is a character manifested in the seed,
a natural cross between green ? and yellow $ would result in a yellow-
cotyledon seed borne on a plant of the green variety. Each pod of
the green plants was examined separately and in a total of 7,480 pods
3 pods, or .04 percent, contained hybrid seeds. Since crossing can occur
between yellow $ and green $ also, this percentage becomes .08. This
is much lower than the percentage given by Piper and Morse,34* but
considerable variation in the percentage of natural crossing may be
expected in different varieties, localities, and seasons. Garber and
Odland8* determined the extent to which different varieties of soy-
beans cross when grown in adjacent rows. Under these conditions
they found that natural crossing was .14 percent in 1922 and .36 per-
cent in 1923. The conclusion appears justified, therefore, that a limited
amount of natural crossing occurs in soybeans but it is considerably
less than 1 percent.

302 BULLETIN No. 384 [November,

Plants whose progeny segregate may also arise as a result of muta-
tion. Since germinal changes usually occur in only one gamete at a
time hybrid plants would be the result of the union of the changed
with the normal gamete, whether the mutation were recessive or domi-
nant to the original. It is difficult if not impossible to state definitely
whether a particular segregating progeny is the result of mutation or
of a natural cross. However, in the case of a mutation there will be
segregation usually for only one character, while in the case of a
natural cross many characters may be segregating at the same time.

Natural crosses are believed to be responsible for many of the
mixtures occurring in our common varieties. At first the mixtures are
mechanical, resulting from the drill or the threshing machine. As these
mechanical mixtures grow alongside and in contact with typical plants
in the same field, there is abundant opportunity for natural crosses to
occur. When natural hybrids are produced, many more off-types are
added owing to segregation and recombination. Hence it is not sur-
prising that our standard varieties as commonly grown become badly
mixed in a few years.

Natural crossing in soybeans is believed to be the work of small
insects. Thrips (Thrips tabaci) have been observed crawling in and
out of soybean flowers. Honey bees, too, have been observed in soy-
bean fields, and it is known that they work on the flowers.

Experiments conducted at the Illinois Station indicate that plants
growing in contact with one another are more likely to be crossed
than plants not in contact but separated by only a few feet. This is
further evidence that natural crossing is due largely to small insects
that travel only between plants that are growing in contact with one
another.

METHOD OF MAKING CROSSES ARTIFICIALLY

The small size of the soybean flower makes artificial crossing a
difficult and tedious operation. The writer has found that this work
can be greatly facilitated by the use of a low-power binocular micro-
scope that can be strapped to the head, thus leaving both hands free.
Only three other instruments are needed; namely, a needle, pair of
fine-pointed forceps, and a small pair of scissors with fine points. Best
results have been secured by hybridizing flowers in the afternoon from
3 to 7 o'clock. Also, it has been found best to emasculate and pollinate
a flower the same afternoon. There appears to be no advantage in de-
laying pollination until the following morning or afternoon.

Emasculation is the most difficult part of the operation. The flower
is so small that great care must be exercised to avoid injuring the

1932] SOYBEAN BREEDING 303

minute and delicate organs. Also the flower may be self -pollinated in
the process of removing the anthers. With the flower held in one hand
the sepals may be pushed down with the needle or forceps held in the
other hand and either broken off with the forceps or cut off with the
scissors. The sepals removed, the corolla may be readily pulled out
with the forceps, thus exposing the ring of ten stamens around the
pistil (Fig. 2, B). The anthers are removed with the needle. With
care, one is able to remove several at a time. It is best to count them

FIG. 2. — STAGES IN THE ARTIFICIAL HYBRIDIZATION OF SOYBEANS

(A) Flower in the advanced bud stage ready for emasculation. (B) Flower
from which the sepals have been trimmed down and the corolla removed to
show the stigma surrounded by a ring of ten stamens ; when the stamens are
removed, the flower is ready to be pollinated. (C) Pollinated flower covered
and protected by fastening a leaf over it with a pin. As the leaf is left attached
to the plant, transpiration continues, thus tending to keep the mutilated parts
from drying out.

as they are taken out so as to be sure that all ten have been removed.
The style is bent like a goose neck and in consequence the delicate stig-
matic surface faces the base of the flower. For this reason it is par-
ticularly easy to self-pollinate the flower, for the anthers, on being
removed, are often broken and the needle, with attached pollen, may
touch the stigmatic surface and thus effect pollination.

Pollination is a relatively simple process, but it is often difficult to
find sufficient pollen in the right stage of development. Fresh-looking
flowers that have just opened are best to use. The flowers are pulled
or cut off the plant, the sepals and corolla removed, and the anthers
examined with the aid of the binocular microscope to see whether
they have burst open and whether the pollen grains seem separate
(not massed together) and in viable condition. If the pollen grains
appear in the right condition, the flower is caught in the forceps and

304

BULLETIN No. 384

[November,

rubbed over the stigma of the emasculated flower. Sometimes a hand
lens with higher magnification than the binocular microscope is used
to determine whether any pollen grains are in contact with the stig-
matic surface. Then a leaf, which is left attached to the plant, is
pinned over the flower to protect the exposed and injured parts from
excessive evaporation (Fig. 2, C). Finally, a small label, bearing the
parent numbers and date, is placed on the stem just below the flower.
The instruments are dipped in alcohol before work is started on the
next flower.

FIG. 3. — SOYBEAN PLANTS GROWN IN GREENHOUSE FOR USE IN HYBRIDIZATION

One-gallon jars were used, and one or two plants were grown in each jar.
Artificial light from 500-watt bulbs was used in the early stages of plant growth
to induce good vegetative development, then the lights were shut off to induce
flowering.

Soybean crosses can be made in the greenhouse as well as in the
field provided artificial light is used to obtain sufficient plant develop-
ment (Fig. 3). Without artificial light the plants are small and dwarf-
like; they bear few flowers, which appear not to develop and open
normally and which fertilize in the very early bud stage. Without
artificial light a successful cross in the greenhouse was a rare occur-
rence, but where light was used, the percentage of successful crosses
compared favorably with the percentage ordinarily obtained under field
conditions.

1932] SOYBEAN BREEDING 305

INHERITANCE IN SOYBEANS

For many reasons the soybean is a good plant to work on from a
genetic standpoint. There are numerous types differing in various
seed and plant characters; the plant is almost entirely self-fertilized;
under favorable conditions a single plant may produce several hundred
seeds ; and hundreds of plants may be grown within a small area. The
main drawback to genetic studies in this plant is the small size of the
flower, making artificial crossing a difficult and tedious operation.
Nevertheless considerable progress has been made in a genetic analysis
of the soybean. The genetic relations of seed-color types have been
worked out fairly completely, and the same may be said of many plant
characters. While much remains yet to be done, much has been ac-
complished during the relatively short time the crop has been studied.

SEED CHARACTERS
Cotyledon Color

In the soybean the cotyledons are of two colors, yellow and green.
Since the cotyledons are a part of the embryo of the seed, they belong
to the next generation and therefore give expression, prior to germi-
nation of the seed, to the character of the next generation with respect
to cotyledon color. Piper and Morse35* as early as 1909 observed the
color differences in cotyledons and noted further that both yellow and
green cotyledon seeds occurred on the same plant. When such plants
were tested, three kinds of plants were found in the progeny; namely,
those bearing only yellow cotyledon seeds, those bearing only green
cotyledon seeds, and those bearing both kinds ; and the ratio was ap-
proximately 1:1:2 respectively. These results, while not conclusive, in-
dicated that yellow was a simple Mendelian dominant to green.

Very different results were secured by Terao.44* In his crosses the
cotyledon color of the hybrid progeny was the same as that of the
female parent in every case, and there was no evidence of segregation
in succeeding generations. He thus found cotyledon color in soybeans
to be maternal in inheritance.

To explain these results Terao suggested that there were two kinds
of chlorophyl represented by the two cotyledon colors ; one that always
remains green, and one that changes to yellow on the ripening of the
beans. The former was designated as (G*) and the latter as (Y). If
the female parent possessed (G) or (Y), the hybrid progeny, down
at least to the F3 generation (as far as the experiment was carried),
would have green or yellow cotyledons respectively.

Maternal inheritance of cotyledon color in the soybean has been

306 BULLETIN No. 384 [November,

substantiated by Piper and Morse35* and more recently by Owen.30*
These workers used the Medium Green variety as the green cotyledon
parent. Owen made a cross between Mandarin (yellow) and Progeny
No. 56 (green), which was a selection of the Medium Green variety;
and between Aksarben (yellow) and Progeny No. 56. In both crosses
the green cotyledon variety was used as the male parent. All the seeds
borne by the hybrid plants had yellow cotyledons. A third cross be-
tween Manchu and Medium Green varieties behaved in a similar
manner.

The observations of Piper and Morse, referred to above, pointed to
the fact that there is, in some cases, a real segregation in cotyledon
color. This was confirmed by Woodworth,57* who found evidence for
two (duplicate) genes for yellow cotyledon. A cross was made reci-
procally between a variety with yellow and a variety with green cotyle-
dons, and in F^ the seeds had yellow cotyledons, showing the yellow
color to be dominant. In F2 a ratio of 15 yellow to 1 green was ob-
tained, and when the F2 yellows were tested by their F3 progeny the
expected three types of progeny were obtained in approximately the
expected proportions; namely, (1) those breeding true for yellow,
(2) those segregating in a 15:1 ratio, and (3) those segregating in a
3:1 ratio. The green cotyledon beans, when tested, bred true for green.

In interpreting these results use was made of an analogous case in
the garden pea. While the seeds of the garden pea are still immature,
the cotyledons of both yellow- and green-cotyledon varieties have both
yellow and green pigments (Bunyard, see Darbishire,4* page 131 of
reference). As the peas ripen, the green pigment fades out in yellow
varieties but persists in green varieties. Yellow-cotyledon varieties,
therefore, are first green, then turn yellow, and differ from the green-
cotyledon varieties in having a gene that causes the green pigment to
fade out as the peas mature. The situation in the soybean is quite
similar. All soybeans are green while immature, and as the ripening
period approaches, the varieties become differentiated by the fading
out of the green pigment in varieties with yellow cotyledons and the
persistence of the green pigment in varieties with green cotyledons.
The yellow varieties, therefore, possess a gene or genes that cause the
green pigment to disappear at maturity, and these are dominant to
those that permit the green pigment to remain.

Soybean varieties with yellow cotyledons differ in the number of
genes for yellow. Some possess only one gene; for example, Auburn
(Woodworth57*), and others possess two such genes. Owen30* recently
used the Mandarin, Aksarben, Ito San, Manchu, and a Japanese gla-

1932] SOYBEAN BREEDING 307

brous variety as parents in crosses and found them all to possess two
(duplicate) genes. Woodworth59* added Midwest, S.P.I. 20406, S.P.I.
65345, Ilsoy, Wea, and twelve other types introduced from China to the
list of the two-gene varieties. Only four varieties have been found that
possess only one gene. These are Auburn (above mentioned), Mi-
kado, Wilson, and an introduction from China designated Progeny
2262. More recently five more varieties have been added to the list
of two-gene varieties and one more to the list of one-gene varieties.
Therefore, of the 33 varieties studied, 28, or 84.8 percent, possess two
genes for yellow cotyledon. This situation is the opposite of what
might be expected. After examining the various possibilities of ac-
counting for the preponderance of two-gene varieties, Woodworth59*
concluded that at that time there was no satisfactory explanation.

The symbols originally used for these duplicate genes for yellow
cotyledons were D and /. Recently these have been changed to D^
and D2 respectively. The corresponding recessives are dl and d2. These
genes are independent in inheritance and hence are borne on different
pairs of chromosomes. So far as the character manifestation is con-
cerned, they appear to be exact duplicates of each other.

There are, therefore, two kinds of green cotyledon varieties;
namely, ( 1 ) a kind which in crosses with yellow types shows no segre-
gation in cotyledon color (maternal green) ; and (2) a kind which in
crosses with yellow types shows segregation and is differentiated from
yellow by genes residing in the chromosomes (genetic green). The first
type is a light or yellowish green, the second, a deep chromium green.
So great is the difference in the intensity of the green color in these
two types that they can be fairly well distinguished (on this basis
alone) without the necessity of making test crosses.

More recent work (Veatch and Woodworth48*) at the Illinois
Station on cotyledon color indicates that the maternal green carries
genes D: and D2 for yellow cotyledon on its chromosomes. Among the
soybean crosses recently made was one between genetic green 9 and
maternal green $ . The crossed seeds were yellow just as tho a yellow
cotyledon type had been used as the male parent. Furthermore, when
the F! plants were grown, segregation for cotyledon color occurred in
a ratio of 15 yellow to 1 green showing that the maternal green parent
had contributed duplicate genes for yellow cotyledon to the hybrid.
Segregation for other characters in which the parents differed proved
that the parents were as indicated above. Also many other crosses of
the same kind have been made always with the same result. There is
good evidence, therefore, that the maternal green soybean is a genetic
yellow so far as chromosomal cotyledon genes are concerned.

308 BULLETIN No. 384 [November,

In the above article Veatch and Woodworth postulated that another
type of maternal green could be produced, tho so far as known it did
not exist at that time. Reference is here made to the type which carries
genes d1 and dz for green cotyledon on the chromosomes, a maternal
green which behaves as a genetic green when used as the male parent
in crosses. It is believed that this type has now been produced. The
procedure outlined in the above article was carried out as follows: A
cross was made between maternal green 2 and genetic green $ .
In the F2 plants of this cross all seeds had green cotyledons because
of maternal inheritance, but we should expect segregation in cotyledon
chromosomal genes, resulting in a difference in the intensity of the
green color. Theoretically, fifteen-sixteenths of the seeds should be
light green and one-sixteenth a deep chromium green. This color !
difference was observed and it was found possible to classify the seeds
into these two groups in approximately a 15: 1 ratio. It was thought that
the deep chromium green seeds represented the type desired because of
their resemblance to the regular genetic greens. Accordingly these
were planted and plants grown. Test crosses were made by applying
pollen from these plants to stigmas of genetic green plants. The re-
sulting seeds were green instead of yellow, thus proving that the $
parent brought genes d^d2 to the cross rather than genes D^D2. We
have therefore produced a soybean that behaves in crosses not only as
a maternal green when used as 2 parent, but also as a segregating
green when used as $ parent.

The above relationships among cotyledon types may be briefly set
forth in the following outline:

Yellow cotyledon

One gene, D\ or D2
Two genes, D\ and Z?2

Green cotyledon
Genetic, d\ d?
Maternal

Genetic yellow, D\ Z)2

Genetic green, d\ dz

As stated earlier, in hybrid plants segregating for cotyledon color
both yellow and green cotyledon seeds occur on the same plant. The
distribution of seeds with yellow or green cotyledons is random over
the plant, and in pods containing two or more seeds the combinations
of yellows and greens follow the laws of chance. These facts were
brought out in a study by the writer of plants segregating in a 3:1
or a 15:1 ratio. Pods were carefully picked off the plants by hand, and
classified into 1-seeded, 2-seeded, 3-seeded, and 4-seeded pods. Each

193Z\

SOYBEAN BREEDING

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310 BULLETIN No. 384 [November,

pod was shelled by hand and the number of yellow and green cotyledon
beans noted. The various combinations of cotyledon colors in the four
types of pods are given in the headings of Table 1. The expected ratio
of yellow to green in 1-seeded pods is 3:1 or 15:1, according as segre-
gation is occurring for one or two genes. The expected ratio of com-
binations in 2-seeded pods is the terms of the binomial raised to the
second power, (3 + 1)2 or (15 + 1)2; in 3-seeded pods, the terms of
the binomial raised to the third power, (3+l)3or(15 + l)3; and in
4-seeded pods, the terms of the binomial raised to the fourth power,
(3 + 1)4 or (15 + 1) 4. A study of the observed numbers compared
with the expected numbers calculated on the basis of these ratios
(Table 1) shows rather close correspondence. The results on the
15:1 ratio plants would have shown closer agreement with the ex-
pected had larger numbers been obtained.

Coat Color

There are four main seed-coat colors in soybeans; namely, black,
brown, green, and yellow. Various combinations of these colors on the
same bean give rise to bicolor types or, more rarely, tricolor types;
and a mottled appearance is presented, sometimes showing definite and
characteristic patterns. The black pigment, according to Owen,32* is a
very intense purple and belongs to the general class of anthocyanins ;
the brown pigment is closely related to quercetin; and the green and
yellow are plastid pigments.

In inheritance the black and brown pigments are genetically inde-
pendent of green and yellow. Black is dominant to brown, and in F2
a ratio of 3 black to 1 brown is obtained. Green is dominant over yel-
low, and in F2 a ratio of 3 green to 1 yellow is obtained.

The situation becomes much more complicated when black or brown
is crossed with green or yellow. Beans are called self -black or self-
brown when the whole bean including the hilum is covered with black
or brown pigment. Most green and yellow beans have black or brown
hilums and may be thought of as black or brown beans in which the
black or brown pigment is confined' to the hilum. There are, however,
a few types in which the hilums are entirely devoid of black or brown
pigment — the hilums are yellow or green, the same as the seed coat.
Hence, when self-black is crossed with a green type with black hilum
there is no gene for pigment involved but rather a gene for restriction
or inhibition of pigment, designated as /. Since the green type carries
I, it is dominant and appears in Fx to the exclusion of self -black, which
carries i. There are genes effecting different degrees of inhibition.

1932] SOYBEAN BREEDING 311

Owen32* has demonstrated the existence of a multiple allelomorphic
series effecting inhibition consisting of /*, /', /* and i. Ih inhibits all
black and brown pigment so that none is visible on the seed coat even
in the hilum. /' permits black or brown pigment to appear in the hilum
but prevents its occurrence on the seed coat. /* restricts black and
brown pigment still less, allowing it to spread out from the hilum as a
center so as to form a "saddle" characteristic of the Black Eyebrow
variety. Finally, i permits pigment to cover the whole seed coat, result-
ing in a "self" colored seed such as we have in black- and brown-seeded
varieties. However, Stewart39* was not able to distinguish between types
carrying Ih and types carrying /', and on the basis of his evidence he
would revise the allelomorphic series thus: 7, /*, and i. Nevertheless
there are yellow and green types that have no black or brown pigment
at all, even in the hilum, as well as those that have these pigments only
in the hilum, and it is important to differentiate between them geneti-
cally, as Owen has done. To make the symbols conform to present
usage it is suggested that they be written /, i*, ik, and >.

A self -black or self -brown seed coat may cover up green or yellow
pigments. In other words, genes G (green) or g (yellow) may be
present but fail to produce a green or yellow seed coat, as the case may
be, because of the presence of i, which permits complete extension of
the black or brown colors. Thus in a cross between a yellow-seeded
strain, designated as A.K. 114, and Ogema, a self-brown variety, the
seeds of the Fj plant were green. The green coat color must have
come from the brown parent. The yellow parent contributed i\ which
in the Fa plant restricted the brown pigment to the hilum, thus per-
mitting the green color to appear. Other brown varieties and a few
black varieties that have been studied with respect to the plastid pig-
ment present are as follows:

BROWN BLACK

Variety Gene Variety Gene

S.P.I. 65388 g Sooty g

Recessive glabrous g Black Eyebrow g

Virginia G Ebony g

412 G g Wisconsin Early Black g

It is clear that the constitution of black or brown varieties with re-
spect to genes G and g can be readily determined by crossing with yel-
low varieties. If the brown or black parent carries yellow, the hybrid
will be yellow; if it carries green, the hybrid will be green.

The genetic relationships of black and brown pigments in the hilum
and the effect of other genes on these colors will be discussed in detail
in the next section.

312 BULLETIN No. 384 [November,

Hilum Color

Nagai,25* Wood worth,57* and Owen32* postulated two complementary
genes for black pigment formation in the seed coat and hilum. These
were designated C and L by Nagai, B and H by Woodworth, and R^
and R2 by Owen. Owen's symbolism appears to be preferable.

In Woodworth's early experiments57* which led to the suggestion
of complementary factors for black hilum, a cross was made between
a strain with black hilum and a strain with colorless hilum. In the F2
of this cross, black and brown hilum plants occurred in the ratio of 9:7.
As pointed out in that publication, the ratio probably .was 9 black: 6\
brown:! colorless, but on account of mottling of the seed coat the
plants with brown hilums could not be distinguished from plants with
colorless hilums, and consequently they were classed together; hence
the ratio 9:7.

The black hilum parent was considered to have the genetic con-
stitution R^Rz, and the colorless hilum parent, r1r2. In F2 the expected
types and proportions would be as follows:

9 RiR2 black hilum
3 R\r2 brown hilum
3 TiRz brown hilum
1 r\r-t colorless hilum

Just recently a cross was made at this Station between Illini [brown
hilum (R1r2)] and a dominant glabrous type [brown hilum (fi/?2)].
The seeds of the Fx plant had black hilums (R^r^r^R^. In F2 of 187
plants 97 had black hilums, 77 brown, and 13 colorless. The expected
numbers on the basis of a 9:6:1 ratio are 105:70:12 respectively. There
was no mottling in this cross to interfere with the separation of color-
less from colored hilum types. In this particular type of colorless hilum
there is no gene for brown pigment, and hence no color can be pro-
duced except the plastid pigment, which is yellow or green according
to whether the seed-coat color is yellow or green. Restriction genes
then have no effect because there is no brown or black pigment for
them to restrict. In appearance this type is the same as the other type
which is colorless because of the action of /, but the two types may
differ greatly in genetic constitution.

Substantiating evidence for the complementary relationship of
R! and R2 has been furnished by recent crosses between types with
brown hilums. Thus the Y1 plant produced by a cross between AK 114
(dark brown hilum) and Illini (light brown hilum) bore seeds with
black hilums. Similar crosses involving these and other varieties with
brown hilum are given below:

1932] SOYBEAN BREEDING 313

Cross No. Parents FI hilum color

2 Chimera X S.P.I. 65345 Black

8 Wea X Virginia Black

31 Wea X 435 B Black

32 Chimera X 435 B Black

36 Dunfield X S.P.I. 65388 Black

37 Illini X 435 B Black

40 Recessive glabrous X S.P.I. 545Q2 Black

47 Illini X Virginia Black

58 435 B X Columbia Black

101 AK 114 X Wea Black

104 Illini X Olive Green Black

128 AK 114 X Illini Black

One parent in each of these crosses has dark brown hilum and the
other parent light brown hilum. The difference in the intensity of
hilum color is due to the genes Tt for tawny vs. gray pubescence.
Types with T have dark brown hilums, and types with / have light
brown hilums. One parent in each cross contributes one complemen-
tary gene Rlt and the other parent contributes the other complementary
gene R2, both of which are necessary for black hilum. The light brown
hilum type is considered to be R-^r^t and the dark brown hilum type,
rtR2T. The Fx plant would therefore be R^rzt • r^R2T.

Obviously not all crosses between types with brown hilum will pro-
duce hybrids with black hilum. Crosses between types with light brown
hilums (R^zt) will produce again this same type; and crosses between
types with dark brown hilums (r^R^t} will produce again this same
type. Black hilum is not produced in either of these crosses, because
either R^ or R2 is involved in both parents and both must be present
to produce black.

In the above reference (Wood worth57*) the cross studied was
RiR2T (black hilum, tawny pubescence) X rj2t (colorless hilum, gray
pubescence). In the F2 of this cross no plants were obtained of the
constitution R^R2t (black hilum, gray pubescence). The results then
obtained indicated that T not only changes gray pubescence to tawny
pubescence, but also reacts with Rt to give black pigment. Conse-
quently T was considered either to be identical to, or completely linked
with, H (now called R2). Since then types with black hilum and gray
pubescence have been found. The hilum, tho black, does not appear
quite the same as it does in the typical black hilum of constitution
RtR-^T. The gene t seems to reduce the amount of black pigment, con-
fining it more to the center of the hilum and leaving the area near the
margin more or less free of this pigment. This type of hilum is called
dilute or imperfect by Owen32* and Stewart,39* and according to these

314 BULLETIN No. 384 [November,

authors has the constitution R^r2t. However, if R^ and R2 are neces-
sary for black, then the so-called imperfect type of hilum must have
both RI and Rz. Furthermore there are types with black coats and
gray pubescence, and these must have the constitution Rt R2 t. They
may be thought of as crossover types. No soybeans representing the
other crossover type of constitution r^r2T are at present known to the
writer, but it is quite likely that they will ultimately be produced, if,
indeed, they are not already in existence. Furthermore, if, according to
Owen and Stewart, r^R2 represents a brown and r^r2 a very light
brown or buff, then a cross of two browns could give only brown again,
not black. In view of the above considerations the following revisions
are suggested even tho it is admitted that the available evidence may
not fully justify them.

Hilum color (with «'•') Symbols Coat color (with i) Symbols

Black RiRtT Black R&T

Black RiRj Black RiR2t

Dark brown r\RtT Dark brown r\R^T

Light brown R\rd Light brown or buff R\rd

Colorless hilum r^T (or /) Yellow or green r^2T (or t)

The genes for purple and white flowers (Ww*) have also been
found to influence black and brown seed-coat or hilum colors. Owen32*
considered W to be complementary to rz, but Stewart39* found it neces-
sary to consider W to be complementary to R^ in order to avoid certain
inconsistencies in the use of formulae. With Rlf W reacts to give
imperfect black in the seed coat, while w with R^ gives a buff color.

Stewart39* found evidence for another series of multiple allelo-
morphs affecting coat color: namely, Rlt rv and r°. Nagai25* had pre-
viously reported that reddish brown (0) was recessive to light brown
(0). Rt with R2 produces black pigment as has been stated above;
rt with R2, light brown; and rf with R2) a reddish brown. Gene rt
is dominant to rx°, 7?x to rx and-r^, thus fulfilling the conditions of a
multiple allelomorphic series.

The production of black mottling on a self-brown seed coat was
explained by Nagai and Saito26* as due to the gene M, m being without
effect. Owen32* described a natural hybrid that segregated in a ratio
of 3 black to 1 brown, but the seeds of the heterozygous plants were
speckled with brown.

Abnormal Hilum

In the soybean the seed normally separates from the pod at the
point of attachment, leaving a clean, straight surface at the hilum or
seed "scar." In the Soysota variety (Fig. 4), however, there is an

1932\

SOYBEAN BREEDING

315

FIG. 4. — NORMAL AND ABNORMAL HILUMS

In most soybean varieties the hilum separates from the pod leaving a clean
smooth surface, as shown by seeds of the Virginia variety (right). In the Soy-
sota variety (left) the seeds have a rough hilum owing to adherence of a por-
tion of the pod tissue to the hilum. In inheritance this is a simple recessive to
the normal condition.

• •

FIG. 5. — BEANS WITH DEFECTIVE AND ENTIRE SEED COATS

In most soybeans the outer seed coat is entire and completely covers the
seed. In others the outer, thicker coat is defective in that it does not completely
cover the seed, exposing the thin, inner coat. The latter is objectionable, as the
seed is thus deprived of the protection that the outer coat should afford. In the
upper row are shown seeds of glabrous type with defective coats ; in the lower
row are seeds of Ogema variety, having entire coats.

316 BULLETIN No. 384 [November,

abnormal hilum owing to the tissues being torn on separation of the
seed from the pod. Owen32* found this type of hilum to be a simple
recessive to the normal. The symbols Nn were used to represent the
genes involved.

Defective Seed Coats

In normal soybeans the seed coats may be said to be entire in the
sense that they completely cover the seed. If, however, one examines
them closely, he will find defects in the outer layer ranging from a
pin point in size to large cracks. Owing to the growth of the cotyle-
dons, the margins of the crack are spread apart in some strains of
soybeans, exposing comparatively large areas of the inner white layer.
Sometimes this happens in such a way as to give a net-like appearance
to the seed coat (Fig. 5). This apparently was the explanation of the
so-called "black and white" soybean, a sample of which was sent to
this Station several years ago by W. J. Morse, of the U. S. Depart-
ment of Agriculture.

Defective seed coats have been observed in black, brown, buff, and
yellow seeds, and rarely in green seeds. In some yellow and green
seeds mottled with brown pigment the defects occur mostly in the
brown areas, as Stewart and Wentz40* found. The character is unde-
sirable, for in the defective areas the very thin inner coat cannot
furnish the protection against unfavorable weather conditions and
organisms causing disease that is afforded by the thick, heavy, strong,
outer coat.

Stewart and Wentz40* observed this character in the seeds produced
by a few F2 plants of a cross between Wisconsin Black and Mandarin
varieties. The F2 ratio was 51 normal to 5 defective. This suggested a
15:1 ratio, which was interpreted as follows: defective seed coat was
found only in plants having i, the gene for lack of restriction of pig-
ment; therefore 7 was assumed to prevent de, the gene for defective
seed coat, from expressing itself. The F2 classes can then be repre-
sented as follows:

9 7 De, normal

3 7 de, normal (except for mottled areas on the seed coat)

3 i De, normal

1 i de, defective

The F3 results gave substantiating evidence for this interpretation.
Since all plants bearing defective seeds had gray pubescence and none
were found with tawny pubescence, it was assumed that there was com-
plete linkage between de and t. On this hypothesis the Mandarin car-
ries the gene de for defective seed coat, but it is prevented from ex-

1932\ SOYBEAN BREEDING 317

pressing itself by / also carried by this variety; and Wisconsin Black
has the constitution iDc. Hence neither parent was characterized by
defective seed coats, but segregation and recombination following the
cross brought about the particular combination of genes which per-
mitted them to occur in certain F2 plants.

Mottling of Seed Coat

A few years ago the seed of many yellow- and green-seeded soy-
bean varieties showed considerable mottling. Seeds with black hilums
were black-mottled, and seeds with brown or colorless hilums were
brown-mottled. The mottling consisted of patches, blotches, or bands
of black or brown pigment, irregular in outline and extent, super-
imposed on a ground color of yellow or green.

In 1924 Wood worth and Cole62* described the character and re-
corded studies on the appearance of mottling on seeds of the same
plant. Seeds in the same pod showed striking similarity in the extent
and even in the particular pattern of mottling, but when these seeds
were planted, the plants grown, and resulting seed studied, there ap-
peared to be no evidence of the inheritance of the parent pattern or
of the amount of mottling. The causal factors were believed to be
physiological rather than genetic. In the same year Hollowell15* re-
ported results of studies on mottling carried on at various points in
Iowa. Rich soils were found to favor mottling, while on thin or poor
soils very little or no mottling developed. Also, crowded plants mottled
much less than plants given ample space in which to grow.

Owen28* concluded after an extensive investigation of this subject
that mottling is due both to hereditary and environmental factors.
Among the environmental factors believed to be important were men-
tioned abnormal physiological conditions resulting in a concentration
of sugars, or at least in an unbalanced nutritive condition, type of
soil, amount of space between plants, fertilizers, inoculation, and
shading. In general the more favorable the conditions for growth the
more the mottling, but there were many exceptions. No definite con-
clusions were drawn regarding the hereditary factors. Selection against
mottling was effective in some cases, and certain strains seemed more
susceptible to mottling than others. In artificial hybrids it appeared
that certain of the factors belonging to the restriction series i and /
were not completely dominant, and mottling was therefore allowed to
develop. The pubescence color was found in one instance to influence
the extent of mottling, tawny pubescence increasing it, gray pu-
bescence decreasing it. However, Owen could not designate any par-

318 BULLETIN No. 384 [November,

ticular factor as being the most important in causing mottling. The
problem had certain genetic aspects, but the environmental effects were
always quite evident.

Some attention was given to the mottling problem at the Illinois
Station, beginning in 1924, to determine whether any genetic aspects
were involved and whether selection would have any effect. Consider-
able variation had been observed in the Manchu variety (Fig. 6) in
the extent of mottling. Some plants showed none at all, other showed

FIG. 6. — MANCHU SOYBEANS: (ABOVE) MOTTLED, (BELOW) NONMOTTLED

Some varieties tend to mottle more than others, but the amount of mottling
is affected greatly by growing conditions. In general the more favorable the
conditions, the more the mottling. Of late years farmers have not complained
much of their soybeans showing this character. Probably one reason is the
wider use of strains that show little tendency toward mottling.

a slight amount, and still others were described as being considerably
mottled. In any plant exhibiting mottling there was considerable varia-
tion in the pattern and amount of the black pigment on individual
seeds, an observation which had been made by the other workers on
this problem. In order to represent the extent of mottling on the seeds
of a single plant by one figure so as to treat the data quantitatively,
the seeds were first placed in the following five classes:

(a) No mottling whatever

(b) Less than ^ of seed coat covered by black pigment

(d) Seed coat more than ^ but not fully covered by black pigment

(e) Seed coat fully covered by black pigment.

The number of seeds in each class was counted and expressed as a
percentage of the total. Then each percentage was multiplied by its
respective factor as follows: Class (a), 0; (b), 1 ; (c), 2; (d), 3; and
(e), 4. These products were added, and the sum expressed in a single

SOYBEAN BREEDING

319

figure the extent to which the seeds of a plant were mottled. This
method admitted of comparing different plants and different progenies
in the amount of the mottling shown.

In the beginning, the plants used as parents were simply dis-
tinguished on the basis of mottling by such descriptive terms as "non-
mottled," "very little mottling," "considerable mottling," and the like.
The progeny of these plants, however, were classified and evaluated
according to the method outlined above. From the results given in
Table 2 it can be seen that two nonmottled plants, 2052-7 and 2052-11,
gave progeny with the lowest amount of mottling; while the progeny of
plant 2064-9, which was also nonmottled, had a mean value of 116.5,
only slightly less than the progeny of 2070-18, which was described as
"very little mottling." Little of significance can be gained from this
table except that the parent plants having no mottled seed produced
progenies that showed the least amount of mottling.

TABLE 2. — COMPARISON OF PARENT PLANTS AND THEIR PROGENIES IN
EXTENT OF MOTTLING OF SEED COAT

Parent

Character of
parent seed

Progeny
No.

Number of
plants

Mean mottling1

202O-2

Considerable mottling

2512

146.75 ± 3.94

2020-12

Very little mottling

2514

180.46 ±3.79

2020-24

Considerable mottling

2517

170.81 ± 4.39

2020-25

Very little mottling

2518

141.78 ± 5.35

2052-7

Nonmottled

2519

75.09 ± 2.52

2052-11

Nonmottled

2520

79.50 ± 1.92

2061-16

Mottled

2523

146.88 ±4.49

2064-9

Nonmottled

2524

116.51 ± 2.49

2070-18

Very little mottling

2528

119.58 ± 3.09

"See text for method of calculation.

To continue the experiment use was made of progenies 2517, 2518,
and 2519 because they represented high, intermediate, and low averages
respectively and their parents stood in almost the same relative position
with respect to mottling. Plants were selected to represent the ex-
tremes as well as the intermediates of each progeny. Also the class of
seeds of each selected plant was kept separate in planting, so as to de-
termine whether any differences could be detected in the offspring. The
results are given in Table 3.

There are several items of interest that may be deduced from this
table. The extent of mottling shown by the offspring of 2519 is the
lowest, and this is significant inasmuch as they trace back to a non-
mottled grandparent. However, there is very little or no difference
between the offspring of 2517 and 2518, in spite of the fact that the
grandparent of the former was considerably mottled and the grand-
parent of the latter very little mottled. Furthermore the several classes

320

BULLETIN No. 384

[November,

TABLE 3. — COMPARISON OF SELECTED PROGENIES WITH ONE ANOTHER AND OF
DIFFERENT CLASSES OF SEED IN EXTENT OF MOTTLING OF SEED COAT

Parents of
1925 crop

Mottling of
parent

Type of seed
planted1

Number of
plants

Mean mottling

2517-8

93.5

0-0

92.38 ± 4.88

2517-9

169.4

o-X

H-w

x-1

0-0

2
2

35.15 ± 4.09
72.65 ± 0.88
75.05 ± 4.36

74.67 ± 0.00

2S17-13

179.3

0-X

x-x
M-l

0-0

11
5
6

66.81 ± 6.18
97.28 ± 4.37
94.92 ± 9.76

91.48 ± 3.03

2517-16

244.5

o-M

H-8

H-l

0-Ji

16
3
5

102.17 ± 7.39
104.86 ± 5.61
130.19 ± 14.33

97.88 ± 9.26

2517-19

219.5

K-H
H-l

0-M

6
16

125.07 + 9.51
105.46 ± 3.63

110.99 ± 7.02

2517-24

94.1

K-J?

y3-i

0-0

4
13

98.65 ± 6.29
76.81 ± 7.53

93.84 ± 5.88

2517-25

212.9

0-Ji

H-H

Ji-1

0-M

2
2

105.03 ± 5.61
66.68 ± 1.98
110.01 ± 1.03

103.41 ± 6.32

2517-27

178.8

Y*-<A
y*-\

o-M

6
9

117.05 ± 15.14
103.47 ± 7.37

108 75 ± 12 95

2517-33

98.3

K-H

M-i

0-0

5
10

106.92 ± 3.57
115.34 ± 6.63

103.25 ± 4.69

2518-6

193.8

o-M
Ji-H
Ji-1

0-M

15
4

116.40 ± 4.14
108.58 ± 3.64
125.45 ± 10.13

115.73 ± 3.55

2518-12

140 1

H-H

j^-i

0-0

8
13

111.78 + 7.89
118.31 ± 5.82

2518-14

141.5

0-Ji

o-M

20
9
2

121. 89 ± 2.71
104. 18 ± 9.47
117.39 ± 6.41

122.94 ± 5.55

2518-15

197 5

H-W

>*-!

0-}i

7
8

115.34 ± 10.61
126.07 ± 7.25

113 55 ± 3 77

2518-19

193 6

H-M

H-I

0-0

8
13

111.69 ± 5.63
116.47 ± 3.21

2518-20

47 8

o-Ji
fc-8

Ji-1

0-0

15
8
9

109.87 ± 4.31
113.28 ± 4.50
113.44 ± 8.75

96 42 ± 8 86

2518-24

51 6

o-Ji

0-0

72.36 ± 11.08
112 78 ± 2 36

2518-26A

00.0

0-M
0-0

13
5

115.48 ± 6.51
100.46 ± 6.36

2518-5

97 9

0-0

93 30 ± 11.56

0-M

99.89 ± 14.83

1932]

SOYBEAN BREEDING
TABLE 3.— Concluded

321

Parents of
1925 crop

Mottling of
parent

Type of seed
planted1

Number of
plants

Mean mottling

2518-34

144.2

0-J4

104.06 ± 5.45

2519-3

74.9

fr*
0-0

112.78 ± 4.15
87.02 ± 6.34

31.61 ± 4.26

2519-8

36.3

o-K

H-H

M-I

0-0

18
3
1

27.94 ± 2.53
52.13 ± 8.40
22.78 ± 0.00

14.27 ± 1.89

2519-9

37.4

o-M

0-0

8
23

25.59 ± 2.10
13.47 ± 1.93

2519-12

92.7

o->i

0-0

10
6

11.98 ± 2.12
26.38 ± 3.25

2519-14

94.7

0-}i

w-8

H-i

0-0

20.83 ± 2.88
7.07 ± 0.61
27.39 ± 0.00

19.89 ± 3.91

o-K

Vi-M

17.16± 2.14

2519-18

93.4

y>-\
o-o

2
8

24.92 ± 4.71
19.83 ± 4.09

2519-21

15.4

0->i
0-0

18
20

26.90 ± 2.68
25.78 ± 2.55

2519-26

77.5

o->i

0-0

29.49 ± 5.38
28.36 ± 1.72

2519-27

75.9

o-M
M-^

0-0

16
4

15.84 ± 1.53
36.37 ± 3.37

19.77 ± 2.52

Bulk Manchu

0-M

0-0
0->i

«-J?

>$-l

20
20
20
20

13.38 ± 1.79

54.55 ± 4.73
52.67 ± 5.76
96.61 ± 2.30
86.41 ± 9.61

10-0 = seeds with no mottling on seed coat.

0-Ji = seeds with Ji or less of seed coat covered with black pigment.

Ji~)4 = seeds with more than J£ but less than J^ of seed coat covered with black pigment.

J^-l = seeds with % or more of seed coat covered with black pigment.

of seeds of any parent plant gave very much the same kind of progeny
with respect to extent of mottling. That is to say, the seeds of a par-
ticular plant, regardless of the amount of mottling each seed exhibited,
are alike genetically and produce the same kinds of plants.

Finally, attention may be drawn to the data on plants produced
from seeds picked out of a bulk lot of Manchu. The seeds were classed
as were those of the individual plants and the progeny of each class
compared in amount of mottling. Plants coming from class (a) seeds
(no mottling) averaged 54.55 and from class (b) seeds, 52.67, while
plants from classes (c) and (d), having considerable mottling, aver-
aged 96.61 and 86.41 respectively.

It would appear from these data that the problem of mottling has
some genetic aspects. In a variety that mottles considerably, as did the
Manchu at the time this experiment was conducted, the selection of in-

322

BULLETIN No. 384

[November,

dividual plants showing little or no mottling may be effective in re-
ducing greatly the amount of mottling. At the outset it might be ad-
visable to pick out from a bulk lot a number of seeds exhibiting no
mottling and to grow plants from these seeds apart from the rest of
the crop. These plants can then be harvested and threshed separately,
and any showing little or no mottling can be continued. Thus a strain
may be developed by selection that lacks the objectionable feature of
mottling exhibited by the original variety.

More recently, for reasons that are not entirely clear, mottling has
not appeared to an extent sufficient to attract attention. Perhaps the
wider use of varieties not subject to mottling has been an important
factor.

Bloom on Seed Coat

Most soybean varieties are characterized by a comparatively smooth
seed coat, tho they differ widely in the degree of smoothness. Some
are rather dull in appearance, while others are bright and shiny. In

••ft

FIG. 7. — SEEDS OF THE SOOTY VARIETY SHOWING "BLOOM" ON THE SEED COAT

On the seeds in the upper row the bloom is unmolested. On the seeds in
the lower row a portion of the bloom is scraped off, showing the smooth seed
coat beneath. In inheritance, bloom is dominant to no bloom, and three genes,
B\, 5», Bt, appear to be necessary for its expression.

some types, however — for example, Wild soybean (S. ussuriensis]
Sooty, Harbin Nos. 222 and 223 (strains secured from Harbin Agri-
cultural Experiment Station, Harbin, Manchuria) and probably others
— there is a distinct "bloom" covering the entire seed coat (Fig. 7). The
bloom can be easily brushed or scraped off, thus exposing the compara-
tively smooth seed coat beneath. All the types mentioned above are
black-seeded, but at the Illinois Station there is a sample of the Wild
soybean that is brown-seeded, and these seeds also show the bloom.

1932] SOYBEAN BREEDING. 323

In a cross between Sooty and Manchu the F1 seeds were black-
hilum yellow and covered with bloom. In the F2 generation the follow-
ing results were obtained:

Number of plants Number of plants Expected

Type observed expected ratio

With bloom 243 241 3

Without bloom 78 80 ±

Total 321 321 4

The data thus indicate that the two parent varieties differed by one
gene for the character.

In another cross quite different results were secured. The parents
were Dunfield, a yellow bean with light brown hilum, and S.P.I. 65388,
a small-seeded, self -brown type. Neither parent showed the bloom.
Seeds produced by the Fx plants closely resembled those of the Fx
plants in the cross above described. They were yellow with black hilum
and were covered with bloom. In the F2 generation the following
results were obtained:

Type
With bloom

Number of plants
observed
119

Number of plants Expected
expected ratio
116 27

Without bloom

155

158 37

Total . .

274

274 64

The actual results obtained fit very closely the expected results on
the basis of a 3:1 ratio in the first cross and a 27:37 ratio in the second.
The facts can be interpreted genetically if we assume that there are
three genes involved, designated as Blt B2, and B3, all three of which
must be present together to manifest the bloom; if any one of these is
not present the character does not develop. On this assumption the
Sooty variety possesses all three genes, the Manchu only two. In the
case of the cross between Dunfield and S.P.I. 65388, one of these
parent varieties must carry two genes and the other the third gene
necessary for the character to manifest itself. It is hoped that further
studies now in progress will help to establish whether this is the correct
interpretation.

While the linkage relations of this character have not been fully
investigated, it is independent of the following character pairs: LI, li,
Gg, Ww, and Tt.

PLANT CHARACTERS

Flower Color

Soybean flowers are purple or white. While variations in intensity
and grade of color are observable in the purple-flowered varieties, no

324 BULLETIN No. 384 [November,

attempt has been made in this country to separate them phenotypically
or genetically. In crosses purple (W) is dominant to white (w), and
a simple ratio of 3 purple to 1 white is obtained in F2 (Wood worth58*).
Takahashi and Fukuyama43* found a dihybrid ratio of '9 purple, 3
purplish red, and 4 white in an F2 generation.

Stem Color

In the seedling stage soybean stems are either purple or green.
The color in purple stems is most abundant just below the cotyledons.
As the plants develop, the color fades out in the stems and accumulates
in the nodes and the petioles of the leaves. Purple-stemmed plants bear
purple flowers and green-stemmed plants bear white flowers (Wood-
worth58*). No exception has been found to this relationship, the same
gene probably being responsible for both characters. Hence stem color
of the seedling is a reliable indication of the flower color to be shown
later by the plant.

Pubescence Color

Practically all soybean varieties are pubescent; that is, the stems,
leaves, and pods are covered with fine hairs which are either brown
(tawny) or gray. Tawny pubescence (T) is dominant to gray (/) in
crosses, and in F2 a simple ratio of 3 tawny to 1 gray is obtained. In
most cases there is no difficulty in distinguishing the two colors : a plant
is either tawny or gray, not both; but in the Morse variety many
plants have been observed that cannot be classified so definitely. The
pubescence color is often intermediate between tawny and gray, and
sometimes both colors appear to be present on the same plant. This
is a problem needing further investigation.

Glabrousness

Two distinct soybean types are glabrous, that is, lack pubescence
or hairiness. These glabrous types are interesting genetically because
of their relation in inheritance to each other and to the pubescent type.
In crosses with pubescent varieties one glabrous type behaves as a
dominant while the other behaves as a recessive. In each case the ratio
is a 3:1, showing that a single factor pair is involved. Xagai and
Saito,26* who discovered the dominant type, designated it by the sym-
bols Plpl, and Stewart and Wentz41* who discovered the recessive type,
used the symbols P2p2. By crossing the two types, Woodworth and
Veatch63* were able to get at the genetic relationship between them.
The Fj plants were glabrous, and in F2 a ratio of 13 glabrous to 3
pubescent plants was obtained. On the basis of these results it was

1932]

SOYBEAN BREEDING

325

postulated that P2 is responsible for pubescence, />, for no pubescence
or glabrousness (recessive), and that Pv is a factor inhibiting the pro-
duction of pubescence, p^ having no effect on P2. This factorial inter-
pretation was substantiated by the F3 results. Hence, factorially, the
dominant glabrous type is P1P1P2P2, and the recessive glabrous type,
piPiP-,p2. The pubescent type is p^p^P-f^. By appropriate crosses a
third glabrous type may be isolated ; namely, PlPlp2p2-

Gene Pv also inhibits plant growth as well as pubescence. Glabrous
soybeans (P^P^ are smaller, shorter, and yield less than most pu-
bescent varieties (piPz}. This fact has been noted by Nagai and
Saito26* and by Owen.31* Veatch47* studied three crosses between pu-
bescent varieties and the dominant glabrous type and found that the
F! plants were below the average of the parents in vigor. Maturity also

Fir,. 8. — PORTION OF Row OF SOYBEAN PLANTS BELONGING TO AN Fa GENERATION
OF A CROSS BETWEEN ILLINI AND A DOMINANT GLABROUS TYPE

The difference in height and vigor is striking. The taller, larger plants are
pubescent, like the Illini parent; the smaller plants are glabrous. The gene Pi
inhibits not only pubescence but also normal vigor and plant development.

was affected by the gene Plf the Fl plants being later than the average
of the parents. In the F2 generation the most vigorous plants were
pubescent and the least vigorous were the glabrous plants. The pu-
bescent plants were 60 to 100 percent taller than the glabrous plants
(Fig. 8), and they were three to five times higher in weight of seed
(yield) and number of seed. It was interesting to note that the F2
glabrous plants averaged higher in most cases fhan the glabrous par-
ents, and that the F2 pubescent plants averaged higher in all cases
than the pubescent parents ; also that certain extreme F2 glabrous
plants had more vigor as measured by plant height and yield than cer-

326 BULLETIN No. 384 [November,

tain pubescent plants, F2 or parent. Evidently the genes for vigor were
not the same in the two parent types.

Furthermore it appeared that /\ did not always inhibit vigor to the
extent it did in the glabrous parent. There probably were modifying
genes involved in the cross, the effects of which are not inhibited or
depressed by P:. Finally, Pl was apparently completely dominant over
its allelomorph plf there being no significant difference in the develop-
ment of the glabrous F2 plants that were heterozygous and those that
were homozygous for /\.

The recessive glabrous type (/>i/>2) discovered and described by
Stewart and Wentz41* is also low in vigor and yield, much lower in
the plantings at the Illinois Station than is the dominant type. Lack
of vigor in the recessive strain cannot be due to P^ since Pl is not
present ; nor to pt since pl does not have such an effect in the presence
of P2; but more likely it is due to the presence of p2. The heterozy-
gous pubescent type (plp1P2P2) appears to be just as vigorous and pro-
ductive as the homozygous pubescent type (A/V^Va)* as Wentz and
Stewart49* have shown. Therefore P2 is apparently completely domi-
nant over its allelomorph p2. Since the recessive glabrous type is lower
in vigor and production than the dominant glabrous type, it would seem
that P2 has a greater effect in producing plant development than Px
has in depressing it.

Leaf Shape

The shape of the leaflets of most soybean varieties has been de-
scribed by Piper and Morse35* as ovate-lanceolate. A few types are
known with leaflets that are described by the same authors as narrowly
lanceolate or almost linear ( Fig. 9) . Takahashi and Fukuyama43* have
studied the hybrids between the normal and narrow types. In their
studies the Fx generation was found to be intermediate, and in F2 a
ratio of 1 broad or ovate to 2 intermediate to 1 narrow or linear was
obtained.

Crosses between normal and narrow-leaf types have been made at
the Illinois Station with essentially similar results except that the broad
shape was partially dominant and the F2 generation was made up of
two main types, broad and narrow, in a 3:1 ratio. There were a few
F2 plants, however, that seemed to be intermediate in leaf shape be-
tween the two parents. The symbols Na, na, are suggested for this
character pair.

Number of Leaflets

As noted in the section on description of the soybean, the com-
pound leaf generally is made up of 3 leaflets, but occasionally 4 or 5

1932]

SOYBEAN BREEDING

327

FIG. 9. — NARROW AND BROAD SOYBEAN LEAFLETS

Most soybeans have broadly lanceolate leaflets as in B, but a few have
narrowly lanceolate, almost linear, leaflets as in A. Narrow leaves are recessive
to the normal in inheritance.

FIG. 10. — NORMAL AND EXTRA LEAFLETS

In soybeans compound leaves with three leaflets are the most common, but
occasionally compound leaves with four or five leaflets are found. A strain of
Manchu is known which breeds true for this characteristic, tho the proportion
of compound leaves with extra leaflets is not high. This abnormality is inherited
but the method of inheritance appears to be rather complex.

328 BULLETIN No. 384 [November,

leaflets are observed (Fig. 10). Takahashi and Fukuyama43* dis-
covered a strain in which 73 percent of the compound leaves on the
main stem had extra leaflets. When this strain was crossed to the
normal, the percentage of compound leaves with extra leaflets on the
main stem of the hybrid was 52. Thus only partial dominance was
shown. However, in F2 a ratio of 3 plants with extra leaflets to 1
normal was obtained. The extra leaflet character has also been ob-
served by the writer in a strain of the Manchu variety. All plants of
this strain show the character, and counts on 6 plants gave 27.12 per-
cent extra leaflets. In the F2 generation of a cross between this type
and Sooty (all normal leaflets) a ratio of 36 plants with extra leaf-
lets to 335 plants with normal leaflets was obtained. This corresponds
to a 1:9.3 ratio. The Fx plants were grown in the greenhouse, and it
was not noted whether they bore compound leaves with extra leaflets
or not. On the F2 plants classed as bearing extra leaflets only one com-
pound leaf with extra leaflets was usually found. This case is evi-
dently different genetically from the case reported by the Japanese
workers.

Occasionally plants in other varieties have been observed with one
or more extra leaflets, but the variation does not appear to be in-
herited.

Height of Plant

Extensive studies on inheritance of size in the soybean have not
been made, but two cases have been reported in which definite segre-
gation in plant height was observed. Woodworth58* described a natural
hybrid that was segregating in the ratio of 3 tall, luxuriantly growing,
late-maturing plants (S) to 1 short, stocky, early-maturing plant (j).
Stewart38* reported the discovery of a dwarf type in soybeans that
behaved in inheritance as a simple recessive to the normal. This type
of dwarf, according to Stewart, seldom reached a height of more than
10 inches, was spindly, weak, and bore only 4 or 5 pods; while the
normal plants in the same segregating line were large and vigorous,
attaining a height of about 2 feet.

Maturity

Soybean varieties differ greatly in the time at which they mature.
This character usually has a complicated mode of inheritance because
it is determined by numerous genes. An exception to this statement
was reported by Woodworth58* in describing a progeny of plants that
conspicuously segregated for two plant sizes, tall and short, in a 3:1
ratio. Coupled with plant size was a difference in time of maturity.

193Z\ SOYBEAN BREEDING 329

The tall type matured usually about two weeks later than the short
type. In this instance late maturity was dominant. Studies made by
Veatch46* tended to confirm this, but Owen31* found the F! of crosses
between early and late types to resemble the early in time of maturity
more than the late, and in F2 the range in maturity due to segregation
covered the entire parental range.

Sterility

Occasionally very small, dwrarf-like soybean plants are found that
bear few or no pods. They are sterile or practically so. In many cases
they appear to be diseased, probably with mosaic, but the writer has
made no particular study of them. Owen27* described a sterile type
in which both ovules and pollen grains were nonfunctional. After
flowering time the plants took on a dark green color, the leaves became
thick, and sometimes the stems became greatly enlarged. The sterile
type wras discovered in a progeny of Manchu soybeans. Segregation
into 3 normal to 1 sterile was definite and clear cut, and it appeared
that a single gene mutation was responsible. Probably many of the
small, dwarf-like plants occurring as rogues in fields of soybeans are
due to genetic factors. If completely sterile they are automatically
eliminated, but their occurrence in the field at all strongly suggests the
presence of normal plants heterozygous for sterility that are constantly
producing sterile plants as a result of segregation and recombination.

Variegation

In the F3 generation of a hybrid a single plant was found at the
Illinois Station with variegated leaves (Fig. 11). Neither parent
possessed this character, and it is supposed that the variegation arose
as a mutation. When the progeny of this plant were grown, all plants
showed the character. In crosses with the normal the variegated type
proved to be recessive. Results in F2 were as follows:

Normal Variegated

Cross No. Parents plants plants

43 Elton X Variegated 123 49

44 Variegated X Recessive glabrous 129 20

46 Variegated X Recessive glabrous 90 20

Total 342 89

While the deviation from expected numbers on a 3:1 basis is rather
large, it is believed that variegation (v^ is a simple recessive to the
normal in inheritance. The wide deviations from expected in crosses
44 and 46 are likely due to errors of classification. The recessive gla-

330

BULLETIN No. 384

[November,

brous plants are small and weak, and their leaves are dull and drab,
appearing to lack the normal amount of chlorophyl. Hence, it was im-
possible to determine whether the F2 glabrous plants were variegated,
and they were all recorded as normal. This type of variegation is best
observed under field conditions in plants that are about half grown.
It does not appear in the leaves first produced on the seedling plant

FIG. 11. — (A) VARIEGATED LEAFLETS COMPARED WITH (B) NORMAL LEAFLETS

The gene responsible for this type of variegation has been designated vt, and
is recessive to the normal V\. This character probably arose as a mutation. It
was found in the F» generation of a cross and it bred true from the beginning.

nor in new leaves put out as the plant is attaining full growth. Hence
only a part of the leaves of a plant are affected and the reduction in
chlorophyl is small. Tho no definite tests have been made, it has not
been observed that variegated plants are any less vigorous or produc-
tive than normal plants.

Takagi42* has reported a type of soybean with greenish yellow
leaves which appeared in one-sixteenth of the F2 progeny of a cross
between two normal, green-leaved parents. Takagi has assumed that
two complementary genes, A and B, for normal leaves are involved,
A being brought in by one parent, B by the other parent ; and that only
in the absence of both factors does the chlorotic type appear. The F2

1932}

SOYBEAN BREEDING

331

ratio was substantiated by growing the Fs progeny. Plants of the
chlorotic type are stunted in growth and bear only small, faintly colored
seeds.

Fasciation

There is, in many different kinds of plants, a peculiar condition
in which the stem is flattened and often abnormally enlarged as if by
the adhering or growing together of two or more ordinary stems ( Fig.
12). This condition is called fasciation. A strain of fasciated soy-
beans, having the characteristically flattened stems has been described

FIG. 12. — STEM AND PODS OF A FASCIATED SOYBEAN PLANT

This type came from Japan (Takagi42*). The stem is wide and
flattened as if two or more stems had grown together. All the pods
were borne at the top of the plant. This character is recessive to
the normal.

by Takagi.42* In crosses with the normal type fasciation was re-
cessive, and in F2 a ratio of 3 normal to 1 fasciated was obtained.

Pod Color

Soybean pods exhibit a wide variety of pod-color types, ranging
from light straw yellow thru various shades of gray and brown to
black. No attempt has been made to describe the many different color
types or to study their inheritance. Two general types have been dis-
tinguished, designated dark and light. Dark pods are mostly black or

332

BULLETIN No. 384

[November,

FIG. 13. — DETERMINATE AND INDETERMI-
NATE TYPES OF GROWTH

A close view of a stem from each of
the two plants shown in Fig. 14. The
stem at the right (determinate) is term-
inated by a long flower stalk bearing
several pods, that at the left (inde-
terminate) has only one pod at the tip.
The pod-bearing habit is an important
factor in yield of seed.

FIG. 14. — (A) INDETERMINATE AND

(B) DETERMINATE POD-BEARING

HABIT IN THE SOYBEAN

These plants came from a progeny
segregating for these habits. The par-
ent of this progeny had the habit shown
by A ; hence the indeterminate habit
is dominant.

nearly so, while by light pods is usually meant pods ranging from a
very light tan thru straw yellow to light brown.

In inheritance dark pods are dominant to light, and in F2 a ratio
of 3 dark to 1 light is obtained (Piper and Morse35* and Wood-
worth58*). The symbols LI are used for this factor pair.

1932} SOYBEAN BREEDING 333

Pod-Bearing Habit

In the main there are two modes of pod formation (Fig. 13) in
the soybean, according to Etheridge, Helm, and King;5* namely
"a dense array of pods on the central stem, terminating there in
a blunt apex, with a thin dispersal on the lateral branches ; and a
sparse and comparatively even distribution of pods over all branches
and stems, a diminishing frequency toward the tip of the central stem
being notable." These authors placed more than a hundred soybean
varieties and strains into one or the other of these classes with respect
to pod-bearing habit.

These two methods of pod formation are very well illustrated in
Fig. 14. The two plants there shown were sibs, members of the same
progeny, the parent of which had the habit of growth shown by the
plant at the left. Hence the pod-formation habit represented by B
is dominant to the habit represented by A. The hereditary behavior
is further indicated by the fact that the progeny, from which the two
plants pictured in Fig. 13 descended, segregated for pod-bearing habit
as follows: type B, 69 plants; type A, 19 plants. Thus a single factor
difference appears to be involved. Type B may be termed indetermi-
nate and type A determinate. The symbols Dt, dt are suggested for this
character pair.

LIST OF GENES IN SOYBEANS

The following list of genes is given for convenience of reference.
No pains have been spared in the attempt to make it complete and
up-to-date. The author has taken the liberty to assign symbols to cer-
tain characters that had been investigated by other workers but not
named and also to change slightly certain other symbols in order to
bring them into line with current usage.

BI, BI, Bi, genes for "bloom" on seed coat

D\, one of duplicate genes for yellow cotyledons; d\, green cotyledons

Dt, one of duplicate genes for yellow cotyledons; dj, green cotyledons

De, normal seed coat; de, defective seed coat

Df, normal ; df, dwarf type

Dt, indeterminate; dt, determinate

E, early maturity; e, late maturity

F, normal stem development;/, fasciated or flattened stem

G, green seed coat; g, yellow seed coat

/, i', *'*, i, multiple allelomorphic series for inhibition of black and brown

pigment in seed coat
/, total inhibition; seeds show no black or brown pigment even in hilum;

identical with /*

*'•', partial inhibition; permits pigment only in hilum; identical with /'
*'*, partial inhibition ; responsible for Black Eyebrow pattern; identical with /*
i, no inhibition; seeds are entirely black or brown

334 BULLETIN No. 384 [November,

L, dark-colored or black pods; /, light-colored pods

Af, responsible for black mottling on a self-brown seed coat; m, no mottling

JV, gene for normal hilum such as is found in most soybean varieties; n, abnormal

hilum such as is found in the Soysota variety
Na, broad leaflet of most varieties; no, narrow leaflet
PI, inhibition of pubescence, causing glabrousness; pi, no inhibition
Pi, gene for pubescence; fa, no pubescence
Ri, f\, r\°, multiple allelomorphic series for seed-coat color
R\, complementary with J?2 for black seed coat or hilum
r\, complementary with 7?2 for brown seed coat or hilum; recessive to RI
n°, reddish brown seed coat; recessive to RI and r\ '

Rt, complementary with RI for black seed coat or hilum; r2, recessive to Rt
S, tall, late-maturing type; s, stocky, early-maturing type
St, normal production of seed; st, sterility
T, tawny or brown pubescence color; t, gray pubescence color
FI, normal chlorophyl development; v\, variegation
W, purple flower color; w, white flower color
X, extra leaflets in compound leaf; x, normal number, three

LINKED CHARACTERS IN SOYBEANS

Artificial crosses are difficult to make in soybeans; hence back
crosses are impractical as a means of determining crossover values in
this plant. A type of soybean carrying a large number of independent
recessive characters would be of great value as a linkage tester. When-
ever the linkage relations of a character or characters are desired, a
cross could be made with the tester, and as large an F2 population as
practicable could be grown and studied. The segregating characters
could then be classified, two at a time, and the extent noted to which
the ratio deviated from that signifying independence. With the con-
venient and accurate formulae and tables now available for determin-
ing crossover values on F2 data, it can no longer be considered a handi-
cap in linkage studies in the soybean to be confined to studies of the
F2 generation.

In the soybean there are many characters which, tho recessive in
inheritance, do not seem to reduce the vigor of the plant. Certain high-
yielding standard varieties possess many recessive characters; as, for
example, gray pubescence, white flowers, light-colored pods, light
brown hilums, and the like. Glabrous plants, however, are low in
vigor, and such types are undesirable for linkage testers because of
the low yield of seed. With the exception, therefore, of such genes as
f>2 (causing glabrousness) and st (causing sterility) it would seem
feasible to accumulate a considerable number of recessive characters in
one type.

A provisional chromosome map of soybeans showing linkage rela-
tions of a few factors is given in Fig. 15.

In Group I, R2 and T are represented as being completely linked.

1932-}

SOYBEAN BREEDING

335

However, since types with the constitution R2t are known, they can be
thought of as crossover types. The linkage then may not be absolutely
complete, tho no crossovers have been observed in crossbreeding ex-
periments. Owen31* found about 6 percent crossing over between E
and T. Also, according to Stewart and Wentz,40* de for defective
seeds is completely linked with /. Hence to date four genes have been

0 6
M

FIG. 15. — CHROMOSOME CHART SHOWING THE THREE GROUPS OF LINKED GENES
THAT HAVE BEEN WORKED OUT THUS FAR IN THE SOYBEAN

In Group I genes De, Rt, and T are all at the same locus, while gene E is
represented as being six units away. Without further information it is not
known on which side of the De, Rt, T locus E lies; hence the semicircle. A
similar plan is followed for the other groups. The idea is borrowed from the
plan used by corn geneticists led by Dr. R. A. Emerson at Cornell University.

identified in Group I, three of which have the same locus, and the
fourth gene is located approximately 6 units away.

In Group II, Pl and Rl are represented as being completely linked
(Owen31*). Factor M for mottling is located 18 units from P1 and Rt.
Thus to date only three factors have been located in this group. Only
two factors, Dl and G^ have been located in Group III. Published data
indicate about 13 percent crossing over (Woodworth,57* Owen30*) in
Group III.

The chromosome number in soybeans is given as 20 for the haploid
and 40 for the diploid condition by Tischler,45* who refers to the work
of Karpetschenko.17*

336 BULLETIN No. 384 [November,

VARIATION IN SOYBEANS

By variation is meant deviation, with regard to a specific character
or quality, from what may be taken as the original type. The type is
usually considered to be the average expression of the character, and
any deviation from this average constitutes a variation no matter how
small or how large. Variation implies a difference in one respect or
another. If a parent plant is heterozygous, there is variation among
the offspring and certain of the offspring may even differ from the
parent. Plants may also vary because of differences in growth con-
ditions. Variations furnish the sole basis for improvement. Without
variation, no improvement is possible.

From a plant-breeding standpoint there are two main kinds of vari-
ation, heritable and nonheritable. The nonheritable variations are often
called modifications or fluctuations, and they are due to the unequal
influences of growth conditions, such as differences in soil type, soil
fertility, moisture, etc. Differences in growth conditions are reflected
in differences in size of plant, branching, height, and rate of growth.
Soybean plants having plenty of space in which to develop send out
many branches, whereas crowded plants usually have only a main stem
with no branches. Such changes are not inherited, and hence affect
only the crop being grown. It would be of great advantage to the
plant breeder in making initial selections and in testing the progeny
of these selections if the growth conditions were identical for every
plant in the field or plot. In such a case the breeder could be reasonably
sure that any variations observed would be due to heritable changes.
However, this ideal situation does not and cannot exist, and the breeder
must devise ways and means of testing his plants in order to demon-
strate heritable differences in spite of the ever-present and universal
lack of uniformity in the plant environment.

Heritable variations are of two kinds ; namely, mutations and those
due to segregation and recombination. Mutations have been defined as
heritable differences that do not result from segregation and recombi-
nation. By far the greater number of mutations affect one gene at a
time. The wild soybean, for example, has purple flowers. By mutation
the element or gene in the germplasm responsible for purple flowers was
changed so that it produced white flowers. Usually such a change oc-
curs in only one gamete at a time, and when the changed gamete unites
with an unchanged gamete, the result is a hybrid which, in the illustra-
tion just cited, would have purple flowers. When the hybrid repro-
duces, however, segregation occurs, and it is to be expected that one-
fourth of the progeny will be white-flowered. It is quite likely that

1932] SOYBEAN BREEDING 337

many of the numerous variations in individual characters observable
today among soybean varieties have originated thru mutation, but it
is certain that in some cases, as in those involving complementary
relationships, hybridization was necessary to bring about visible differ-
ences.

Piper and Morse35* describe an instance in which a brown-seeded
variety arose as a mutation from a yellow-seeded variety. "The Tren-
ton is a brown-seeded variety found in a field of the yellow-seeded
Mammoth grown in Kentucky. Grown side by side at the Arlington
Experimental Farm, the two sorts were indistinguishable by any other
character than the seed color." This could be readily explained by
assuming that the gene i* carried by the Mammoth variety mutated to
i, thus permitting the brown pigment of the Mammoth hilum to extend
over the whole seed coat to produce a brown bean.

Many of the characters the mode of inheritance of which was
described under the section on inheritance in soybeans have been due,
according to the various authors, to mutation. Thus Stewart and
Wentz41* believe that the recessive glabrous type designated by the
symbol />2 likely arose as a mutation. Stewart38* found a dwarf soy-
bean which was most probably a mutation. The list could be greatly
extended. Below are a few mutations that have come under the obser-
vation of the writer during the course of several years:

Dark pod to light pod

Normal plant size to dwarf

Normal green plant to variegated

Black hilum to brown hilum (this occurred in Ebony, a black-seeded variety)

In addition to the above a few somatic mutations have occurred
resulting in "chimeras" of various kinds (Fig. 16). One of the most
interesting of these was a small plant the stem of which was half
purple and half green from base to tip (Fig. 17). This plant was dis-
covered in one of the plantings at the Illinois Station by Dr. Leo Clem-
ente, now of the University of the Philippines. The line separating
the two stem colors was very sharp and distinct. The purple side of
the plant bore purple flowers and the green side white flowers, a result
to be expected because of the close relationship between stem color and
flower color. A few seeds have been found with one green and one
yellow cotyledon, also the result of somatic mutation. These are, of
course, of no importance from a plant-breeding standpoint since they
cannot be perpetuated, but they are of considerable genetic interest.

The cause of mutations is not known. Various attempts have been
made by different means in different organisms to induce mutative

338 BULLETIN No. 384 [November,

changes artificially. The most successful of these has been the use
of X-rays. The changes induced by the X-rays are recessive and detri-
mental, but it is significant that these are the same in nature and ap-
pearance as those occurring naturally from time to time in the same
material. It appears, therefore, that X-rays may be a means of speed-
ing up the process. Quite recently Patterson and Muller33* have suc-

FIG. 16. — A TYPE OF VARIEGATION IN THE SOYBEAN

This was discovered for the first time in 1930. A few leaves were entirely
white and a few were entirely green, but most of them were part white and part
green, as shown above. The plant from which these leaves came is a chimera.

ceeded in inducing dominant changes in the fruit fly (Drosophila
melanogaster}. It appears that the use of X-rays in improving plants
has possibilities for the future, but for the present it must be considered
to be in the experimental stage. So far as the writer knows, no work
of this kind has been done on the soybean. The soybean breeder is
fortunate in having an abundance of variations on which to work.
Much remains to be done in the way of utilizing the varieties and
strains at present available for improvement thru hybridization and
selection without resorting to the use of X-rays.

Not many years ago the idea was entertained by some people that
selection, if carried on continuously for a period of years, caused herit-
able changes in the direction of selection. This idea has special refer-
ence to normally cross-fertilized material. While accepting the fact
that such changes may have accompanied selection, they have now been
interpreted in other ways. A character is the manifestation of a gene

1932}

SOYBEAN BRKEDING

339

or genes resident in the germplasm, and a character must appear be-
fore selection can become operative. Hence selection can scarcely be
said to induce mutations when such heritable changes had already
taken place in the germplasm before the plant was subjected to
selection.

Furthermore the occurrence of mutations cannot be ascribed to a
need for them on the part of the plant. Proof of this statement

FIG. 17. — A YOUNG SOYBEAN PLANT WITH HALF ITS STEM
PURPLE AND HALF GREEN

The purple side (left) later produced purple flowers and the green side
(right) white flowers. This is a chimera, and probably arose as a mutation
which occurred at a very early stage in the development of the embryo. It was
discovered by Dr. Leo Clemcnte, now of the University of the Philippines, while
at the University of Illinois.

is furnished by some results obtained at the Illinois Station in an ex-
periment on breeding soybeans for high and low oil content. This
experiment will be described more fully in a later section, but suffi-
cient mention will be made here to illustrate the point. Several hundred
soybean plants of the Manchu variety were analyzed for oil content.
Considerable variation was found, most of which was probably due to

340

BULLETIN No. 384

[November,

growth conditions. That some of this variation was genetic was indi-
cated by the fact that progeny tests of high and low parents revealed
significant differences of 2 to 3 percent oil. No correlation was found
between oil content and yield of seed. Therefore slight differences in
oil content can have no selective value from the standpoint of the sur-
vival and well-being of the plant. In this variety at various times in
the past and for no known reasons mutations have occurred affecting

FIG. 18. — Two SOYBEAN PODS THAT HAVE GROWN TOGETHER

The soybean flower usually has a single pistil, but this double pod may have
developed from a double ovary. It is probably not inherited.

oil content, and as a result the variety has become a mixture of types so
far as this character is concerned. The occurrence of heritable varia-
tions for oil content would never have been detected had analyses not
been made.

The same may be said concerning resistance to plant diseases. Mu-
tations occur from time to time regardless of whether the variety is
ever subjected to attacks of the causal organism or not. Consequently,
when such attacks occur, plants are found that are partially or wholly
resistant and hence survive and propagate their kind. Thus resistant
varieties are produced. Resistance to a plant disease, then, is not a
character that has been produced by a need for it on the part of the
individual plant, for the character existed prior to such need. Also, if
after the first attacks of the causal organism further mutations occur
in the direction of more complete resistance, it seems reasonable to sup-
pose that the same cause or causes responsible for the initial variations
have brought about the further germinal changes, rather than that
these changes are due to the presence of the causal organism.

Thus mutations occur from time to time, producing certain new
characters or quantitative changes in others without provocation, need,
or apparent cause. All that the breeder can do is to watch for them
and when they occur isolate and study them to see if these mutant
forms are more valuable than the type from which they came.

1932] SOYBEAN BREEDING 341

In soybeans very few abnormalities, such as albinos and yellow
plants, are found when compared with the number observed in corn
following self-fertilization. This should not be taken to mean that such
mutations rarely occur in soybeans, but rather that when they occur
they are almost immediately subjected to natural selection and they
either perish or survive on their merits. In a cross-fertilized crop like
corn, on the other hand, such abnormalities can be carried along in-
definitely in the heterozygous condition, and as additional mutations
are constantly occurring and none is eliminated, they tend to accumu-
late in the variety. It may be said, then, that self-fertilization is good
for the soybean plant in that it tends continually to purge it of defec-
tive germplasm, and thus to keep it remarkably free from abnormal
types.

It is extremely difficult, if not impossible, to get at the rate of mu-
tation for various characters in the soybean. The number of mutations
in any organism is usually so small that several hundred thousand or
even millions of cases must be observed in order to determine the rate
of change with any degree of accuracy. Stadler36* has made a study
of the mutation rate in corn for factors affecting aleurone color and
endosperm texture, and he finds significant differences in the rate for
different genes. His procedure may be illustrated by the method used
to determine the rate of mutation of R, one of the genes for aleurone
color in corn. Crosses on a quantity basis were made between a color-
less aleurone type ACrPr ? and a purple aleurone type ACRPr $.
All the kernels on the crossed ears should have been purple, and this
was the case except when the gene R mutated to r, giving rise to color-
less aleurone. The ratio of such colorless aleurone kernels to the purple
aleurone kernels gives the mutation rate of this gene. Crosses can be
very easily made in corn, and large numbers can be obtained particu-
larly in cases involving endosperm characters. On the other hand, in
the soybean crosses are made with great difficulty. This in itself pre-
cludes the use of Stadler's method for getting at mutation rates in soy-
beans. Furthermore cotyledon color is the only character so far known
for which cases running into the thousands can be easily obtained in
segregating generations. This fact, however, does not help in getting
at mutation rates even for cotyledon genes, owing to the above-men-
tioned difficulty of making crosses and to the inability to distinguish,
in segregating generations, green cotyledon beans due to mutation from
those due to recombination. Hence there appears to be at present no
adequate method by which mutation rates for various genes in soy-
beans can be determined.

342

BULLETIN No. 384

[November,

Other heritable variations are those due to hybridization, which
result in segregation and recombination. The breeder desires to find
ways of inducing variation so as to make it occur more frequently

FIG. 19. — TWIN SOYBEAN PLANTS AND SEED PRODUCED BY THEM

These plants grew together as one plant on the same root but
bore different kinds of seed. Hence these are not identical twins.
There were probably two embryos within the same seed coat, and
they produced different kinds of seed because of a different genetic
constitution.

than it would occur ordinarily by mutation. Hybridization is one of
these ways. Hayes and Garber13* state that the only practical means
possessed by the breeder of inducing variation is thru hybridization.
Bettering the conditions of growth induces greater development but
these changes are not inherited. Since, as stated above, the cause is
unknown, the breeder does not know how to produce them. Also, the

1932~] SOYBEAN BREEDING 343

use of X-rays in causing mutations, while a promising method for the
future, must be considered experimental probably for many years to
come. Hence hybridization will likely continue for some time to be
the only practical means in the hands of the breeder of producing
heritable variations.

How does hybridization induce variation ? This is brought about as
a result of segregation and recombination. When a hybrid, such as
AaBb, is self-fertilized, four kinds of progeny will result, two of which
will be like the original parents, namely, Ab and aB. The other two will
be different. One will possess both genes A and B, and the other
neither of these genes (ab). Thus new types have resulted from the
cross. Both these new types might have been produced by mutation,
but the breeder might have waited in vain for them to be produced
in that way. By crossing, nature is given a chance to produce them
in a relatively short time.

In the illustration given above only two factors were used. Segre-
gation and recombination following a cross between two soybean varie-
ties is much more complex since there are not only the numerous genes
involved for characters, such as color of flower, pod, pubescence, and
seed, but also genes for such characters as type of plant, maturity, size
of seed, and height of plant. The quantitative characters just men-
tioned are quite complicated in inheritance, many genes being involved
for each. Various combinations of all these genes give rise to numerous
types which are different not only from the parents but from one
another. An idea of their complexity may be gained by referring to
the statement made later in this bulletin regarding the number of types
resulting from selfing, starting with hybrids heterozygous for varying
numbers of genes. By reference to the general formula there given
(page 345), it can be seen that by crossing two varieties differing by
only ten genes as many as 1,024 different types will result. Thus hy-
bridization is a practical means of inducing heritable variations by re-
combining in various ways characters already in existence.

Furthermore under certain conditions hybridization may give rise
to entirely new characters. This comes about thru the complementary
relationship of certain genes. It was shown under the section on in-
heritance that by crossing two brown hilum varieties, a black hilum
type could be produced. This was explained as follows: two genes are
necessary to produce black ; namely, RT and R2. When one parent car-
ries RI and the other R2 the hybrid, containing both Rt and R2, will
be black. Then in addition to recombining in different ways characters
already present in the parents, hybridization may give rise to entirely
new characters, not of course as mutation does it, by a change in the

344 BULLETIN No. 384 [November,

germinal material, but by combining two sets of germplasm, one of
which contains genes complementary to those of the other.

SELECTION AS A METHOD OF BREEDING

Selection is a choice of the best. It is one of the most powerful
tools in the hands of the breeder for effecting improvement. Effective
selection presupposes the occurrence of variations. One discovers a
plant that is different in some particulars from its neighbors. It may
be taller, have more branches, more pods, or it may have merely a
different color of flower, pod, or pubescence. It is selected, isolated
from the group, and when planted separately it may breed true to the
particular difference or differences for which it was selected, and thus
a new type is produced.

COMPOSITION OF VARIETIES

Varieties of soybeans are, as a rule, relatively pure; that is, all
the plants of a given variety are alike. This is due to the fact, men-
tioned earlier, that the soybean is a self-fertilized plant. However, if
one walks thru a field of soybeans all planted to the same variety and
examines the plants, he will find a greater or less number of off -type
plants, so called because they are different in one or more of the char-
acteristics that distinguish the growing plant of one variety from that
of another. There may be a difference in flower color, pubescence
color, pod color, or habit of growth. If later one were to examine the
threshed seed, he might also discover seeds having coat or hilum colors
that are not typical of the variety.

How do these mixtures occur? There are at least three different
ways: (1) By mechanical means. Where several varieties are grown
in a neighborhood, the threshing machine carries seeds from one farm
to another. If farmers change from one variety to another, they are
often not careful to clean out the drill or seed room thoroly. (2) By
natural crossing. When mechanical mixtures have occurred, and the
off-type plants are growing alongside and in contact with the typical
plants of the variety, natural crosses occur to a certain extent, as was
pointed out earlier in this bulletin. When the hybrids grow and re-
produce they will not breed true but, owing to segregation and recom-
bination, will give rise to numerous types. (3) By mutation. As
pointed out under the heading of variation, mutations may occur tho
they are probably rare. Nevertheless instances are recorded of germ-
inal changes taking place in soybeans, thus giving rise to new types.

If mechanical mixtures are avoided, no natural crosses can give

I
1932} SOYBEAN BREEDING 345

rise to mixed types. Mutations, tho beyond the control of the grower,
occur too rarely to be an important factor in causing mixtures. Hence
the grower can keep his variety relatively pure by taking care that no
off-type seeds get into the variety by mechanical means, and by rogue-
ing them out if by chance some do get in, thus reducing greatly the
chances for natural crossing.

If the variety is pure for its characters, self-fertilization results in
its breeding strictly true. The composition of the variety then remains
the same so long as the condition of purity is maintained. If, however,
hybrids are produced by natural crossing and thereafter propagated
by self-fertilization, the tendency is for the progeny of the hybrid, after
several generations, to consist almost entirely of pure types.

This principle can perhaps be made clearer by an illustration. Sup-
pose we start with a plant which is heterozygous for gene A, having,
therefore, the constitution Aa. When self-fertilized, it will produce the
following types in the following ratio: I AA: 2 Aa: 1 aa. Now, if
each plant of this progeny produces four individuals, the next genera-
tion will consist of the following types and proportions: 3 A A: 2 Aa:
3aa. The pure types, AA and aa, make up 75 percent of the total, and
the heterozygous type, Aa, only 25 percent. In the preceding genera-
tion the ratio of homozygous to heterozygous was 1:1. If this problem
is carried further it can be readily determined that the homozygous
type will increase and the heterozygous type will decrease, until it can
be said that the progeny of the original hybrid, for all practical pur-
poses, consists of only two types, A A and aa, in equal numbers. Stated
in general terms the proportions of the pure and impure types in the
progeny after n generations wrould be 2n — I A A: 2 Aa:2n — 1 aa, where
n is the number of generations, counting the F2 generation as the first.

It should be emphasized that self-fertilization does not reduce the
progeny of the hybrid to homozygosity, but rather to a mixture of
homozygous types. The number of homozygous types resulting from
continued self-fertilization depends on the number of heterozygous
factors in the original hybrid parent. In the example cited, there were
two pure types. If the original parent were heterozygous for B as well
as A (AaBb) there would be four pure types: namely, A ABB, A Abb,
aaBB, and aabb. The general formula 2m, where m is the number of
heterozygous factors in the original parent, can be used to calculate
the number of different pure types that would result. This is better
shown as follows:

Number of heterozygous genes 1 2 3 4 5 m

Number of homozygous types 2 4 8 16 32 2m

346

BULLETIN No. 384

[November,

Thus if a plant were heterozygous for 10 pairs of genes, 210 or 1,024
pure types would be produced as a result of self-fertilization. This
emphasizes the importance of natural crossing in causing mixtures
in the variety.

NATURAL PURIFYING METHOD OF SELECTION

There is a method of handling hybrids which is based on the prin-
ciple just explained. This has been variously called the "Svalof
method," the "bulked population method," and the "natural purifying
method." The plan (Fig. 20) generally followed is to bulk the plants
of the first segregating generation instead of harvesting and threshing

FIRST
YEAR

VARIETY A

VARIETY B

SECOND
YEAR

THIRD FOURTH
YEAR YEAR

Fi
PLANTS

Fz
PLANTS

FROM |

ERA- — s
ION

PLANTS GLN
BULKED ""

FIFTH
YEAR

— *

SIXTH
YEAR

SEVENTH
YEAR

SELECTED
PLANTS •
PLANTED iq,
SEPARATE
Rows

EIGHTH
YEAR

^-» GENERA-
TION

GENERA-
TION

) "

I a uj

}--!:! K

) Q! to

FIG. 20. — STEPS IN THE NATURAL PURIFYING METHOD

Hybrid plants from natural or artificial hybrids are bulked together and a
plot grown each year from seed produced by the hybrid population of the pre-
vious year. This process is continued for several years. Then single plant
selections can be made with fair assurance that such plants will be homozygous
for their characters and will therefore breed true, the heterozygous types having
been gradually eliminated as a consequence of the principle of self-fertilization.
The selected plants can then be carried along by the pedigree system.

them separately. The seed is planted in bulk, the plants again bulked,
and the same procedure is repeated for several generations. After
6 to 10 generations the breeder can make selections from such a popu-
lation of plants with fair assurance that any plants selected will be
pure for the combination of characters carried, and will therefore breed
true when tested by their progeny. Furthermore, during the purifying
period all the plants are subjected in each generation to natural selec-
tion and the very poorest are eliminated. Hence any plants selected
may be considered not only to be pure but also to be among the best
so far as yield and vigor are concerned.

1932] SOYBEAN BREEDING 347

This method has certain advantages. One of these concerns the
time element. It requires little of the breeder's time and attention to
carry along the material during the purifying period. Also, the hy-
brids from many different crosses can be put together and continued as
one population. No time is required for a careful study till the selec-
tions are made and tested. Another advantage is that the lot of seed
can be divided after the second or third generation has been grown
and the smaller lots sent to parts of the state which differ in soil type
and climatic conditions. During the purifying period natural selection
would tend to favor those types that are best adapted to the particu-
lar locality. Hence the task of selecting good, desirable types will be
lightened by whatever nature has done in the way of preserving the
best and eliminating the poorest.

The "natural purifying method" of selection can be used for the im-
provement of soybeans, since the soybean is a self-fertilized crop. The
method has not been used to any extent at the Illinois Station because
the pedigree system, in which the progeny of each hybrid is studied
carefully for several generations, has been preferred. Such a careful
study is necessary when one wishes to learn about the mode of inherit-
ance of characters. Furthermore selections can be made at any time
and tested to see whether they breed true for the combination of char-
acters desired. However, the "natural purifying method" can be used
with natural or artificial hybrids, and soybean growers who are in-
terested in soybean improvement may use it on the progeny of natural
hybrids that occur in their fields from time to time.

PEDIGREE SELECTION METHOD

In a variety which has become mixed in any one or all of the ways
described above, numerous types exist that differ in size, maturity, and
yield as well as in color of flower, seed, or pod. For the most part
these types can be considered pure for their characters, and the differ-
ences they exhibit are inherited. Such a mixed variety can be con-
sidered a mixture of pure types. If now the breeder selects from such
a mixture a number of single plants, keeps the seed of each plant
separate, grows a row of plants from each parent plant, and continues
to keep the progenies separate, he is practicing pedigree selection (Fig.
21), so-called because the pedigree of the strain can be traced back
to a single parent plant.

The success of this method depends greatly on the first or initial
selection. If the parent plant is pure for its characters, the progeny
it produces is spoken of as a pure line, because it is produced as a

348

BULLETIN No. 384

[November,

result of self-fertilization. When the pure line is separated or isolated
from the group, it is useless to select further within the line, since
such selection has been found to have no effect. All that selection can
do, therefore, in a self- fertilized crop like soybeans, is to separate the
pure lines of which the original variety or type was composed. This
is why so much importance is placed on the initial selections, and why

SlXTHjStVtHTH*
E»MTH YtAOS
DniLL-PLOTlESTS

NINTH

YtAR

TENTH
YCAP

FIELD

. FIELD

FIELD

FIG. 21. — STEPS IN THE PEDIGREE SELECTION METHOD

Selected plants from a variety or hybrid population are planted in separate
rows. The best are chosen by inspection and tested in rod rows for three or
more years. The poorest are discarded and the best are advanced to drill plot
tests. Finally the best strain is multiplied in a multiplying plot for general field
planting. Many superior strains have been isolated from existing varieties by
some such plan as this.

as large a number of plants as possible should be selected at the be-
ginning. If 1,000 plants are chosen for test, the chances of finding
an improved type are ten times as great as if only 100 plants are
selected.

Variation in growth conditions in different parts of the field makes
selecting plants for test difficult and uncertain. Plants that appear
superior to the rest in yield may be so because of more space in which
to grow or more fertility in that particular spot of soil, and not be-
cause of a better genetic constitution for yield. In other words, two
plants may appear different but breed the same, or appear alike but
breed differently. Differences in appearance will not be reproduced in
the progeny unless they are the result of a difference in genetic con-
stitution.

Pedigree selection is responsible for the discovery of many im-
proved types of soybeans. The Illini variety originated from a single
plant selected from the A.K. variety (Fig. 22). The parent plant bred
true from the start, and since the resulting strain had certain desirable

1932]

SOYBEAN BREEDING

349

characters, such as high yield and early maturity, it was multiplied and
distributed. Mansoy is a selection from Manchu, Ilsoy from Ebony,
and Virginia from the Morse variety.

FIG. 22. — A FIELD OF ILLINI SOYBEANS ON THE UNIVERSITY OF ILLINOIS

SOUTH FARM

One of the outstanding characteristics of this variety is its uniformity in
growth habit.

MASS SELECTION METHOD

Mass selection differs from pedigree selection in that the selected
plants are bulked and threshed together and the seed planted as one
lot of seed (Fig. 23), instead of the seed of each plant being kept
separate as in the latter method. Oftentimes a seed plot is maintained
which is planted to the seed of the selected plants ; this may be a corner
of the general field. Before harvest the breeder goes thru the seed plot
and again selects the best plants, which are again bulked as before,
and another seed plot planted. The remainder of the plants in the
first seed plot can be harvested together, threshed, and the seed used
for commercial planting. This process may be repeated indefinitely.

Mass selection is simple and has the further advantage of requir-
ing little time on the part of the breeder as compared with that neces-
sary in the pedigree method. If, however, hybrid plants are selected,
they will segregate and cannot be eliminated from the seed plot except
by further selection. In the case of pedigree selection the whole row
coming from the hybrid plant can be eliminated. Furthermore, if we
think of the population as being a mixture of pure lines which vary
in yielding capacity, each around its own respective average, continued

350

BULLETIN No. 384

[November,

mass selection of the higher yielding lines tends, theoretically at least,
to eliminate first the lowest ones, then those near the average of the
whole group, and finally all but one line which is the very best. When
the population thus is made up of a single line, further selection would
have no effect, as explained in the preceding section. To reach this
point under mass selection requires a considerable number of genera-
tions, many more, likely, than the practical results would justify. That
is to say after a period of five or six years the breeder may have been
able to eliminate, by mass selection, all but the best yielding half dozen

FlDST

YEAR

SECOND
YEAR

FIG. 23. — STEPS IN MASS-SELECTION METHOD

This method is simple and easily carried out. It is particularly effective in
purifying a variety that has become badly mixed with other types. It may also
be used to isolate the better yielding line or lines from the plant population, but
a long period of time is required for this, and furthermore improvement is very
slow if it can be noticed at all within a period of a few years.

lines. The difference between these, if any, may be so slight as to make
it immaterial whether the population is reduced to a single line. It
should be kept in mind that the lines we are concerned with here are
alike with respect to plant characters, so that when mixed together
they present a uniform appearance, and that any differences they ex-
hibit are due to genes affecting size, vigor, yield, and the like. Such
quantitative differences are very generally confused with differences in
growth due to soil fertility or soil type, and under field conditions it
is difficult and oftentimes impossible to distinguish between the herit-
able and the nonheritable quantitative variations. For these reasons it
may be a waste of time to continue mass selection beyond a certain
point.

Aside from any improvement in yield which may result, mass selec-
tion is quite effective in purifying a mixed population of soybeans or

1932]

SOYBEAN BREEDING

351

of any other self-fertilized crop. For example, if for various reasons
a field of Illini beans becomes badly mixed and it is desired to purify
them, the grower can select a large number of plants which are typical
of the variety in every particular, thresh them together, and plant the
bulked seed separate from the rest of the field. If none of the selected
plants is a natural hybrid, the next crop will be pure and true to type ;
but if a few natural crosses have occurred, as may have been the case,
a few mixtures will be observed in the next crop. If these mixtures
carry plant characters which make them easily distinguishable from
the typical plants, they can be pulled and removed from the field. In
case, however, plants off-type in seed characters occur, they cannot be
distinguished from the rest until harvest, and then they can be picked
out and discarded. It may be necessary to repeat this process in order
to make sure that all mixtures have been removed.

Pedigree selection is still more effective than mass selection in puri-
fying a variety of soybeans. Each selected plant is planted in a sepa-

FIRST
YEAR

SECOND
YEAR

THIRD
YEAR

FOURTH
YEAR

MULTI-

^«

-^/

PLOT

-SJv yj

FIELD

FIG. 24. — STEPS IN A MODIFIED METHOD OF MASS SELECTION

Instead of bulking the seed from selected plants as in the mass-selection
method, the selected plants are harvested and threshed separately and the seed
planted in separate rows the following year. The rows that are inferior for any
reason or are segregating for one or more characters can be eliminated and the
rest can be harvested together for planting a multiplying plot the next year.
This is a good way to purify a variety having many dominant characters.

rate row and every plant of the progeny examined with respect to
plant and seed characters. Any row found to be different from the type
in any way or to be segregating for any character is discarded, and the
typical true-breeding rows are massed together to make a pure stock
of seed (Fig. 24). Thus only one generation with pedigree selection
is required to attain a high degree of purity in the stock.

352 BULLETIN No. 384 [November,

CROSS-FERTILIZATION OR HYBRIDIZATION AS
A METHOD OF BREEDING

It was shown earlier that self-fertilization tends to separate the
genes present in the hybrid and distribute or assort them to different
strains. Hence self-fertilization may be looked upon as an analytical
method which breaks or tears down the combination of genes piled up
or assembled in the hybrid. By way of contrast cross- fertilization may
be considered to be a synthetic method which brings together or syn-
thesizes the genes that have been separated or assorted to different
lines. Cross- fertilization offers an opportunity for the greatest number
of favorable genes to be combined into one type. Hence types may be
said to be synthesized by cross- fertilization and analyzed by self-
fertilization.

Plant-breeding has for its major problem the bringing together into
one type of all the characters that are considered desirable from the
standpoint of production. No type now known contains all these char-
acters expressed to the highest degree. For example, a strain selected
from the A.K. variety of soybeans is a good yielder, stands up well,
and is early maturing, but in certain seasons it shatters badly. Two
methods are available for improving this strain. One method is to
examine the strain for plants that show little or no shattering in the
hope that such plants, if any are found, represent heritable variations
in the direction of nonshattering. If, however, the strain is pure for
the shattering, no progress can be made by selection. The other
method is to cross the strain with another variety that is quite resistant
to shattering in the hope of being able to isolate from among the
hybrids pure types that are good in yield, early, and erect as well as
nonshattering. All these characters involved in the cross are probably
determined by a large number of genes. It is a difficult problem, there-
fore, to grow and study enough plants in the F2 and F3 generations to
justify the hope of finding types having the particular combination
desired, especially since the expression of these characters may be
modified to so great an extent by differences in soil fertility and soil
type. Nevertheless progress is being made in overcoming these diffi-
culties by learning more about the soybean plant itself, the mode of
inheritance of the characters, and ways of testing plants in the field
to obtain more accurate comparisons.

Hybridization makes it possible oftentimes to produce types that
are superior or inferior to either parent with respect to a given char-
acter. This is referred to as transgressive segregation. The phenom-
enon of transgressive segregation occurs when the character involved

1932} SOYBEAN BREEDING 353

is determined by many genes some of which reside in one parent, some
in the other. In such a case the parents are not thought of as being
at the extremes for the expression of the character, that is, one parent
very low or small, the other very high or large ; but rather as inter-
mediate or nearly so, tho between this situation and that in which one
parent represents the extreme in one direction and the other parent
the extreme in the other direction all possible gradations in inter-
mediacy may occur.

A cross between two soybean varieties involving the hard seed-coat
character is a good example of transgressive segregation. Seeds of
the Dunfield variety when placed in water imbibe water very quickly
and swell. Seeds of a strain designated S.P.I. 65388 have very hard
seed coats, and imbibe water only after being soaked for several days.
When, however, the seed coats are cut or scratched, water enters and
the seeds swell. Hence the varieties differ in something that tends to
make the seed coat impervious to water. In the crosses between these
varieties it was found that the seeds of the Ft hybrid were inter-
mediate between the two parents in this respect, tho resembling the
Dunfield somewhat more than the other parent. In the F2 plants con-
siderable variability occurred, as might have been expected. There
were not only all possible gradations between the parents in the ability
to imbibe water, but a few plants were even harder than the hard
parent and a few even softer than the soft parent. Even tho it might
be considered that the parents stood at practically the extremes for this
character, yet a few.F2 plants were found that transgressed the limits
of the parents. Thus hybridization offers opportunity not only for
combining desirable characters into one- type, but also for bringing
together or piling up genes for a given character, some of which are
contributed by one parent and some by the other, resulting in the ex-
pression of the character to a higher or lower degree than it appeared
in either parent.

Another phenomenon often accompanying hybridization is called
hybrid vigor. This phenomenon may be denned as the stimulating
effect resulting from a cross between different types, causing the
hybrids to excel the parents in general vigor, size, or other character-
istics. It has been variously called "vigor due to crossing," "hybrid
vigor," "stimulus of heterozygosis," "heterozygotic stimulation," and
"heterosis."

Hybrid vigor is particularly noticeable in hybrids between inbred
strains of corn, a normally cross-fertilized crop. As a result of con-
tinuous self-fertilization strains have been produced that are inferior

354 BULLETIN No. 384 {November,

to ordinary varieties of corn in general vigor, size, and yield. When
however, these selfed lines are crossed, the hybrids usually excel the
parents in these qualities and often even the ordinary varieties from
which the parent lines originated. Hence this method of breeding corn
is receiving considerable attention at the present time, particularly by
the corn-belt experiment stations and the U. S. Department of Agri-
culture.

The phenomenon of hybrid vigor is not, however, confined to corn
and other cross-fertilized crops. It is exhibited to a certain extent in
self-fertilized crops as well. The first report of heterosis in soybeans
was given by Wentz and Stewart,50* who found in the hybrid (Fj) of
some crosses considerable increases in height of plant over the average
of the parents. A few hybrids, however, were even below the parental
average in this character. In crosses exhibiting heterosis the hybrids
were below the parents in height during the first half of the growing
period, but during the latter half and particularly during the last three
weeks of the period the hybrids grew faster than the parents and finally
exceeded them. Still greater evidences of hybrid vigor were shown by
the hybrids in yield of seed, the percentage increases over the parents
ranging from 59.58 to 394.37. The number of hybrid plants grown
and studied was small, ranging from 1 to 3, and the number of parent
plants, 1 to 5. In some cases the male parent plants were not available
for comparison, but the authors stated that even if they be assumed to
show a high yield of seed, there would still be good evidence for the
occurrence of hybrid vigor.

Studies on hybrid vigor involving more crosses and more characters
than the preceding were made by Veatch46* at the Illinois Station.
Sixteen crosses were compared with the average of the two parents
and with the better parent in the following characters: yield of seed,
number of seeds, average seed weight, percentage of abortive seeds,
number of pods per plant, number of seeds per pod, plant weight,
straw-grain ratio, plant height, total stem and branch length, number
of nodes, average internode length, and number of days from planting
to flowering. As might be expected, there was considerable variation
among the hybrids as to the extent of hybrid vigor. The average of
all the hybrids was higher in all characters studied than the average
of all the parents ; but in the following characters the average of the
hybrids was exceeded by the average of all the better parents: average
seed weight, number of seeds per pod, straw-grain ratio, and average
internode length. The characters in which the hybrids exceeded, on
the average, even the better parents, and therefore the characters in

1932]

SOYBEAN BREEDING

355

which hybrid vigor was shown, were the following: number of pods
per plant, plant weight, plant height, total stem and branch length,
number of nodes, days from planting to flowering, seed weight or yield,
and number of seed (Fig. 25). While these data are also based on
only 1 to 4 hybrid plants of each cross and the same number of parent
plants, yet, taken together with those of Wentz and Stewart just de-
scribed, it is clear that a certain amount of hybrid vigor is shown by
many soybean varietal crosses.

Granting that the phenomenon of heterosis is exhibited in certain
soybean crosses, we have the problem of the utilization of this vigor

Pods

Average seed
weight

Number seeds
per pod

Seed

number

Seed

weight

Days from
planting
to flowering

Plant
weight

Straw-grain
ratio

Plant
eight

Average
internode lengt

Total stem
and branch length

Number
of nodes

Average percent increase over average of parents

Average percent increase over higher parent

FIG. 25. — EXTENT OF HYBRID VIGOR IN SOYBEANS

Hybrid vigor occurs in certain characters and certain crosses in soybeans
but it has not yet been utilized for increased production. In this diagram the
light innermost circle represents 20 percent below the average of the parents;
the next, a heavy line, represents the average of the parents; the next light line
represents 20 percent above the average, and so on. The position of the hybrids
is represented by the two polygons. The average of the hybrids exceeded the
parental average for every character, but it exceeded the average of the higher
parents only in number of pods, plant weight, plant height, total stem and branch
length, number of nodes, number of days from planting to flowering, weight of
seed, and number of seed. (From Veatch4**)

356 BULLETIN No. 384 [November,

for increased production. The soybean hybrids will not breed true.
Unlike the horticulturist who is working with a plant that can be prop-
agated by budding, cuttings, etc., thus passing on the benefits of hybrid
vigor from generation to generation in undiminished extent, the soy-
bean breeder must either first render his material homozygous for
whatever genes are responsible for the hybrid vigor or else he must
produce the hybrids anew each year. The latter method is out of the
question because of the difficulty of making soybean crosses. The
former method with various modifications that may be devised is the
only one that has promise. With respect to yield the question may be
put thus: What are the possibilities of isolating a type homozygous
for genes responsible for the high Fx yield by selecting the highest
yielding plants from among those of the F2 generation? The answer
is that such a type should be expected, provided the Mendelian inter-
pretation of heterosis as being due to dominant growth genes is the cor-
rect one, and provided further that linkage of genes does not exist to
prevent random assortment and recombination of the genes involved.
On account of the large number of genes probably involved, it would be
impossible to grow and study enough F2 plants to have any chance of
securing the particular combination which is homozygous for all the
genes for which the Ft was heterozygous. Aside from this, there is
the matter of random gene assortment. With 20 pairs of chromosomes
in the soybean we should not expect linkage to hinder this process to
the extent that it probably does in corn, which has only 10 pairs of
chromosomes. At any rate, the chances are better in the soybean than
in corn for the occurrence of independent pairs of genes and hence for
independent assortment of these genes to the gametes. For the practi-
cal utilization of hybrid vigor, however, it is not necessary that the all-
homozygote should be located and isolated. If a type could be ob-
tained from among the hybrids that would breed true for 10, or 5, or
even 2 favorable genes more than were carried by the better parent,
some improvement would thereby have been brought about.

With the object of determining to what extent the F2 plants ex-
hibited the vigor of the Fx parent, Veatch46* made a study of the F2
generation of four crosses in yield of seed, number of seeds per plant,
height of plant, and number of days from planting to flowering. Un-
fortunately the F-L and F2 generations could not be grown the same
season, but comparisons of the two hybrid generations were made thru
the parental types by interpolation. With respect to yield the F2 plants
were found to extend from the lowest variate of either parental line
to the highest variate or above. In one cross a particular F2 plant

1932} SOYBEAN BREEDING 357

yielded 67.38 grams, while the best plant of either parent yielded 58.27
grams. This difference was probably due to soil conditions. In
another cross, however, the extreme yield of 111.17 grams was given
by an F2 segregate, and this yield was more than twice as great as that
of any plant in the parental lines or of any other plant in the F2 popu-
lation. It would seem that this was too great a difference to be ac-
counted for by soil differences, yet when this extremely high yielding
Fo segregate was tested the next year by its progeny, it failed to trans-
mit its marked reproductive propensity.

It must be granted that the method of comparing Ft and F2 genera-
tions by interpolation is far from satisfactory, but when this was done
in the above case, it was found that three of the four F/s were above
their respective F,'s in average yield. In all but one of the F2 popula-
tions, however, there were extreme variates that yielded twice as much
as the Fx plants (interpolated yields). This situation held very well
for each of the other characters studied tho the excess of the extreme
F2 variates over the interpolated . Ft results was not quite so much.
The next year many other extreme F2 segregates were tested by their
progeny, in addition to the two mentioned above, but the results were
disappointing. They performed no better than their parents. Prob-
ably soil variation is responsible for these results. More accurate meth-
ods of testing and comparing hybrid parents and progenies are needed
in order to make definite progress in the utilization of hybrid vigor for
increased production.

BREEDING FOR SPECIAL FEATURES
OIL AND PROTEIN CONTENT

One of the most important considerations in connection with the
improvement of the soybean is that of modifying the composition of
the bean, especially with regard to its oil content. The Illinois Station
has demonstrated the possibility of effecting profound changes in the
composition of corn with regard to protein and oil by continuous selec-
tion. The question arose as to whether these same methods used so
effectively in modifying the composition of corn could be applied to
the soybean, recognizing the fact that corn is a cross-fertilizing plant
while the soybean is a self-fertilizing plant.

To test this point an experiment" was started with the Ebony va-
riety. Seed from ten plants from each of several plant rows grown in
1914 were analyzed for oil and protein. The row showing erect type,

'This work was done by Dr. L. H. Smith and Dr. A. M. Brunson.

358 BULLETIN No. 384 [November,

vigor, and the greatest variability in protein content, Strain 13-13, was
selected as the foundation of the High- and Low-Protein strains, and
similarly the row exhibiting erect type, vigor, and the greatest vari-
ability in oil content, Strain 13-29, was selected as the foundation of
the High- and Low-Oil strains. Thus the two protein strains origi-
nated from a single plant and the two oil strains originated from a
different single plant of the same variety.

In the spring of 1915 seed from the 5 plants showing the highest
percentage of protein of the 10 plants analyzed from Strain 13-13 were
planted in Rows 1 to 5, comprizing the High-Protein selection. Sim-
ilarly seed from the other 5 analyzed plants of Strain 13-13 showing
the lowest percentage of protein were planted in plant Rows 6 to 10,
and this comprized the corresponding Low- Protein selection.

In like manner seed from the 5 plants of Strain 13-29 showing
highest percentage oil was planted in Rows 11 to 15 to form the High-
Oil planting, and the seed from the 5 plants of the same strain showing
lowest percentage oil was planted in Rows 16 to 20 to form the Low-
Oil planting.

The later procedure was as follows: The 20 best plants of each
row were selected by inspection, cut, and bagged separately, then later
threshed, weighed separately, and the seed stored in glass jars. Com-
posites of each of the 20 rows were then made and analyzed for protein
and oil. The 20 plants of the row in the High-Protein selection having
the highest percentage of protein were then analyzed individually, and
the 5 highest of these were selected to plant the 5 High-Protein rows
for the following year. Similarly the highest or lowest 5 plants of the
row showing highest or lowest composite were selected in each of the
other 3 strains to plant the following year. In this manner all 5 rows
of one strain any year were planted from 5 mother plants, all grown
in one plant row the year before.

The data obtained in this experiment are summarized in Tables 4
and 5. Table 4 shows the composition each year of the seed planted
and the corresponding crop harvested with respect to protein content
in the High-Protein and Low-Protein strains, and Table 5 shows
similar results with respect to oil content in the oil strains.

In general the results show rather conclusively that selection has
had no effect in these strains, either in the high or the low direction.
As an average, the six crops of the Low-Protein strain were even
higher than the six crops of the High-Protein strain, tho the difference
was only .37 percent. In oil content the average analyses for the two
strains showed a difference of only .10 percent. These results tend to

1932]

SOYBEAN BREEDING

359

TABLE 4. — PROTEIN CONTENT OF SEED AND OF CROP THEREFROM FOR THE
HIGH-OIL AND LOW-PROTEIN STRAINS

Year

High protein

Low protein

Seed

Crop

Seed

Crop

1915

42.85

41.47

40.29

42.23

1916'

1917

43.11
41.49
47.56
40.70
42.04

42.96

38.96
44.34
39.28
39.45
41.41

40.82

40.53
41.43
41.24
37.85
38.72

40.01

42.36
43.55
39.03
39.11
40.84

41.19

1918

1919

1920

1921

Average

'No crop.

confirm those obtained in similar investigations on other species in
indicating the futility of attempting to modify characters by selective
breeding within pure lines propagated by self-fertilization.

Since selection for protein and oil within pure lines of soybeans
appeared to have no effect, an experiment was started by the writer in
1922 to determine if strains differing in oil content could be isolated
from a single variety. The Manchu variety was chosen for this ex-
periment. On examining the beans it was found that two types ex-
isted, namely, those with black hilums and those with brown hilums.
In 1922 over 200 plants were grown from each type of seed, the plants
were threshed separately, and the seed of each analyzed for oil. In
Tables 6 and 7 respectively the plants from black-hilum beans and
from brown-hilum beans are classified with respect to oil content. The
means, standard deviations, and coefficients of variability were not very
different for the two groups.

Correlation coefficients were calculated for yield per plant and per-
centage of oil. These were, for the black-hilum group, r = .119 ± .042,
and for the brown-hilum group, r = .037 — .041. The results indicate
that in this material no relation exists between the percentage of oil
in the bean and the yield per plant.

TABLE 5. — OIL CONTENT OF SEED AND CROP THEREFROM FOR THE

HlGH-OlL AND LOW-OIL STRAINS

Year

High oil

Low oil

Seed

Crop

Seed

Crop

1915

17.85

17.39

16!i9
16.53
18.83
19.09
18.38

17.74

16.94

16.'62
15.44
15.63
17.95
18.38

16.82

17.29

\6\26
16.37
18.41
19.23
18.33

17.64

1916>

1917

18.04
16.89
16.67
19.18
19.68

18.05

1918

1919

1920

1921

Average

'No crop.

360

BULLETIN No. 384

[November,

TABLE 6. — FREQUENCY DISTRIBUTION OF 256 SOYBEAN PLANTS IN

PERCENTAGE OF OIL
(Progeny 1590, black hilum Manchu)

Range

Mid-value

Frequency

17.75-18.24

18.0

18.25-18.74

18.5

18 75-19.24

19.0

19.25-19.74

19.5

19.75-20.24

20.0

20.25-20.74

20.5

20.75-21.24

21.0

21.25-21.74

21.5

21.75-22.24

22.0

22.25-22.74

22.5

256

Mean = 20.50
Standard deviation = .90
Coefficient of variability =

4.39

In order to determine whether high or low percentages of oil, as
the case may be, shown by the parent plants would be transmitted to
the progeny, several of the extreme variates in each group were planted
in the spring of 1923. The high parents chosen were all but one over
21 percent in oil content and the low parents were all below 19 percent.
In Tables 8 and 9 are given the means and variation constants of the
progenies along with the analyses of the parent plants. In general
high parents produced relatively high progeny and low parents, low
progeny, tho there were a few exceptions. For example, Plant 1590-10
analyzing 21.35 percent gave a progeny of plants varying around a
mean of 17.85, and Plant 1590-162, with an analysis of 17.82, produced
a progeny giving the high mean analysis of 19.70 percent. On the
whole, however, it can be stated that the initial selection was the im-
portant one, and that after the high and low strains were separated
from the population no further increases or decreases were accom-
plished by selection.

TABLE 7. — FREQUENCY DISTRIBUTION OF 271 SOYBEAN PLANTS IN

PERCENTAGE OF OIL
(Progeny 1591, brown hilum Manchu)

Range

Mid-value

Frequency

17.75-18.24

18.0

18.25-18.74

18.5

18.75-19.24

19.0

19.25-19.74

19.5

19.75-20.24

20.0

20.25-20.74

20.5

20.75-21.24

21.0

21.25-21.74

21.5

21 75-22.24

22.0

22 . 25-22 .74

22.5

271

Mean =20.10
Standard deviation = .68
Coefficient of variability =

3.40

1932]

SOYBEAN BREEDING

361

TABLE 8. — ANALYSES OF PARENT PLANTS SELECTED FOR HIGH OIL CONTENT AND
ANALYSES OF THEIR PROGENIES

Parent

No.

Analysis
of parent

Progeny
No.

Number
plants
analyzed

Mean
percent
oil

1590-5

21.23

2018

18.66 ± .08

21.52

2019

19. 15 ± .11

21.35

2021

17.85 ± .22

21.61

2022

19.50 ± .08

21.56

2023

18.97 ± .08

21.78

2024

19.20 ± .09

22.59

2025

19.57 ± .07

21.54

2026

• 24

18.62 ± .07

21.53

2027

19.33 ± .10

21.96

2028

19.86 ± .11

21.07

2029

19.97 ± .13

44...

21.56

2030

19.96 ± .09

21.91

2031

19.44 ± .10

21.20

2032

19.61 ± .09

21.84

2033

20.10 ± .11

21.75

2034

19.50 ± .17

21.88

2036

19.74 ± .11

21.98

2038

20.18 ± .11

21.57

2039

20.20 ± .09

21.82

2040

19.54 ± .15

21.79

2041

19.19 ± .08

21.77

2042

19.58 ± .08

136

21.82

2045

20.35 ± .08

171

21.79

2047

19.89 ± .10

175

21.72

2049

20.60 ± .10

191

21.62

2050

20.19 ± .13

197

21.67

2052

18.99 ± .11

204

21.68

2053

20.15 ± .07

208B

21.77

2054

19.85 ± .08

258

21.63

2055

18.95 ± .09

1591-12...

20.98

2056

18.73 ± .12

21.16

2057

19.64 ± .09

21.64

2058

19.19 ± .12

21.99

2059

19.64 ± .13

21.10

2060

19.22 ± .08

21.17

2062

19.92 ± .10

21.09

2063

19.73 ± .07

21.35

2064

19.00 ± .91

112

21.24

2065

19.41 ± .11

119

21.17

2066

19.38 ± .11

T.ABLE 9. — ANALYSES OF PARENT PLANTS SELECTED FOR Low-OiL CONTENT AND
ANALYSES OF THEIR PROGENIES

Parent

No.

Analysis
of parent

Progeny
No.

Number
plants
analyzed

Mean
percent
oil

159O-8 .

18.33

2020

15.29 ± .09

18.38

2035

17.49 ± .11

72A

18.16

2037

18.10 ± .07

17.99

2043

15.88 ± .10

162

17.82

2046

19.70 ± .08

172

17.96

2048

16.40 ± .25

195

18.09

2051

17.00 ± .11

1591-57

18.73

2061

15.57 ± .19

For planting in 1924 seven progenies were chosen. To show how
they compared with the original parents in percentage of oil, and the
reason for using them to continue the experiment, Table 10 is pre-
sented. Significant differences are apparent in the mean analyses of
the progenies described as high and low. It was obviously impossible,

362

BULLETIN No. 384

[November,

TABLE 10. — PEDIGREE, PERFORMANCE, AND OTHER INFORMATION ON PROGENIES
SELECTED FOR STUDY OF OIL CONTENT

Original
parent

Parent

analysis

Progeny
No.

Mean percent oil
of progeny

Hilum
color

Reason for
selection

1590-8

18.33
17.99

21.82

17.96
21.72
18.73
21.17

2020
2043
2045

2048
2049
2061
2062

15.29 ± .09
15.88 ± .10
20.35 ± .08

16.40 ± .25
20.60 ± .10
15.57 ± .19
19.92 ± .10

Black

Black
Black

Black
Black
Brown
Brown

Low analysis
Low analysis
High analysis
/Medium analysis
(.High variability
High analysis
Low analysis
High analysis

9<5

136

172

175. .

1591-57

on account of the expense, to continue all progenies, but the seven
given in Table 10 were continued another generation by using a few
plants as parents that represented the lowest, middle, and highest
points of the range. The results are shown in Table 11. Again low
progenies produced low, high progenies, high. The progeny described
in Table 10 as being of medium oil content with high variability

TABLE 11. — COMPARISON OF ANALYSES OF PROGENIES WITH THOSE OF THEIR
PARENTS WITHIN THE SAME LINE, 1924 CROP

Progeny
No.

Mean analysis of
progeny (1923)

Analysis of parent
plants of 1924 crop

Mean analysis of
progeny (1924)

2020

15.29 + .092

15.70

14.84 ± .06

2043

15.88 ± .102

16.30
14.44
14.54
14.77
16.15

15.92

15.45 ± .06
15.00 ± .09
14.84 ± .06
14.96 ± .07
14.95 ± .06

15.57 ± .07

2045

20.35 ± .086

16.48
19.98

15.27 ± .04
17.79 ± .08

2048

16.40 ± .252

21.05
20.54

19.96

18.24 ± .11
18.44 ± .07

17.16 ± .11

2049

20 60 ± .109

16.45
14.27
14.99
15.58
18.00
19.41

21.32

14.88 ± .05
15.19 ± .06
14.63 ± .05
14.95 ± .06
16.03 ± .12
16.89 ± .10

18.22 ± .10

2061

15.57 ± .194

20.70
19.96
21.10

16.60

18.23 ± .13
17.76 ± .14
17.93 ± .13

15.17 ± .08

2062

19 92 ± .108

15.49
14.29
17.03
14.87
15.58

19.63

14.78 ± .08
15.28 ± .09
15.53 ± .08
14.58 ± .05
15.42 ± .14

18.01 ± .09

18.44
21.05
20.52
19.13
19.93

15.31 ± .10
18.40 ± .07
19.13 ± .10
18.06 ± .07
17.59 ± .07

19321

SOYBEAN BREEDING

363

(Progeny 2048) gave variable results, but in general high variates
gave rise to high progenies, low variates to low progenies, thus giving
evidence of segregation for oil content. The parent plant was likely
heterozygous for factors affecting oil content and in consequence the
progeny showed segregation. From such a segregating progeny it is
possible to isolate types which differ in percentage of oil, but after
these have been isolated selection is powerless to effect further im-
provement.

It is of interest to follow further the behavior of this variable prog-
eny. Plants from three progenies of the 1924 crop were tested in 1925.

TABLE 12. — COMPARISON OF ANALYSES OF PARENTS AND ANALYSES OF PROGENIES

DESCENDED FROM LINE 2048, WHICH SHOWED HIGH

VARIABILITY IN OIL CONTENT

Percent oil
of parents

Mean percent
oil of 1924
progenies

Analysis of
parents of
1925 crop

Mean percent
oil of 1925
progenies

Analysis of
parents of
1926 crop

Mean percent
oil of 1926
progenies

19.96

17.16 ± .11

16.42

18.70 ± .09

16.42

19.10 ± .12

17.05
18.98
16.72

19.94 ± .10
20.19 ± .09
19.36 ± .07

/21.24
"\18.94

19.62 ± .12
19.63 ± .05

18.86

19.18 ± .06

17.20

19.60 ± .06

14.99

14.63 ± .05

14.67

16.75 ± .07

14.21
14.52

16.58 ± .06
17.15 ± .07

/18.07
••\16.36

16.96 ± .07
17.17 ± .11

15.03

16.83 ± .05

14.23

16.64 ± .05

15.01
14.35

16.91 ± .07
16.18 ± .09

M7.39
"\14.89

17.03 ± .07
16.82 ± .06

18.00

16.03 ± .12

17.60

18.26 ± .23

119.43

19.97 ± .04

16.56

19.13 ± .14

14.88

15.54 ± .19

16.34
14.97

16.14 ± .08
16.29 ± .07

17.08

17.52 ± .36

16.45

14.88 ± .05

14.27

15.19 ± .06

15.58

14.95 ± .06

19.41

16.89 ± .10

The results are given in Table 12. It is clear from this table that the
progeny giving an average analysis of 17.16 in 1924 differed genetically
from the progeny giving an average analysis of 14.63, based on the
behavior of the progeny. It is also clear that the progeny analyzing
16.03 in 1924 must have been segregating for factors affecting oil con-
tent, as the resulting progenies in 1925 differed significantly in average
analyses. Furthermore, when certain of these were carried further by
using the highest and the lowest plants of certain progenies as parents,
the result was the production of high-analysis progenies and low-
analysis progenies differing by about 2 percent. Also, in the case of

364

BULLETIN No. 384

[November,

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1932\ SOYBEAN BREEDING 365

highest and lowest parent plants complete regression to the mean of
the type was shown by both progenies in every case, indicating homo-
zygosity for factors for oil content. Hence it may be concluded that
in a mixed or segregating population of plants for oil content, selection
of single plants is effective in separating out the various types, but
after they have once been isolated from the group and from one
another, no further effect is obtained.

After the analyses were obtained on the 1924 crop, certain prog-
enies were composited which could be traced back to a single original
plant. Four lines or strains were thus formed, designated as A, B, C,
and D. Lines A and C were characterized by black hilums ; Lines
B and D, by brown hilums. Lines A and B exceeded Lines C and D
by about 2 percent of oil on the average, and this difference has been
maintained quite uniformly until 1930 (Table 13) when Line A
showed less than 1 percent more oil than Lines C and D. Seasonal
variations have been quite marked ; however, in a season favoring
high oil, as in 1928, both the high and low lines were increased, but
the same relative difference was maintained between them. Likewise
in 1924 the conditions favored low oil content ; consequently, both high
and low lines were lowered but they still differed by 2 to 3 percent.
The unselected Manchu beans which have been carried on for com-
parison have varied considerably from season to season, but in general
their oil content has been between the analyses of the high and low
lines. An exception to this occurred in 1930, when the oil content of
the unselected Manchus was higher even than that of the high lines.

The application of these findings is clear. Since selection within a
pure line is without effect, some progress may be made by isolating
different lines from a variety that has not undergone previous selection.
In such a variety germinal changes for oil content may have occurred
at various times in the past, remaining, of course, unknown until chem-
ical analysis is applied. After the strains have been isolated from the
group and found to breed true, it is needless to attempt further im-
provement by reselecting within the strain.

Since selection was powerless to effect any further improvement
in oil content within the strain, hybridization was resorted to. The
high-analysis strains, A and B, were approximately 2 percent higher
in oil than the low-analysis strains, C and D. Would it be possible by
crossing A and B or C and D to produce a type higher in oil than the
parents in one case and lower in oil than the parents in the other case,
on the principle of transgressive segregation discussed earlier in this
bulletin? This would be possible only in case the oil content of Strains

366

BULLETIN No. 384

[November,

A and B or of Strains C and D were due to different genetic factors.
Since selection brought about no further improvement, it seemed worth
while to try the method of crossing.

Accordingly crosses were made between Strains A and B and
between C and D. Crosses were also made between A and D, A and C,
and C and B in order to get some information on the inheritance of
oil content. The oil content of the Fx plants (Table 14) was in most
cases between the analyses of the two parents. An exception to this
occurred in the cross A X C, as the Fx progeny were higher even than

TABLE 14. — COMPARISON OF Fi PLANTS WITH PARENT STRAINS IN
PERCENTAGE OF OIL, 1927

Type

Number of
plants

Average
analysis

Type

Number of
plants

Average
analysis

A...

19.50

17.95

20.08

20.61

Fi (A X B)

19.63

Fi (C X B)

19.16

A...

19.58

16.93

18.63

17.74

Fi (A X D)

19.50

Fi (C X D)

17.67

19 28

17.97

Fi (A X C)

19.85

the higher parent. This fact might suggest that the parents contributed
complementary genes for oil content so that in the Fx generation there
was a combination of genes more favorable for oil production than
existed in the higher parent in the homozygous condition. However,
this suggestion was not confirmed by the F2 results (Table 15). The
average of 51 F2 plants in Rows 1050 and 1051 was 22.03 percent, and
the average of 33 F2 plants of the same cross in Rows 1053 and 1054
was 21.72 percent. The range of the entire 84 F2 plants was from
19.78 to 23.76. Ten plants of A, Row 1049, averaged 22.77 percent
and 9 plants of A, Row 1055, averaged 21.98 percent; while 8 plants
of parent C, Row 1052, averaged 19.84 percent. The parents ranged
in oil content from 19.33 to 24.17 percent. Thus even with 84 F2
plants no evidence was obtained to indicate that a type higher than the
higher parent in oil content could be produced from this cross. Hence
the Fj of this cross was higher than the higher parent owing probably
to environmental influences rather than to a more favorable combina-
tion of genes for oil content than existed in the higher parent.

Much the same could be said about the F2 progeny of the other
crosses grown. The extremes of 31 F2 plants of cross A X B, both
high-analysis strains, were within the fluctuating variations of the two

1932-}

SOYBEAN BREEDING

367

parents. In crosses between high and low strains, as A X D, A X C,
C X B, and B X C, the same thing was true, with the single exception
that an F2 plant in cross C X B, with an analysis of 18.68 percent, was
lower than any plant of the low-analysis parent C. However, in the
cross C X D, low-analysis parents, both the upper and lower extremes

TABLE 15. — COMPARISON OF F2 PROGENIES WITH THEIR PARENTS IN
PERCENTAGE OF OIL, 1928

Cross

Row

Type

Number
plants
analyzed

Average
percent
oil

Range

From

A XB
A XD

A XC

C XB
B XC

C XD

1037.

A
B
Ft
Ft

A
A
D
D

Ft
Ft

A
A
C
Ft
Ft

C
B

B
B
C
C
Ft
Ft

C
D
Ft
Ft
F»

3
10
10
19
12

10
10
11
9
23
37

10
9
8
51
33

10
10

10
10
10
9
43
37

10
10
38
29
11

21.36 ± .35
23.72 ± .22
23.48 ± .09
22.84 ± .15
21.94 ± .10

23.72 ± .22
22.46 ± .15
20.94 ± .08
21.07 ± .05
22.27 ± .12
22.28 ± .08

22.77 ± .12
21.98 ± .15
19.84 ± .10
22.03 ± .07
21.72 ± .09

19.70 ± .06
21.91 ± .11
20.68 ± .10

21.87 ± .12
21.38 ± .13
18.79 ± .08
19.06 ± .12
20.91 ± .64
20.87 ± .11

19.45 ± .12
19.81 ± .12
19.80 ± .09
20.07 ± .14
19.86 ± .23

20.14
22.22
22.32
20.99
21.23

22.22
21.39
19.86
20.79
20.46
20.17

21.90
20.44
19.33
19.78
20.25

19.29
20.71
18.68

21.07
20.47
17.93
18.19
19.05
18.07

18.20
19.17
17.91
17.46
18.05

22.08
25 . 36
24.51
24.45
23.09

25.36
23.54
22.32
21.38
23.38
24.25

24.17
22.67
20.74
23.76
23.16

20.48
23.01
22.06

22.82
22.15
19.51
19.62
22.63
22.62

20.39
20.88
21.39
22.26
21.95

1040

1034

1035.1036
1038.1039

1040

1048

1043

1044

1045

1046.1047
1049. . .

1055

1052

1050.1051
1053.1054

1065

1067

1066

1068. ..

1072

1071

1075

1069.1070
1073.1074

1080. . .

1083

1081.1082
1084. 1085
1086

of the parents were exceeded by certain plants of the F2 generation.
There appeared, therefore, to be good indication of transgressive segre-
gation in this cross, particularly when these facts are considered along
with the results obtained in the F3 generation.

For the purposes of determining whether any segregation occurred
with respect to genes for oil content, and to secure further evidence
for transgressive segregation indicated in a case or two in the F2 gene-
ration, the extreme F2 segregates were tested by their F8 progeny in
comparison with plants of the original parental strains. The results
are shown in Table 16. In the F8 plants of the cross A X B, both high
strains, there is no evidence of genetic differences and none of trans-

368

BULLETIN No. 384

[November,

TABLE 16. — COMPARISON IN PERCENTAGE OF OIL OF F3 PROGENIES WITH
THEIR PARENTS, 1929

Cross

Type of

Analysis

Number
plants of

Row

Average
percent

Rar

ige

parent

of parent

progeny
analyzed

No.

oil of
progeny

From

A X B

330

20.13 ± .14

19.51

21.97

336

20.21 ± .06

19.36

20.68

333

21.06 ± .09

20.17

21.91

339

20.11 ± .13

19.12

21.44

A X D

High
Low
High
Low
High
Low

24.33
21.69
24.45
21.23
24.10
21.43

10
10
10
10

10
10

331
332
334
335
337
338

342

20.22 ± .16
20.27 ± .13
20.26 ± .16
20.56 ± .20
20.31 ± .19
19.96 ± .09

20 27 ± .08

18.50
19.34
18.77
18.79
19.70
19.40

19.37

21.67
20.97
21.53
21.66
20.98
20.61

20.92

A X C

A
A
D
D

High
Low
High
Low
High
Low
High
Low

24! 34
20.68
24.00
20.60
24.58
20.46
24.25
21.76

10
10
10
10
10
10
10
7
10
10
10
10

348
354
345
351
343
344
346
347
349
350
352
353

354

21.00 ± .10
20.26 ± .14
16.08 ± .19
17.17 ± .11
19.19 ± .14
19.38 ± .18
19.11 + .12
18.17 ± .15
20.66 ± .27
18.01 ± .13
19.77 ± .13
18.75 ± .17

20.26 ± .14

19.66
18.88
13.60
16.09
18.28
17.64
18.44
17.07
20.33
17.27
18.68
17.13

18.88

21.88
21.03
16.95
17.90
20.21
20.34
20.17
18.63
21.10
19.82
20.61
20.58

21.03

360

20.64 + .06

20.18

21.13

357

17.16 + .15

15.55

17.92

363

17.99 ± .15

16.59

18.71

C X B ..

High
Low
High
Low
High
Low

23.76
19.78
23.32
20.72
23.16
20.25

10
10
10
10
10
10

355
356
358
359
361
362

377

20.04 ± .16
17.54 ± .16
20.36 ± .05
19.22 ± .13
20.91 ± .09
18.36 ± .16

17 05 ± 13

18.24
16.72
19.85
18.20
20.34
16.75

16 09

20.93
19.35
20.97
20.51
21.64
19.49

17 81

383

16.63 ± .14

15.77

18.15

380

20.00 ± .24

17.24

21.59

386

20 28 ± .09

18.13

19.60

C X D

High
Low
High
Low
High
Low

22.06
19.50
22.15
19.38
22.62
18.07

10
10
10
10
10
10

378
379
381
382
384
385

387

19.25 ± .14
18.68 ± .13
20.88 ± .10
18.25 ± .17
19.02 ± .20
18.68 ± .29

17 64 ± .04

18.29
17.64
19.84
17.46
17.55
15.81

17.16

20.19
19.48
21.71
20.04
20.78
20.89

18.16

D
D

High
Low
High
Low

2l!39
17.91
22.26
17.46

7
10
8
10
10
10

390
394
388
389
391
392

17.22 ± .03
16.77 ± .07
17.32 ± .06
17.15 ± .10
17.19 ± .11
15.82 ± .16

16.99
16.39
15.71
16.19
16.36
14.66

17.66
17.25
19.07
17.85
18.08
17.30

gressive segregation. There was complete regression to the same mean
in progenies from both high and low selections. It may therefore be
concluded from these data that Strains A and B have the same genetic
constitution with respect to oil content.

In crosses between high and low strains, as A X D, A X C, and
C X B, there were distinct evidences of segregation. Many progenies
from high F2 segregates were significantly different from those from
low F2 segregates. However, no progenies were lower than the low

1932] SOYBEAN BREEDING 369

parent or higher than the high parent. Hence these crosses, so far as
they were carried in this experiment, were of no value in producing
types with either lower or higher oil content than that of the parents.
To express these facts genetically we may assume that the high strains
possess the genetic complex X + AB and the low strain the genetic
complex X + Ab or X + aB. The ¥l hybrid between these would
likely be intermediate between the parents in oil content, and this was
found to be the case. Segregation and recombination, furthermore,
cannot produce any F, segregates that are higher or lower in oil con-
tent than the parents, and this also appeared to be the case in the
crosses mentioned above.

As stated above, the only cross that showed evidence of trans-
gressive segregation in the F, generation was that between C and D.
Data on the F3 generation of this cross furnished a small amount of
confirmatory evidence on this point. The highest F2 segregate of this
cross contained almost 5 percent more oil (Table 15) than the lowest
F2 segregate (22.26 percent as compared with 17.46 percent) and about
1.38 percent more oil than the highest variate of the parents; and the
lowest F2 segregate contained .74 percent less oil than the lowest
variate of the parents. In the F3 generation the progeny of the highest
F2 segregate (22.26 percent oil) varied about a mean of 17.19 percent,
about the same as the average of the parent strains ; and the progeny
of the lowest F2 segregate (17.46 percent oil) varied around a mean
of 15.82 percent, about 1.5 percent lower than the parental average.
There was, therefore, more than 1 percent difference between the
means of these two progenies, a difference of some statistical signifi-
cance. Thus there appeared to be a slight indication of transgressive
segregation in the direction of low oil content but not in the direction
of high oil content.

Soybeans with low oil content are desired especially for hog feed-
ing, as too much oil tends to produce "soft" pork. It has not been
definitely determined how low the percentage in the soybean should be
to prevent this effect, but indications point to a range of tolerance of
5 to 8 percent. Since in the cross C X D, both low oil strains, there
seemed to be evidence of transgressive segregation for oil content on
the low side (one of the progenies in F3 varying around a mean signifi-
cantly lower than the parental analyses, 15.82 percent as compared
with 16.77 to 17.64 for the parent strains), there seemed to be some
hope of making more progress in producing types with lower oil con-
tent by hybridization than by selection. When, however, several indi-
vidual plants in this F3 progeny were tested in 1930, the resulting prog-

370

BULLETIN No. 384

[November,

enies (Table 17) were quite similar and varied around a mean quite
close to the analyses of the parent strains grown adjacent to them.
These results were not only disappointing from the standpoint of de-

TABLE 17. — COMPARISON OF F4 PROGENIES WITH THEIR PARENTS IN
PERCENTAGE OF OIL, 1930

Number of

Rai

ige

Parent

parent

progeny
analyzed

of progeny

From

93812-1

17.30

17.97 ± .01

17.11

19.43

15.81

18.16 ± .01

16.64

19.28

14.70

17.89 ± .00

KS.82

18.74

16.26

17.37 ± .01

16.31

18.68

15.99

17.65 ± .00

17.22

18.14

14.66

17.63 ± .01

16.17

18.37

16.33

18.21 ± .01

17.26

19.34

16.02

17.47 ± .00

16.71

18.35

15.68

17.97 ± .01

17.05

18.55

17 93

Line DJ parents

17.98

veloping a soybean strain with a low oil content, but they also cast
some doubt on the conclusions above drawn that transgressive segre-
gation actually occurred in this cross on the low side.

Hence it cannot be definitely stated that the results obtained in
these crosses demonstrated the occurrence of transgressive segregation
for oil content, tho the F2 and Fs data on cross C X D seemed to indi-
cate it. Hybridization, therefore, was not effective in producing true-
breeding types of soybeans with higher or lower oil content than that
of the parents so far as these experiments were concerned.

In the interpretation of these results certain limitations are recog-
nized and admitted. The number of plants analyzed of any one strain
or hybrid progeny was inadequate, but, owing to the expense and time
required to make the analyses, it was impossible under the circum-
stances to handle a greater number. This limitation is felt especially
when it is desired to carry forward and analyze the progeny of several
extreme F2 segregates. The number of plants that should be analyzed
each successive year thus increases in geometrical ratio, and the experi-
ment spreads out like a fan, expanding with each succeeding genera-
tion. In the case of a character like height of plant or color, which
can be taken and noted quickly, the limitation of numbers is scarcely
felt.

Furthermore there is considerable variation in the analyses of indi-
vidual plants of the same pure strain due to unequal effect of growth
conditions. With the limited number of plants which it was possible

1932] SOYBEAN BREEDING 371

to analyze, the parent strains sometimes varied more than their hybrid
progenies. However, no method of removing this ever-present varia-
tion presented itself; hence the amount of variation was calculated so
that the progeny means could be compared in the light of their prob-
able errors. In this connection too it is recognized that 10 or less is
too small a number on which to calculate the probable error, but this
was the only practical means of indicating the amount of variation
since to publish the individual analyses would require an undue amount
of space.

Finally, oil content in soybeans is a very complex character prob-
ably determined by a number of genes. For this reason the data do
not admit of determining the mode of inheritance of oil content. It is
believed that the high strains, A and B, differ genetically from the low
strains, C and D, because (1) their means differed significantly, and
(2) the hybrids between the high and low lines showed segregation in
oil content. No attempt was made to estimate the number of genes
which were responsible for the difference between the high and low
strains.

The percentages of oil and protein have been determined for a large
number of soybean varieties by various workers (Fellers6*; Piper and
Morse35*; Stark37*; Cole, Lindstrom, and Woodworth.2*) The pub-
lished analyses indicate that varieties differ greatly in oil and protein
content. It is not known, however, to what extent these differences
are genetic and therefore of value from the plant-breeding standpoint.
Considerable evidence is at hand showing that the environmental fac-
tors greatly affect the composition of a variety. Indeed Stark37* states
that such factors may be responsible for greater differences in composi-
tion within the same variety than are usually found between varieties.

QUALITY OF OIL

The use to which soybean oil may be put depends greatly on its
quality. Quality may, of course, include many things, but in soybean
oil quality refers particularly to its drying property since so large a
proportion of the soybean oil produced is used in paint manufacture.
The capacity for rapid drying depends on the ability to combine with
atmospheric oxygen, and this in turn, on the degree of saturation or
unsaturation of the fatty acids in the oil. The more unsaturated the
oil, the more quickly it combines with oxygen.

Drying quality is measured by the amount of iodin which the oil
will absorb. The percentage of iodin absorbed is called the iodin num-
ber of the oil. The iodin number of linseed oil is about 180 while that
of soybean oil is about 125 to 130. To make soybean oil a better dry-

372 BULLETIN No. 384 [November,

ing oil, its iodin number must be raised. One method by which this
may be brought about is by breeding.

In 1912 a selection experiment was started at the Wisconsin Station
(Cole, Lindstrom, and Woodworth2*) to determine whether any prog-
ress could be made in the direction of a high iodin number by selection
within a pure line. Seeds of a single plant were analyzed for quantity
and quality of oil. The progenies of this plant were grown in 1913,
and each plant analyzed. Then the highest and lowest of these were
selected in order to start a "high" line and a "low" line. Each year
therefore for seven years the plant analyzing highest in the high line
and lowest in the low line were used to continue these lines.

Selection resulted in the development of differences between the
high and low lines in quality of oil as measured by the iodin number.
The averages for the last three years of the experiment were as fol-
lows: high line, 133.7; low line, 124.9. These results were interpreted
by the authors as due to the separation of two different genotypes
within the original variety rather than as due to selection being effec-
tive within a pure line. That this interpretation is probably the correct
one is indicated by the fact that while the individual plants of the
original strain showed high variability in iodin number, they also
varied in flower color, maturity, and type of plant.

Selection was based only on iodin number and at the end of the
experiment the high line was purple-flowered, tall, late-maturing;
while the low line was white-flowered, short, early-maturing. Hence
the original type was probably heterozygous for genes affecting quality
of oil as well as for genes responsible for plant characters; or else
was a mixture of types, and selection did no more than to isolate or
separate the types that were present. This wras also the case, appar-
ently, in the selection experiments at the Illinois Station for oil content
described above.

Cole, Lindstrom, and Woodworth2* reported that late maturity in
soybeans seems to be correlated with high-quality oil. Probably this
accounts for a certain amount, at least, of the difference between the
two lines mentioned above. Another important point was that there
seemed to be no relation between the quality and quantity of the oil.
Therefore selection for quality did not tend to depress the quantity
of oil present.

Leith23* reported a similar effect of selection in isolating strains
differing in iodin number. The difference between the two strains,
32-4-1 and 32-12-1, was small but probably significant. Leith empha-
sized the fluctuations shown by soybean strains in composition and

1932] SOYBEAN BREEDING 373

quality of the oil from year to year. This is in line with what other
workers have found, and helps to bring out the extent to which the
plant environment modifies the behavior of the soybean. However,
strains differing in composition or quality of the oil owing to genes in
the germplasm tend to maintain these same differences regardless of
seasonal conditions.

RESISTANCE TO DISEASE

The soybean is attacked by a large number of bacterial and fungous
diseases but no one disease has as yet assumed serious proportions in
this country. As the culture of the soybean continues, however, we
may expect diseases to increase in number as well as in prevalence and
in destructive effect on the crop. Hence breeding for disease resistance
may become in the future a very important phase of soybean improve-
ment.

Fortunately numerous instances are known of varietal resistance
to certain bacterial and fungous diseases.

Bacterial Diseases

Clinton1* reported observations on the occurrence of a bacterial leaf
disease on the following varieties: Medium Yellow (Midwest), Wilson,
Manhattan, Quebec 92, Quebec 537, and Ito San. The last-named
variety was affected worse than the others. Unfortunately he did not
mention other varieties which were presumably examined and not
found infected. This was probably the same disease as the one to be
mentioned next.

Woodworth and Brown61* reported results of observations and ex-
periments on varietal resistance and susceptibility to bacterial blight
(Bacterium glycineunt, n. sp.) of the soybean. Of 47 varieties grown
and artificially inoculated under field conditions in 1918 about half
were completely resistant and the remainder ranged from complete
susceptibility to partial resistance. The experiment was repeated with
a number of varieties in the greenhouse and similar results obtained.
"Varieties Ebony, Elton, Habara, No. 8 (S.P.I. 20406), Mammoth
yellow, Virginia, Cloud, Wilson, Medium yellow, and Ito San were
under trial. All plants were sprayed with a water suspension of the
bacterial blight organism shortly after the first compound leaf ap-
peared. An examination made three weeks later showed about half
of the plants of the Wilson variety, all but three of Medium yellow,
and all of Ito San infected. The other varieties were completely re-
sistant."

Observations made by Wolf51* on the natural occurrence on a num-

374 BULLETIN No. 384 [November,

her of varieties of a closely related disease caused by Bacterium sojae,
n. sp., failed to disclose any evidence of varietal resistance or suscep-
tibility.

The most extensive study made with soybeans on varietal resistance
and susceptibility to a disease was that made by Lehman.22* As a
result of random field observations and carefully executed artificial
inoculations both on field and greenhouse plantings, Lehman was able
to classify 56 varieties of soybeans with respect to their reaction to the
bacterial pustule disease (Bacterium phaseoli sofense). This classi-
fication is as follows, the varieties being arranged in order of decreas-
ing resistance, the least resistant at the bottom:

Highest Intermediate Lowest

(resistant) (susceptible)

Columbia Laredo Hoosier

Mandarin Chiquita Midwest

Old Dominion Mammoth Yellow Medium Green

Tarheel Black Virginia

Biloxi Herman

Otootan Haberlandt

Goshen Prolific Pine Dell Prolific

Southern Prolific Hollybrook

Minsoy
Merko
Yokoten

The Columbia variety, while the most resistant of all, was not
immune, but showed some lesions when the conditions for infection
were made as favorable as possible. However, the lesions were fewer
in number, smaller, and slower in development than those of less re-
sistant varieties, in addition to lacking certain features, such as the
yellow halo, which were typical of lesions in other varieties. Hence,
as pointed out by Lehman, the Columbia variety may prove to be a
good parent to use in crossing experiments for the purpose of combin-
ing resistance to the bacterial pustule disease with desirable characters
of other varieties.

Fungous Diseases

The foliage of soybeans is also attacked by a fungous disease called
"brown spot" (Septoria glycines Hemmi). It was first described in
Japan by Hemmi.14* Later Wolf and Lehman52* in this country made
a careful study of the disease and established the fact that the fungus
causing the disease in America was identical with that causing the same
disease in Japan. Wolf and Lehman noted differences among soybean
varieties in relative resistance and susceptibility. The most susceptible
were Black Eyebrow, Virginia No. 12, and several hybrids of Virginia.

1932] SOYBEAN BREEDING 375

Those showing only a moderate degree of infection were Austin, Wil-
son, Midwest, and Ito San. The most resistant varieties, that is, those
showing only a slight amount of infection, were Mammoth Yellow,
Haberlandt 38, Laredo, Biloxi, Lexington, Tokyo, Tarheel Black, and
Chiquita.

Another fungous disease of the soybean is called Fusarium blight
by Cromwell,3* who made an extensive study of the disease as it oc-
curred in North Carolina. In variety tests for indications of resistance
the following varieties were used: Brown, Black Eyebrow, Virginia,
Mammoth Yellow, Early Dwarf Green, Wilson, Barchet, Jet, Austin,
Arlington, Guelph, Chiquita, Auburn, Manchu, Tokio, Peking, Tar-
heel Black, Haberlandt, and Medium Yellow (now called Midwest).
All these varieties were susceptible except Black Eyebrow which, in
two tests, showed considerable evidence of resistance. The Brown
variety, however, tho infected, seemed to be able to tolerate infection
better than any of the others. This variety, according to Cromwell, is
the same as Mammoth Yellow except in seed color. Haberlandt also
seemed to be able to develop well in spite of infection.

Evidence of varietal resistance and susceptibility to still another
disease attacking the foliage of the soybean, namely, frog eye leaf spot
caused by Cercospora diazu Miura, was furnished by Lehman.20* Va-
rieties which were attacked and therefore susceptible were Laredo,
Otootan, Biloxi, Manchu, Mammoth Yellow, Goshen Prolific, Virginia,
Austin, Tarheel Black, Wilson, Tokyo, Haberlandt, and Chiquita. "Of
this group, Otootan and Biloxi are most susceptible ; Chiquita, Tarheel
Black, Wilson, and Mammoth Yellow are somewhat less susceptible.
Early maturing varieties such as Dixie, Manchu, and Virginia, escape
serious injury; while such late maturing varieties as Otootan and
Biloxi suffer most."

Mosaic Disease

Clinton1* reported observations on a disease which from his de-
scription was probably the mosaic disease. The following varieties
were found to be infected: Medium Green, Wilson, Swan, Kentucky,
Mikado, O'Kute, Ito San, and Midwest. The last-named vaciety ap-
peared to be infected worse than the others. Varieties, if any, which
were not infected, and therefore considered resistant, were not men-
tioned.

More extensive studies and experiments were made on the mosaic
disease of soybeans by Gardner and Kendrick10* and Kendrick and
Gardner.19* These authors report that, "Mosaic has been noted on the
following varieties of soybeans at La Fayette, Indiana: Midwest, or

376 BULLETIN No. 384 [November,

Medium Yellow, Haberlandt, Manchu, Ito San, Mongol, Hurrelbrink,
Mammoth Black, Habara, A.K., Arlington, Hoosier. Elton, Wea, Lex-
ington, Black Eyebrow, Pinpu, 36847, Feldun, Dunfield, Soysota, Wil-
son Black, Mammoth Yellow, Brown, Virginia, and Tar Heel Black.
The disease seems to be most prevalent in the Midwest, Haberlandt,
and Black Eyebrow varieties, and the symptoms seem to be most con-
spicuous in the Midwest variety." The varieties above named may be
considered susceptible.

That the disease may be transmitted thru the seed has been proven
by Gardner and Kendrick.10' 19* In work reported in 1921 13 percent
of the seed from mosaic plants transmitted the disease. In 1922 a plot
was planted with seed from mosaic plants, and a similar plot with seed
from healthy plants. In the former, of a total of 993 seedlings, 172,
or 17 percent, showed mosaic ; while in the latter there were no mosaic
plants in 590. These results were confirmed by further studies.
Furthermore, when seeds were taken from diseased plants of different
varieties, and plants grown, it was found that varieties differed in their
ability to transmit the disease. "The Midwest, Haberlandt, Black Eye-
brow, A.K., and Arlington varieties apparently transmit the disease
more readily than Feldun, Manchu, Lexington and Dunfield."

Studies made by these authors on secondary spread of mosaic
brought out further evidences of varietal differences. In variety test
plots which were equally exposed to infection and which showed no
mosaic among the seedlings, counts made on August 7 to 14 ranged
from .6 percent for Virginia to 90 percent for Midwest. Soysota also
largely escaped infection, having only 2 percent. Among the most
susceptible were Midwest (90 percent), Manchu (79 percent), Haber-
landt (73 percent), Elton (64 percent), and Feldun (63 percent). It
therefore seems clear from these results that varieties of soybeans
differ greatly in relative resistance and susceptibility to this disease.

An important point brought out by Gardner and Kendrick was that
seed from plants apparently free from mosaic produced practically
100 percent healthy seedlings. This was not always the case, however.
For exa/nple, seed from 42 supposedly healthy plants was saved, and
of these, 3 showed mosaic when tested. Probably such variable results
were due to the difficulty of choosing plants absolutely free from the
disease. Also single plants of the same variety seemed to vary con-
siderably in the amount of mosaic in the progeny. For example, in
the progenies of six single plant selections from the Midwest variety
the amount of mosaic varied from 0 to 33 percent. This would seem
to indicate that the variety is a mixture of types with respect to relative

1932} SOYBEAN BREEDING 377

susceptibility, and that selection may be effective in isolating types
from the same variety differing in resistance and susceptibility or in
ability to transmit the disease.

Thus there is considerable evidence that varieties of soybeans differ
greatly in relative resistance and susceptibility to certain diseases. Also
sufficient information is available to show that selection of the variety
or of the plants within the variety is an important factor in disease
control.

YIELD OF SEED

Seed yield is probably the most sought-after character in soybeans
at the present time. There are, of course, other characters that must
be considered important ; for example, ability to stand erect, early
maturity, good seed quality, color of seed, resistance to disease, and
the like. Extensive yield tests are being conducted at all experiment
stations in states where soybeans are an important crop, for the pur-
pose of determining the best yielding varieties. New strains developed
by the plant breeder by selection or hybridization are carried thru a
long series of nursery and field-plot tests, and their ranking with re-
spect to yield determines whether they shall be distributed or not pro-
vided, of course, that no distinctly undesirable characters are present.
Hence breeding for high yield of seed is a major problem in soybean
improvement.

Seed yield is a very complex character. It is the end-result and
sum total of the activities of the plant. Two main forces determine
the amount of seed produced. These are growth conditions (environ-
ment) and heredity. Soil fertility, amount of space per plant, soil type,
and moisture are examples of environmental influences. Heritable
influences are concerned with the internal yield factors of the plants.
They are responsible for yield differences between varieties produced
under identical growth conditions. It is with the heritable factors that
the plant breeder is most concerned.

The complexity of seed yield as a plant character makes it very
difficult to study. The character must be broken down into its com-
ponent parts and each studied separately as well as in combination with
each other. Even with this simplification the problem is difficult be-
cause the component parts or attributes are also complex and do not
lend themselves readily to genetic analysis. This is because they are
quantitative in nature with a complicated mode of inheritance and are
affected more or less by environmental influences. The complexity of
each attribute of yield emphasizes the complexity of yield itself, which
is the end result of all attributes working together.

378 BULLETIN No. 384 [November,

The components or attributes which are thought to determine yield
of seed in soybeans are as follows: number of nodes per plant, number
of pods per node, number of seeds per pod, percentage of abortive
seed, and size or average weight of seed. These attributes were studied
for 26 different varieties at the Illinois Station in 1930. The results
of this study are here presented.

One object of this study was to determine why one variety is a'
better yielder than another. To what yield component or components
is the superiority of variety A due ? Why is variety X at the bottom
of the list? How do our standard varieties compare in the five yield
attributes mentioned? Very little information is available which en-
ables us to answer such questions as these. An attempt was therefore
made to evaluate our varieties with respect to these internal yield fac-
tors in order to learn in what things they are superior and in what
inferior and also to learn what varieties to use as parents in crosses
in breeding for yield.

The common method of breeding for yield thru crossing has been
to cross different varieties, grow the hybrids, select from among the
F2's the plants that are apparently the best yielders, then test these in
plant rows or rod rows and finally in drill plots for yield. The weight
of seed produced per row or per plot is the criterion of superiority or
inferiority. The original crosses were made without regard to the
contribution of yield factors that each parent might make to the cross.
This method appears rather haphazard, and founded on too little in-
formation of the parent types to justify the hope of securing improve-
ment commensurate with the time and money expended. It would
appear that if more information were available on the internal yield
factors of our standard varieties, the selection of types to use as
parents would be greatly facilitated and also the probability would be
greater of producing types by hybridization that are superior to those
we now have.

The same observations may be made with respect to selections for
yield from ordinary varieties. The selection is based on appearance,
and the plants selected are tested in plant and rod rows and later in
drill plots, just as the hybrids are. A knowledge of the internal yield
factors and their mode of inheritance should be helpful in making
better selections and in judging their superiority or inferiority by
progeny tests.

As a basis for comparing the 26 varieties of soybeans mentioned
above, an attempt was made to secure a planting arrangement that
would equalize growth conditions for all varieties. The following plan

193Z\ SOYBEAN BREEDING 379

was adopted. The rows were planted 2 feet 6 inches apart, and the
plants stood 2 inches apart in the row. Two seeds were planted in
each hill, and later the seedlings were thinned to one plant to a hill.
Twelve hills of one variety were planted, then 12 hills of another, and
so on till one planting of all varieties was completed. Then where the
last variety left off, the second replication began and the same order
was observed. This was continued till 15 replications had been com-
pleted. The rows were about 30 feet long, thus accommodating 13
varieties. Two full rows and 2 feet of the third row were thus re-
quired for each replication (starting with 27 varieties, one was dis-
carded owing to poor stand). With this number of replications and
this arrangement of varieties, the conditions of growth were fairly well
equalized for each variety. This particular arrangement meant that
successive replications of each variety were placed diagonally across
the plot. The soil appeared to be quite uniform. Since it was planned
to study individual plants, the plot was small as plots go (30 feet by
75 feet) and its very smallness resulted in lowered variability. There
was practically a perfect stand, so that each plant of each variety can
be considered to have had the same opportunity for development so
far as plant environment was concerned. Finally, the first and twelfth
plants of each replication were considered as border plants and not
used in the study, thus eliminating the influences of end-to-end com-
petition between varieties.

The individual plant was made the basis of yield determinations
and of factors determining yield. Each plant was tagged and num-
bered. The plants in all replications of a particular variety were pulled
when mature, tied together, and hung in a screen shed to dry. As soon
as the plants were dry and before any pods shattered, the nodes were
counted on each plant and the pods were picked off and placed in a
numbered envelop on which was recorded also the number of nodes.
Later the pods were classified and other data were taken and cal-
culated, as will be described later. Because of the time and expense
involved in taking the records on so many plants, only about 100 plants,
or those in replications one to ten inclusive, were used in obtaining the
results reported here.

An important phase of this study was that of evaluating the several
varieties with respect to the measurable yield factors. This is shown
by the means calculated for the following characters: number of
nodes, number of pods per node, number of seeds per pod, percentage
of abortive seed, and average weight of 100 seed. In Table 18, page
384, these means are given and also the probable errors of the means.

380 BULLETIN No. 384 [November,

It is recognized that these data are for one year only, and that
much more importance could be attached to them if they represented
averages of studies extending over a period of three or more years.
Data from another year's study may change the rankings of certain
varieties with respect to one or more yield attributes. The writer's
long experience and observations on soybeans, however, confirm him
in the belief that the conclusions drawn from this study are, in the
main, justified. Moreover, in the general problem of breeding for
yield in soybeans the method of attack is important, and so far as the
writer is aware, the method here described is new and untried for this
crop and may have considerable promise for the future.

The varieties will be discussed further with respect to one character
at a time.

Number of Nodes per Plant

The mean node number for each variety as given in Table 18 is the
mean total nodes for the plant, counting from the ground line to the
tip. In soybeans, pods are usually borne at all the nodes except those
nearest the ground. The particular node up from the ground at which
the first pods are borne varies for different varieties, and for different
plants in the same variety owing to a combination of hereditary and
environmental factors. If plants have plenty of space the first pods
are borne close to the ground, but if the plants are crowded the first
pods are usually borne from one-half to a foot or more above the
ground. This is an important factor in harvesting, for if pods are
borne too close to the ground it is impossible to run the cutter bar low
enough to get them and they are consequently left on the field. The
measured yield is reduced below what is actually produced. Since pods
are very seldom borne at the first few (2 to 4) nodes above the ground,
it is perhaps not correct to include these in the total count, but it
seemed simplest to use the node at the ground line as a starting point.
Furthermore, on some plants branches arise on the main stem below
the lowest pod, and if one counts only from the lowest pod he is
omitting nodes that are more fruitful than if they bore a pod or two
instead of branches. All things considered, it is believed that the total
nodes, counted from the ground line to the tip, including also those on
the branches, is fairest for all plants, and that method of counting was
used in this study.

Comparing the varieties with respect to node number, we find that
there are very significant differences between certain of them. The
hay types Wilson 5, Peking, and Ebony have the highest number of
nodes; and the seed types Illini, A.K. 114, and Manchu have the

1932} SOYBEAN BREEDING 381

lowest number per plant. However, the seed types S.P.I. 54592,
Morse, and W. Virginia 8 were also high, higher than the hay-type
varieties Ilsoy, Virginia, and S.P.I. 65388. It is also interesting to
compare the varieties in variability of number of nodes as indicated
by the value of the probable error. Some varieties, A.K. 114, Illini,
Mandarin, and Mansoy, were quite low (about .33) ; whereas other
varieties, S.P.I. 54592, Wilson 5, Peking, and Ebony, were high (about
1.0). This point is clear when one compares the frequency distribu-
tions of the varieties for this character. In some the plants are
grouped rather close together within a narrow range, while in others
the plants are spread over a wide range. The highly variable varieties
may be a mixture of types with respect to node number. This, how-
ever, can be determined only by a careful study of the progeny of
selected plants within each variety.

Number of Pods per Node

The number of pods per node was calculated for each plant by
dividing the total number of pods by the number of nodes. The plants
of each variety were then classified for this character and the mean
for all plants determined. The results are given in Table 18.

Two main plant characters affect the number of pods per node ;
namely, number of nodes and pod-bearing habit. The first has already
been discussed. Pod-bearing habit is determined largely by the growth
habit of the plant. The two general types of growth in plants are
called determinate and indeterminate. The distinction between these
is concerned mostly with the kind and location of the inflorescence.
Growth may be said to be indeterminate when there is no terminal in-
florescence formed, when flower clusters are formed in the axils of
the leaves from the base to the top of the plant, and when in the same
flower cluster the lower flowers bloom first, followed by the next higher
in regular order. In the determinate type there is a terminal inflores-
cence and the older flowers are at the center or top of the inflorescence
and the younger flowers appear in order toward the outside. Strictly
speaking then, growth habit in all soybeans is indeterminate.

However, many soybean varieties show certain features that us-
ually belong to the determinate habit. While in the flower cluster the
older flowers are at the base and the younger at the top, the stems of
such varieties are terminated by a flower cluster (Fig. 26), and this
of course stops further stem growth in length. In such types the axial
as well as the terminal flower cluster contains many flowers, and the
flower stalk may be and often is an inch or more in length. This

382

BULLETIN No. 384

[November,

results in a plant which is rather short and stocky with comparatively
few nodes but having a proportionately large number of flowers per
node.

In contrast to this there are soybean varieties that more nearly ap-
proach the indeterminate type. They have no terminal inflorescence.
When the stem reaches the height of the determinate-like type just
described, it does not terminate in a flower stalk but continues as a
stem, bearing a leaf at the next node and flower buds between the leaf

FIG. 26. — TERMINAL INFLORESCENCE OF A SOYBEAN PLANT
AS GROWN IN THE GREENHOUSE

This is characteristic of many soybean types. In the field the terminal
.flower stalk or peduncle may bear as many as 20 flowers, but many flowers may
drop off and never develop pods. The determinate type of inflorescence pre-
vents further extension in length of the stem.

and the stem, and thus the stem continues to extend itself. This type
of growth is characteristic of most hay-type soybeans. The stem often
becomes fine and twiney toward the tip, and the internode length usu-
ally increases as the stem lengthens. Thus this type may have more
nodes than the determinate-like type, is often taller, and the flower
stalks are often shorter with fewer flowers, the number diminishing
rapidly toward the tip of the stem, which usually bears a single pod.
Fig. 14 brings out the important features of these two habits of
growth, and Fig. 26 shows the long terminal flower stalk of the
determinate-like type.

1932] SOYBEAN BREEDING 383

The growth habit and hence the flower-bearing habit in soybeans
is worthy of more study than has hitherto been given to it. As stated
above, some types are characterized by long flower stalks or peduncles
at each node. In 1930 counts were made of the number of flowers on
the terminal flower stalk of several plants of each of two strains. The
results were as follows:

Plant No 1 2 3 4 5 6 7 8 9 10 Average

Strain A 15 16 23 17 15 23 19 14 17 16 17.5

Strain B 10 17 10 17 17 24 13 9 14 . . 14.55

Tho no counts were made it was observed that the axial flower
clusters of these plants appeared to be about as long and large as the
terminal ones, and therefore to contain approximately as many flowers.
Other strains have very short flower stalks and sometimes, especially
toward the end of the stem, there is no flower stalk at all but simply
a pedicel bearing a single flower at its tip.

Unfortunately for seed yield, the pod number of a plant seldom or
never equals the flower number; it may be only half as much or even
less, owing to dropping of flowrers and small pods soon after fertiliza-
tion occurs. For this reason it is a common sight to see long flower
stalks, which bore 15 to 20 flowers, bearing only 2 to 5 pods at the
base, all the flowers above having dropped off. Probably the plants
started out to produce much more than they could properly nourish,
and the curtailment was an adjustment to growth conditions. The in-
teresting point is that the potential capacity for production was much
greater than the realization. This is an important problem from the
yield standpoint and merits careful study.

A comparison of soybean varieties with respect to number of pods
per node may be made from results given in Table 18. The averages
are not so high as might be expected from the appearance of the soy-
bean plant itself. This is due to the fact that many nodes at the base
of the plant were included in the total, for reasons explained above,
tho normally they bore no pods at all. Also in many varieties, as
pointed out above, the set of pods per node is rather sparse, especially
toward the tip of the stem. However, there are numerous significant
differences between varieties in this character. Peking, the variety
with the highest average, 1.503, is a type which has large, many-
flowered, many-podded peduncles (determinate-like habit). The
Manchu, on the other hand, with a low average, .84, has small short
flower stalks (indeterminate habit). The pod-bearing habit is prob-
ably the most important factor influencing the number of pods per
node, but the rating of a variety on the basis of this character is

384

BULLETIN No. 384

[November,

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SOYBEAN BREEDING

385

affected also by (1) the height above the ground at which the lowest
pods are borne, and (2) the relative number of flowers dropping off
and therefore not developing into pods.

Number of Seeds per Pod

Soybean pods contain 1 to 4 seeds. The majority of pods produced
by a plant contain 2 or 3 seeds. Whether the 2's are more numerous
than the 3's, or vice versa, depends on the variety (Fig. 27). So much
is this a varietal characteristic that one can generally refer to a variety
as a 2-seeded or as a 3-seeded type, as the case may be. This is clear
from Table 19, which shows the proportions of 1 -seeded, 2-seeded,
3-seeded, and 4-seeded pods for all varieties under study. There can
be no question that seed number per pod is a varietal characteristic
(Fig. 28).

TABLE 19. — COMPARISON OF 26 SOYBEAN VARIETIES IN PERCENTAGE OF 1 -SEEDED,
2-SEEDED, 3-SEEDED, AND 4-SEEDED PODS

Variety name or No.

Total num-
ber pods
classified

1 -seeded
pods

2-seeded
pods

3-seeded
pods

4-seeded
pods

Mandarin

2495

perct.
16.2

perct.
53.3

perct.
30.1

perct.
.2

Brown T30

2805

7.4

37.2

55.0

A.K. 114

2402

5.7

38.5

55.7

Ito San

3476

12.7

64.0

22.9

Illini

2899

1.8

21.5

76.4

1884

4.4

36.7

58.7

Wea

2896

6.2

47.4

46.1

Black Eyebrow

2505

12.0

64.4

23.4

2451

7.9

64.7

27.2

Harbinsoy

2906

12.5

64.2

23.1

Ebony

3970

14.9

54.6

30.2

S.P.I. 65394

2557

2.3

30.8

66.1

Ohio 13-177

2993

6.2

50.2

43.5

West Virginia 8

4003

15.8

52.2

31.9

Peking

6701

8.4

60.4

30.9

Virginia

3362

24.2

49.5

26.2

Ilsoy

4029

12.3

53.8

33.8

S.P.I. 04002 B

2434

2.9

24.8

72.1

S.P.I. 54610-3

2754

2.9

22.7

74.3

S.P.I. 65388

3116

2.4

20.3

74.5

2.5

S.P.I. 54592

2692

3.3

21.9

74.7

2591

8.2

49.9

41.8

Morse

2577

17.0

52.3

30.5

65345

2910

8.3

75.8

15.7

Dunfield

2243

7.5

47.4

44.9

Wilson 5

5038

14.4

52.4

33.1

The 26 varieties examined ranged from 2.00 to 2.77 seeds per pod,
including abortive seed (Table 18). Those that had an average of
approximately 2, or a little above, were predominantly 2-seeded ; those
with an average of 2.65 to 2.80 were predominantly 3-seeded ; and
those ranging from 2.35 to 2.52 had about as many 2's as 3's (Table
19). Thus soybean varieties can be placed roughly in one or another
of those three classes. Illini, S.P.I. 65394, S.P.I. 04002-B, S.P.I.

386

BULLETIN No. 384

[November.

54610-3, S.P.I. 65388, and S.P.I. 54592 stand at the top in number of
seeds per pod, being predominantly 3-seeded types. Ito San, Mandarin,
Black Eyebrow, Mansoy, and others are predominantly 2-seeded types.

FIG. 27. — SOYBEAN PODS ARRANGED ACCORDING TO NUMBER OF SEED CONTAINED

Soybean varieties differ in the proportion of the kinds of pods they bear
with respect to number of seeds. A, C, and E represent pods from one plant of
Illini: (A) 3-seeded pods; (C) 2-seeded pods; and (E) 1-seeded pods. B, D,
and F represent pods from two plants of Ito San; (B) 3-seeded pods; (D)
2-seeded pods; and (F) 1-seeded pods. This is only one of the important com-
ponents influencing yield of seed in the soybean.

Percentage of Abortive Seed

The occurrence of abortive or undeveloped seeds in soybeans
was studied many years ago by Halsted,11' 12* who classified pods of
three varieties, namely, Early Brown, Wilson, and Ito San. Of 4,382
pods containing 9,584 seeds, 1,259 seeds or 13.14 percent were abortive.
The percentage of abortiveness in Early Brown was 15; in Wilson,
6.75 ; and in Ito San, 14.2.

Also in these early studies Halsted gave attention to the relation
of abortiveness to position in the pod. A very interesting fact dis-
covered by Halsted was that the basal seed (that nearest the point of
attachment of the pod to the plant) showed by far the highest propor-
tion of abortiveness. He states, "Twenty-six and two-tenths percent
of 4,382 pods have one or more abortive ovules, and of these 1,149
pods, 71.5 percent have an abortive basal ovule, and 31.9 percent are
with aborts at the tip."

In an extensive study of abortiveness in soybeans conducted re-

79J2]

SOYBEAN BREEDING

387

SEEDED
Poos

FIG. 28. — COMPARISON OF SOYBEAN VARIETIES IN NUMBER OF SEEDS PER POD

Varieties of soybeans differ greatly in the proportion of 1-seeded, 2-seeded,
3-seeded, and 4-seeded pods. In this diagram four varieties are represented.
About 75 percent of the pods borne by the Illini variety were 3-seeded, while
about 75 percent of the pods borne by the strain S.P.I. 65345 were 2-seeded.
Fifty percent of the Virginia pods were 2-seeded, about 25 percent 1-seeded, and
25 percent 3-seeded. The importance of this character in seed yield is obvious.

cently at the Illinois Station (Wood worth55*) the above findings were
in large measure confirmed. In a study of eight different varieties, most
of which were selected strains from the A.K. variety, there was a
range in mean percentage of abortive seeds of 9.4 for A.K. 114 to 22.2
for Illini. Many of these strain differences were large enough to be sig-
nificant. Hence there appeared to be varietal differences in amount
of abortiveness. Also, the author's data on the relation of abortive
seeds to their position in the pod were substantially in agreement with
those of Halsted. With respect to counts on the Illini variety it was
stated that "of 115 two-seeded pods having abortive seeds, 87, or 75.65
percent, are in the basal position ; 16, or 13.9 percent, in the tip posi-
tion, and 12, or 10.45 percent, had both seeds abortive. Similarly, of

388 BULLETIN No. 384 [November,

the 569 three-seeded pods having one seed abortive, 498, or 87.52 per-
cent, are abortive in the basal position; 39, or 6.85 percent, in the
middle position; and 32, or 5.63 percent, in the tip position. Also
certain combinations of positions are more favored for abortiveness
than others. Thus, of 118 three-seeded pods with two abortive seeds,
82, or 69.49 percent, are abortive in base and middle positions ; 26, or
22.03 percent, are abortive in base and tip positions ; and only 10, or
8.48 percent, in the middle and tip positions" (Fig. 29).

There were certain other interesting facts gleaned from this study.
No relation was apparent between the number of seeds per pod and
the amount of abortiveness. That is to say, 2-seeded pods had about
the same percentage of abortion as 1 -seeded or 3-seeded pods. There
was considerable difference in the size or degree of development of
the aborts with reference to their position in the pod. Thus the abort
in the tip of the pod was found to be the least developed of all, both
in 2-seeded and 3-seeded pods, and the abort in the basal position was
the larger in 2-seeded pods and only slightly below the size of the
middle abort in 3-seeded pods. Comparisons between normally de-
veloped seeds in pods containing one or more abortive seeds and in
pods containing no abortive seeds revealed the fact that the latter class
of pods contained the heavier seeds. Finally a relation was found
between the amount of abortiveness and position on the plant. Hal-
sted11* found that the percentages of abortive seeds increased from
the lower to the upper parts of the plant. Results at this Station dif-
fered somewhat from those of Halsted in that the lower part of the
plant showed only slightly less abortiveness than the tip, and the
middle of the plant showed the least amount.

In the present study the mean percentage of abortive seeds was
calculated for each of the 26 varieties in the following manner: After
the pods were classified, and the potential number of seeds determined
for each plant, the pods were threshed and the "good" or normally
developed seeds were counted. The number of good seeds divided by
the potential number and the quotient multiplied by 100 gave the per-
centage of good seeds. This percentage subtracted from 100 gave the
percentage of abortive seed. In Table 18 the mean percentage for each
variety is given, together with the probable error.

In general it can be said that the percentage of abortiveness in soy-
beans is higher than would appear to be the case on casual observation
of the plants as they are approaching maturity in the field. The lowest
mean percentage in Table 18 is 14.94 for the Harbinsoy variety and
the highest is 31.6 for the Mandarin. In the case of the Mandarin,

SOYBEAN BREEDING

389

F 6

FIG. 29. — APPEARANCE OF ABORTIVE SEEDS OF THE SOYBEAN
AND THEIR LOCATION WITHIN THE PODS

(A, B) One-seeded pods: (A) nonabortive, (B) abortive. (C) Abortive
seeds ranging from small shriveled masses to half-grown seeds. (D-G) Two-
seeded pods: (D) nonabortive, (E) basal, (F) tip, abortive, (G) both seeds
abortive. (H-O) Three-seeded pods: (H) nonabortive, (I) basal, (J) middle,
(K) tip, (L) middle and tip, (M) basal and tip, (N) basal and middle, abortive,
(O) all three seeds abortive. (P-V) Four-seeded pods: (P) nonabortive, (Q)
basal, (R) basal and second, (S) basal and tip, (T) basal and third, (U) basal,
second, and third, abortive, (V) all four seeds abortive.

390 BULLETIN No. 384 [November,

therefore, almost a third of the seeds were abortive or undeveloped.
A majority of the varieties, that is, 16 of the 26, were 20 percent or
above. Tho the probable errors are fairly high, many of these means
show differences great enough to be significant. It is believed there-
fore that while growth and seasonal conditions may, and probably do,
affect the amount of abortiveness, the character may be expressed to
different degrees in different varieties owing in part at least to genetic
factors.

Size of Seed

A glance at the seed of different soybean varieties reveals the fact
that they vary in size as well as in shape. Each variety has its own
typical seed size (Fig. 30). Some varieties, such as Black Eyebrow,
are large-seeded and others, such as S.P.I. 65388, Peking, or Wilson 5,
are small-seeded. Growth conditions, such as soil fertility, inoculation,
and the like, modify the seed-size, increasing it if favorable, decreasing
it if unfavorable ; but if these conditions are the same for all varieties,
as they should be if comparisons are to be made, distinct and signifi-
cant differences in seed size will be shown by different varieties.

For purposes of the present study the average weight of 100 seeds
was calculated for each plant of each variety. This was obtained in
the following manner: The seeds of each plant were weighed to tenths
of a gram. The number of seeds per plant was counted and this num-
ber divided into 100. The factor thus obtained was multiplied by the
weight of seeds for the plant to give the weight of 100 seeds. The
plants of each variety were then classified on the basis of this char-
acter, and the mean weight of 100 seeds of each variety, together with
its probable error, was obtained. The results are given in Table 18.

The varieties range in average weight of 100 seeds from about 5
grams for S.P.I. 65388 to a little over 18 grams for Black Eyebrow
and Manchu. Most of them, however, range from about 12 to 18
grams. Strains of soybeans are known which bear seeds much larger
than those of any variety in this experiment, and there are seeds of
wild types of soybeans that are much smaller than the smallest used
in this study.

Seed size is mentioned in numerous published descriptions of soy-
bean varieties and it has even been used as a part of the key in classi-
fying varieties. The relative size of soybean seed is often expressed
as so many in a pound or bushel. Many farmers seem to prefer small-
seeded types because they go farther in seeding and hence the seed-
cost per acre is smaller. The large seed contains more food material,
however, and thus is able to give the seedling a better start than is the
small seed.

1932]

SOYBEAN BREEDING

391

Correlation Between Yield Components and Yield
One would naturally expect that the yield components that have
been discussed would show a direct and important relation to yield of
seed. This would be true of each yield component in turn if all other
yield factors were kept constant. For instance, if number of pods per
node, number of seeds per pod, percentage of abortive seed, and size

FIG. 30. — SOYBEANS OF DIFFERENT SIZES

Seeds of soybean varieties differ greatly in size and hence in weight. The
seeds here pictured represent in general the five groups into which the varieties
used in an analysis of yield could be classified for this character. (1) S.P.I.
65388, 4.94 grams per hundred seeds; (2) Peking, 7.64 grams; (3) West Vir-
ginia, No. 8, 11.47 grams; (4) S.P.I. 65345, 14.91 grams; and (5) Black Eye-
brow, 18.21 grams.

of seed were the same for all varieties and the varieties differed only
in number of nodes, then of course there would be a very close rela-
tionship between yield and number of nodes. But the true situation is
quite different from this. There is involved the problem of the rela-
tionship between the yield components themselves, which will be dis-
cussed later.

That a variety may rank high in one yield component, such as num-
ber of nodes, and may be low, intermediate, or high in one or more of

392 BULLETIN No. 384 [November,

the other yield components, such as number of seeds per pod, is shown
in Fig. 31, which is built from the data in Table 18. The figures at the
bottom of the columns in this graph indicate the following varieties:

1 S.P.I. 54592- 14 Morse

2 Illini 15 Harbinsoy

3 Ohio 13-177 16 West Virginia 8

4 S.P.I. 04002 B 17 Wea

5 Aksarben 18 S.P.I. 65345

6 Ito San 19 Manchu

7 Peking 20 Dunfield

8 A.K. 1 14 21 Ebony

9 Ilsoy 22 Wilson 5

10 S.P.I. 54610-3 23 Brown Type 30

11 Black Eyebrow 24 Virginia

12 S.P.I. 65394 25 Mandarin

13 Mansoy 26 S.P.I. 65388

It is apparent from this study that inferiority in one yield com- ,
ponent may entirely counterbalance superiority in another in the case
of a single variety. S.P.I. 65388, for example, ranked first in number
of seeds per pod but was lowest in yield of seed on a plant basis.
Furthermore the variety that ranked highest in yield, S.P.I. 54592, did
not rank highest in any single yield component; indeed in average
number of pods per node it was one of the lowest. The lower yielding
varieties appear to have higher percentages of abortive seed and
smaller seed, in general, than the higher yielding varieties ; however,
the other yield components appear to show little if any relation to yield.
These facts are also evident from the correlation coefficients.

These relationships will probably be better understood if we bear
in mind that we are concerned here with genetic, not nongenetic, cor-
relations. All the varieties were so planted and so grown that the
growth conditions were very much the same for each. Since this is so,
we can leave the environmental or nongenetic influences out of con-
sideration. If we were to correlate number of nodes, for example,
with yield of seed for plants within the variety, we should no doubt
obtain a correlation coefficient high enough to be significant. This
would be so because both number of nodes and yield are influenced by
the same cause, namely, growth conditions ; consequently the two
characters would tend to vary together. A condition that increases the
number of nodes likewise increases yield, and a condition that de-
creases the number of nodes decreases yield. Now these environ-
mental effects are not inherited and therefore are of no importance to
the plant breeder except as they modify the results obtained. He is
vitally concerned, however, with genetic correlations, that is, with cor-
relations that tend to cause two characters to be inherited together

19321

SOYBEAN BREEDING

393

§51
Z43

o35
037

Z 19
II

5 6 7 8 9 10 II 12 13 H 15 16 17 18 19 20 Zl ZZ Z3 Z4 Z5 26

FIG. 31. — RELATION OF YIELD COMPONENTS TO YIELD OF SEED

In the top diagram 26 different varieties of soybeans are ranked accord-
ing to yield. In the other diagrams each variety maintains the same relative
position. From these figures a good idea of the relation between yield com-
ponents and yield may be obtained.

394 BULLETIN No. 384 [November,

because they are determined by the same genes in the germplasm or
by different genes located near together on the chromosomes, thus pre-
venting random assortment to the gametes.

There are two methods of determining genetic correlations. One
is by the method followed in this study; namely, that of growing and
testing as many different types as possible under comparable growth
conditions, calculating the means for the various characters on a plant
basis, and using these means as separate items in calculating the cor-
relation coefficients. The plant breeder wishes to know how important
these separate yield components are in producing high yield. Do all
types, for example, with a high number of nodes give high yields, and
all types with a low number of nodes give low yields? If this question
can be answered in the affirmative, then the breeder knows that to get
high yields he must have types with a high number of nodes. He
knows what plants to select and what to reject, and he knows what
types to use as parents in crosses and what types not to use for this
purpose. A similar line of reasoning could be used with respect to all
the other yield components studied.

The second method of determining whether genetic correlations
exist is by the method of hybridization. Crosses can be made between
strains in which the characters are expressed to different degrees and
the F2 plants studied and classified with respect to these characters.
For example, S.P.I. 65388 has the highest average number of seeds
per pod, 2.765, and also the lowest yield, 3.304 grams per plant (Table
18). Ito San, on the other hand, has a low number of seeds per pod,
2.113, and yields 8.3 grams a plant, more than twice as much as S.P.I.
65388. These facts in themselves might suggest that there is no genetic
relation between this yield attribute and yield. If now a cross be made
between these varieties, will there be a tendency for these characters
to stay together in the hybrids, thus maintaining the same combinations
as existed in the parents, or will there be segregation and recombina-
tion according to chance? That is to say, by recombination will the
nonparental types having (1) a high number of seeds per pod and
high yield, and (2) a low number of seeds per pod and low yield be
obtained in the F2 generation, in addition to the parental types; and
if so, will they be obtained in proportions which suggest independence
or partial linkage? If these new combinations are obtained, the link-
age, if any exists, cannot be complete; and if they are obtained in
proportions expected on a chance basis, then it can only be concluded
that no linkage at all exists.

Data are not now available which will enable us by a study of

193Z\ SOYBEAN BREEDING 395

hybrids to get at the genetic relationship between these yield com-
ponents and yield tho some hybrid material is on hand which it is
hoped will furnish data for this study in the near future. Hence we
must confine ourselves for the present to the first method ; namely,
that of correlation using the varietal means. Accordingly correlation
coefficients have been calculated, using the means of the varieties as
separate items and the formula commonly employed when the data are
ungrouped. The results are given in Table 20.

TABLE 20. — CORRELATION COEFFICIENTS BETWEEN YIELD COMPONENTS AND YIELD,
AND BETWEEN THE COMPONENTS THEMSELVES

Characters correlated

Pearso
(r)

Number of nodes and plant yield

.019 ±

.132

Number of pods per node and plant yield

.191 ±

.127

Number of seeds per pod and plant yield

.200 ±

.127

Percentage of abortive seed and plant yield

-.521 ±

.096

Average weight of 100 seeds and plant yield

.519 ±

.096

Number of nodes and number of pods per node

-.184 ±

.128

— .193 ±

.127

Number of nodes and percentage of abortive seed

.347 ±

.116

Number of nodes and average weight of 100 seeds

-.592 ±

.086

Number of pods per node and number of seeds per pod

-.101 ±

.131

Number of pods per node and percentage of abortive seed

.159 ±

.128

Number of pods per node and average weight of 100 seeds

-.382 ±

.112

Number of seeds per pod and percentage of abortive seed

-.238 ±

.125

Number of seeds per pod and average weight of 100 seeds

-.047 ±

.103

Percentage of abortive seed and average weight of 100 seeds

-.520 ±

.096

It is recognized that certain limitations obtain in the interpretation
of these coefficients. The numbers are small, there being only 26 va-
rieties and therefore 26 pairs of items to correlate. Statisticians are
wary of correlations calculated on so small a number. One reason is
that when the numbers are small, the items might become grouped by
chance in such a way as to appear to indicate correlation even when
none is to be expected. As the number of items increases, the prob-
ability of occurrence of correlation by chance decreases. If we take
this into account, and apply Fisher's criterion,7* with a level of sig-
nificance of P = .01, the correlations between yield and average weight
of 100 seeds and between yield and percentage of abortive seed are the
only ones high enough to indicate with reasonable certainty that cor-
relation exists. Correlations between the other yield components and
yield are too low to indicate relationship.

Furthermore, even tho most of the correlations were too low to
indicate relationship, it should not be concluded that such yield com-
ponents have nothing to do with yield, but rather that their individual

BULLETIN No. 384

[November,

100

70
u
9 wi

< ou

z „.

6538J

•"""^

NODES Poos SEEDS PERCT WT. YIELD
PEP PER ABORTIVE 100 PER
NODE POD SEEDS SEEDS PLANT

FIG. 32. — COMPARISON OF HIGHEST AND LOWEST YIELDING STRAINS OF SOYBEANS
IN THE YIELD STUDY, WITH RESPECT TO THE COMPONENTS OF YIELD OF SEED

Each yield component for each strain is expressed as a percentage of the
highest value in the test. These two strains were quite similar in all components
except in weight of seed as expressed in grams per hundred seeds. This dia-
gram illustrates the important influence of seed weight on plant yield, other
things being approximately the same.

100

XMAN30Y

\
\

/ .

"-~b5S94

T|~l

0
No

DES PODS SEEDS PCRCT. WT YIELO
PER PER ABORTIVE 100 PEP
NODE Poo SEEDS SEEDS PUW

FIG. 33. — Two VARIETIES OF SOYBEANS WITH THE SAME YIELD PER PLANT BUT
DIFFERING GREATLY IN THE SEVERAL COMPONENTS THAT MAKE UP YIELD

Each yield component for each strain is expressed as a percentage of the
highest value in the test. The superiority of S.P.I. 65394 over Mansoy in num-
ber of pods per node and number of seeds per pod is counterbalanced by
Mansoy's superiority in seed size. Thus the same yield is attained by two dif-
ferent varieties by different routes.

1932}

SOYBEAN BREEDING

397

effects may have been counterbalanced by the influence of others. For
example, S.P.I. 65388 is the highest in average number of seeds per
pod, but it is lowest in plant yield. If we seek for the cause of its
extremely low yield we find it in the very small seed which it produces
(Fig. 32). Numerous other cases may be cited. If each yield com-
ponent could vary in turn while all the other components were held

100

k
\

•-l^Omc

^~^*^J

13-ITT^y

P^— — — -

10
u

5 M)

\
\

/*-
/

— M0023

^•^

\
\

/
/

ui 50

a
u 40
a

30
20

0
Nc

•*

>DES Poos SEEDS PERCT. WT. YIEU
PER PER ABORTIVE 100 PER
NODE Poo SEED SEEDS PLAN

FIG. 34. — Two VARIETIES OF SOYBEANS THAT PRODUCED THE SAME YIELD PER
PLANT BUT ATTAINED THEIR YIELDS IN DIFFERENT WAYS

Each yield component for each strain is expressed as a percentage of the
highest value in the test. This diagram shows the importance of number of
seeds per pod in producing high yield. Thus superiority in one component may
compensate for inferiority in one or more other components. In breeding for
high yield, therefore, a promising method would be to attempt to combine into
one type, by cross-breeding, all components expressed to the highest degree
possible.

constant, yield of seed per plant would vary also and in the same direc-
tion, and hence a close relationship would be shown. But this does not
happen in nature. Other yield components are also varying at the
same time, being expressed to different degrees in different varieties.
Thus it is difficult to get at the relative influence of each component
on the resulting yield — to find which has the strongest influence, which
the least. A variety may be thought of as a biotype exhibiting a certain
characteristic yield capacity which is due to a particular combination
of the various yield attributes. These combinations admit of almost
endless diversity, and probably no two varieties have the same com-
bination tho they may give the same yield (Figs. 33 and 34).

398 BULLETIN No. 384 [November,

Finally, there is evidence for a slight negative relationship be-
tween percentage of abortive seed and actual yield per plant in grams
(r = — .521 ± .096). That is to say, varieties with high percentage of
abortive seed tend to be low yielders of good seed, and varieties with
low percentage of abortive seed tend to be high seed yielders. This
appears reasonable, but there are many exceptions. A high-yielding
plant may have a high percentage of abortion, and a low-yielding plant
a low percentage. That is, high percentage abortion does not always
accompany low yields, and low percentage does not always accompany
high yields. This is evident from a comparison of varieties with re-
spect to these characters in Table 18.

Percentage of abortive seed is an important character in a given
variety from a yield standpoint. This is illustrated by the fact that if,
in a given variety with 30 percent abortion, something could be done
to lower the abortion to 15 percent without affecting the other yield
components at all, the yield would be increased 20 percent.

Correlation Between Yield Components Themselves
In breeding for yield it is desirable not only to be able to evaluate
varieties with respect to the several yield components, but also to know
whether the components themselves are related. To what extent, for
example, do number of nodes and number of seeds per pod go together
in inheritance? Can high number of pods per node be combined with
low percentage of abortive seed?

The present study on yield attributes in soybeans involving many
varieties has demonstrated that no variety ranks first in all yield com-
ponents. A variety that ranks well in one or more attribute may rank
low or medium in others. Manchu soybeans, for example, rank high
in average weight of seed but low in number of pods per node. On
the other hand, Peking soybeans rank high in number of pods per node
and low in seed size or average weight. What are the chances of
isolating from a cross between these two varieties a type that is high
in both characters? If these yield attributes are independent in inheri-
tance, the chances should be good of securing this recombination type
if sufficient numbers of plants are grown. But if the characters are
genetically associated so that they tend to stay combined in the same
way they are in the parents rather than be distributed to the hybrid
plants independently, then the probability is lessened of securing types
carrying the desired combination of these characters.

As in the case of the genetic relationship of yield attributes with
yield, there are two methods of getting at the genetic association of
the yield attributes with each other. One is to calculate the correlation

1932} SOYBEAN BREEDING 399

coefficients between each component and every other component using
the varietal means as separate items. This was done and the results
are given in Table 20.

Without placing too much reliance on these coefficients because of
the small number of items used (only 26), we may be justified in
making the following statements: Varieties with a low number of
nodes tend to have large seed and a low percentage of abortive seed,
and varieties with a high number of nodes tend to bear small seed but
to have a high percentage of abortive seed. High number of pods per
node appears to go with small seed, and small number with large seed.
Finally, varieties with a high percentage of abortive seed tend to bear
small seeds, and those with a low percentage, tend to bear large seeds.
This last-mentioned relationship is of interest in the light of a state-
ment made above in the discussion of the occurrence of abortive seeds
in soybeans. When the average weight of good seeds in pods con-
taining no abortive seeds was compared with the average weight of
good seeds in pods containing one or more abortive seeds, it was found
that the former were the heavier. Thus absence of abortion seemed
to accompany better seed development. Whether there is any connec-
tion between these two instances of relationship, the future will de-
termine.

It does not appear from these coefficients that the yield components
are strongly associated. While there seemed to be certain tendencies,
as was pointed out, yet there were many exceptions in the case of indi-
vidual varieties, and almost any combination of degrees of development
of two components appeared possible. If this is the true situation, it is
just what the plant breeder desires, for it means that there is high
probability that he will be able to build up a type which is superior in
more attributes than any type he now has.

The other method of determining the genetic relationship between
the yield attributes is by crossing. This method is theoretically sound,
but there are serious practical difficulties in its use, such as the con-
fusing effects of growth conditions and the complexity of inheritance
of yield components, which are themselves quantitative in nature.
Data are not now available which can be used for this study, but the
F2 generation of certain crosses will be tested in the near future with
this problem in mind.

SUMMARY

Soybeans are normally self-fertilized. Natural crosses rarely occur,
usually fewer than 1 percent. Hence the same principles of breeding

400 BULLETIN No. 384 [November,

apply to the soybean as to other self-fertilized crops such as wheat,
oats, and barley.

Considerable work has been done on the genetics of the soybean.
Cotyledon color exhibits both maternal and Mendelian types of inheri-
tance. Other seed and plant characters exhibit the Mendelian type
only. Most of the characters so far studied are concerned with dif-
ferences in color or structure, and very few have anything to do with
yield or performance. The soybean has twenty pairs of chromosomes,
and hence presumably, the possibility of twenty groups of independ-
ently inherited characters. To date only three groups of linked genes
are known. Much, therefore, remains to be done, not only in identify-
ing other linkage groups, but also in learning more about the inheri-
tance and relationships of economically valuable characters such as
pod- formation habit, yield of seed, and percentage of oil.

Variations are necessary for improvement. The only practicable
means the plant breeder has of inducing variations is by hybridization.
Treatment with X-rays has induced changes in other organisms, but
these changes are detrimental for the most part. The occurrence of
mutations and natural crosses at various times in the past has fur-
nished the soybean breeder with an abundance of different types for
use in an improvement program.

Mass selection is quite effective in purifying a variety that has be-
come mixed mechanically or thru mutation and natural crossing.
Pedigree selection may be resorted to, particularly for isolating the
better yielding line or lines from a mixed variety. Thus Illini and
Mansoy are pure-line selections from the A.K. and Manchu varieties
respectively. Selection within pure lines of soybeans is ineffective.
The progeny of artificial or natural crosses may be bulked in the F2
generation and carried along as an ordinary variety, with fair assur-
ance that after six to ten years the majority of plants selected will be
pure for their characters.

The advantages of hybridization consist in the possibility of com-
bining desirable characters from different varieties into one type, in
obtaining plants expressing a character to a greater or less extent than
it was expressed in either parent (transgressive segregation), and in
securing hybrid vigor which may be utilized for increased production
in the soybean if F2 segregates can be isolated possessing more domi-
nant growth factors in the homozygous condition than were possessed
by either parent. While many crosses have been made in the soybean
primarily for genetic studies, no varieties of commercial importance
have yet been produced at the Illinois Station as a result of hybridiza-

1932] SOYBEAN BREEDING 401

tion. It is believed, however, that this method has promise for the
future.

In breeding for quantity and quality of oil in the soybean, the best
procedure appears to be to analyze adapted varieties and then to isolate
the best line from the best variety. After the best line has been
isolated, further selection is ineffective. Differences in oil content are
believed to be inherited, but attempts to secure types with increased oil
content by crossing lines from the same variety were unsuccessful.
Crosses between varieties differing significantly in oil content may
have more promise, particularly if large numbers of plants can be
analyzed and tested in the F2 and later generations.

Varieties of soybeans differ greatly in relative resistance and sus-
ceptibility to plant diseases.

An attempt was made to analyze yield of seed into its component
parts, that is, number of nodes, number of pods per node, number of
seeds per pod, percentage of abortive seed, and size of seed, and to
evaluate each variety with respect to these components. The general
situation was that any particular variety was found to rank well in one
or more components and low in others. No variety was found to rank
highest in all. Significant negative correlation (r = — .521 ± .096)
was obtained between percentage of abortive seed and yield, and sig-
nificant positive correlation (r = .519 ± .096) between average weight
of 100 seeds and yield. Hence varieties with a low percentage of abor-
tive seed and large seed tend to give the better yield. With respect to
the components themselves the following statistically significant cor-
relations were found:

_ Number of nodes and percentage of abortive seed r = .347± .116

"" Number of nodes and average weight of 100 seeds r = — .592 ± .086

Number of pods per node and average weight of 100 seeds r = —.382 ± .112

Percentage of abortive seed and average weight of 100 seeds r = — .520 ± .096

Hence varieties with small seeds tend to have a high number of nodes,
a high number of pods per node, and a high percentage of abortive
seed ; and varieties with large seed tend to have a low number of
nodes, a low number of pods per node, and a low percentage of abor-
tive seed. Also, varieties with a low number of nodes tend to have a
low percentage of abortive seed, and varieties with a high number of
nodes tend to have a high percentage of abortive seed. These relation-
ships, however, are not so close as to prevent the occurrence of re-
combination types as a result of crosses. Consequently the method of
crossbreeding that has for its object the production of types with all
yield components expressed to a higher degree than in the parents
appears to be a promising method of breeding for increased seed yield
in the soybean.

402 BULLETIN No. 384 [November,

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404 BULLETIN No. 384

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UNIVERSITY OF ILLINOIS URBAN*

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