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PLANT GENETICS 



BY 

JOHN M. COULTER 

Head of the Department of Botany 
in the Untvemty of Chuaga 

AND 

MERLE C. COULTER 

Instructor in Plant -Gtitetu* 

in the University of Chicago 




THE UNIVERSITY OF CHICAGO PRESS 
CHICAGO, ILLINOIS 



COPYRIGHT 1918 BY 
THE UNIVERSITY OF CHICAGO 



All Rights Reserved 



Published July igi8 



Composed and Printed By 

The University of Chicago Press 

Chicago, Illinois, U.S.A. 



PREFACE 

There is developing an increasing demand for courses 
in genetics adapted to the needs of botanical students, 
and teachers of botany have asked repeatedly for a 
suitable text. A number of texts on heredity and evo- 
lution have appeared, but these have contained far too 
little genetics. On the other hand, there are several 
very thorough texts on certain ^phases of genetics, but 
these have been more in the nature of monographs than 
texts. They have focused on too limited a field to give 
the reader any general appreciation of genetics as a 
whole and frequently have been too difficult for the 
uninitiated to appreciate at all. WALTER'S Genetics 
is an admirable elementary text, but is not exactly 
adapted to the needs of the young botanist. All texts, in 
fact, have emphasized animal genetics more strongly 
than the student of botany needs, except certain books 
on plant breeding, which, however, are general in nature 
and give little attention to theoretical genetics. In 
short, the subject has been changing so rapidly that no 
one has ventured to write a general authoritative text. 1 

The group that the present book is intended to serve 
primarily comprises those who intend to make botany 
their profession and who, although not as yet specialists, 

1 After the manuscript of this text had gone to the printer, there 
appeared a book by BABCOCK and CLAUSEN, entitled Genetics in relation 
to agriculture. This is "a general authoritative text/' for it contains a 
treatment of both plant and animal genetics, well suited to a thorough 
year's course in genetics. 



vi Preface 

have had general training in the fundamentals of botany. 
Such students, for example, are commonly found in the 
last undergraduate year or first graduate year of their 
work with no distinct purpose to become geneticists, but 
wishing to be able to appreciate the important current 
work in plant genetics. 

Such a group of young botanists became evident at 
the University of Chicago. In their attack upon current 
botanical literature they frequently encountered papers 
dealing with plant genetics, but through lack of prepa- 
ration were unable to grasp their significance. This 
type of literature seemed too important to be relegated 
entirely to the specialist, and therefore the authors of the 
present text organized a course of lectures to meet the 
need. The purpose of the lectures was not to develop 
professional geneticists, but merely to initiate students 
of botany into the point of view of working geneticists, 
so that they could appreciate an important phase of 
botanical literature. With such a purpose there was 
no attempt to give a complete presentation of modern 
genetics, but rather to introduce the student to genetics 
in the simplest way. As a consequence, for pedagogical 
reasons, certain perplexing facts were omitted, while 
others were slightly adjusted so as to convey the funda- 
mental ideas without confusion. The result of the 
course was so successful as to suggest the desirability of 
putting the lectures into text form for the benefit of 
teachers who wish to profit by this experience. As a 
reference book it is entirely inadequate, much represen- 
tative material having been omitted and only enough 
bibliography given to put the student upon the trail. 
As a textbook also it has two disadvantages: (i) it 



Preface vii 

is avowedly not exact in some of the details; (2) it is 
adapted definitely to young botanists with fairly thor- 
ough elementary training. The excuse for the inexact- 
ness of certain details is the pedagogical necessity. The 
preparation of a text for the students referred to is 
explained by the fact that it represents a very important 
group which has not been provided for. In brief, the 
book is neither a technical presentation of genetics nor a 
general text, but a course of general lectures adapted to 
a special purpose. 

In connection with the laboratory phase of this course 
the authors wish to make grateful acknowledgment to 
E. M. EAST, ALBERT BLAKESLEE, R. A. EMERSON, and 
G. H. SHULL for very useful illustrative material. 

J. M. C. 

M. C. C. 



CONTENTS 



CHAPTER 

I. INTRODUCTION 



II. EARLIER THEORIES OF HEREDITY ..... 4 

III. THE INHERITANCE OF ACQUIRED CHARACTERS . 16 

IV. MENDEL'S LAW .......... 28 

V. NEO-MENDELISM .......... 4 

VI. NEO-MENDELISM (Continued) ...... 52 

VII. NEO-MENDELISM (Continued) ...... 63 

VIII. NEO-MENDELISM (Continued) ...... 76 

IX. NEO-MENDELISM (Concluded) ...... 86 

X. NON-MENDELIAN INHERITANCE ...... 97 

XI. MODIFICATION OF UNIT CHARACTERS .... 104 

XII. PARTHENOGENESIS AND VEGETATIVE APOGAMY . 115 

XIII. INHERITANCE IN GAMETOPHYTES ..... 123 

XIV. SELF-STERILITY ........... 133 

XV. THE ENDOSPERM IN INHERITANCE ..... 144 

XVI. HYBRID VIGOR ........... 158 

XVII. SEX PL TERMINATION ......... 177 

XVIII. THE BEARERS OF HEREDITARY CHARACTERS . .198 

INDEX ................ 209 

ix 



CHAPTER I 

INTRODUCTION 

It should be realized that genetics is a natural out- 
growth from the study of organic evolution. The con- 
ception of evolution began as a speculation, but became 
scientific in connection with the work of LAMARCK and 
DARWTN. In such work the method used was that of 
observation and inference. Facts were observed and 
an explanation was devised that would relate them. 
Resemblances and differences among species were 
noted, and it was inferred that these indicated degrees 
of relationship. It was assumed that closely related 
species must have had a comparatively recent common 
ancestry and that more distantly related species must 
have had a more remote common ancestry. 'Through 
comparisons of structure and of geographical distribution 
systems of phylogcny have been inferred, and an outline 
of the evolution of the plant and animal kingdoms has 
been the result. All of these conclusions are based upon 
comparison and inference. 

This method reached its extreme application in the 
work of DARWIN, whose observations included a range 
of forms and an extent of time unequaled by any pre- 
ceding student of evolution. It may be said that in 
DARWIN'S work the method of comparison and infer- 
ence reached the limit of its possibilities. The students 
of evolution were chiefly concerned with explaining 
the changes that resulted in phylogeny. In other words, 



2 Plant Genetics 

by comparison and inference they related certain forms 
and then tried to explain why these forms had become 
different. For example, LAMARCK'S explanation of the 
changes was that they are the results of "use and dis- 
use "; DARWIN'S explanation was that they are due to 
"natural selection.^ 

It was in 1900 that a new method for the study of 
evolution was announced with the appearance of The 
mutationtheory by DE VRIES. The new method was 
experimentation. Instead of comparing two species 
and inferring that one of them had produced the 
other, species were bred through successive generations, 
under rigid control, and were observed to produce new 
species. The old method inferred that a certain thing 
occurred; the new method saw it actually occurring. 

In developing this experimental method the facts 
of inheritance began to accumulate. Presently the 
facts accumulated sufficiently to be organized into 
theories of inheritance, and the special field of genetics 
was the result. Jn brief lt _the.rcfQre J gengtics is the 
experimental study of inheritance. Aside from the 
interest of genetics itself, its possible applications are 
even more important. For example, it will probably 
eventually explain organic evolution; and, most impor- 
tant of all considerations, it will probably enable us to 
control plant and animal breeding in a way that will 
be of the greatest practical importance in agriculture. 

It should be realized that, in so complex a subject as 
heredity, increasing experimental work must greatly 
increase the range of known facts and make explanations 
increasingly difficult. The facts of inheritance obtained 
from experimental work with a few simple forms 



Introduction 3 

may permit satisfactory conclusions; but when greater 
numbers and more complex forms are included, con- 
clusions become not only difficult but sometimes impos- 
sible. It is necessary, however, to organize facts, 
as they are obtained, into some consistent theory that 
will relate them to one another and that may be the 
basis of further investigation. Progress in any complex 
subject is marked by a series of explanations, each in 
turn proving inadequate as facts multiply, but each in 
turn helping to further progress. It should be under- 
stood, therefore, that in the subsequent chapters the 
theories presented are not to be regarded as final, but 
rather as suggested explanations of the known facts. 
It is certain that new facts will continue to be discovered 
and that explanations will have to be modified to fit 
them; but the present explanations are necessary for co- 
ordinating the facts we have. In other words, a sharp ; 
distinction must be drawn between established facts 
and proposed explanations. The former are permanent, 
the latter are temporary. 



CHAPTER II 
EARLIER THEORIES OF HEREDITY 

It is probable that men have thought of heredity from 
the earliest times, but so far as is known there was 
no formulation of any definite theory of heredity before 
the time of j\RiSTQJtE. Since that time there are 
records of numerous theories of inheritance that may 
well be regarded as wild speculations. They were 
evolved with little or no basis of fact and of course are 
not to be regarded as scientific. Some of these early 
theories are interesting but not profitable for our pur- 
pose. They developed from superstition rather than 
from observation. 

It is evident also that inheritance in man first 
attracted attention; later, animals were taken into 
consideration. The study of inheritance in plants is of 
comparatively recent development, due chiefly to the 
fact that ,sX was not observed in plants until late in the 
seventeenth -century, and heredity was thought of only 
in connection with sex. Even after sex in plants was 
announced by CAMERARIUS JJ^L 1694 tjiere was no general 
belief in the~cHmi until much later. For this reason 
plant inheritance became a subject of observation and 
theory, and the history of plant genetics began long 
after the discovery of sex in plants. 

The earliest distinct theory of heredity which falls 
within the period of modern biology was formulated 
by DARWIN (i). This was published in 1868 in his 

~~~~~ 4 



Earlier Theories of Heredity 5 

book entitled Animals and plants under domestication 
and was called pangenesis. (In a certain sense DARWIN 
apologized for pangenesis, realizing that knowledge had 
not advanced far enough for the construction of any 
adequate theory.) The theory of pangenesis, therefore, 
is to be regarded more as a suggestion than as a formula- 
tion of belief. 

The theory may be summarized as involving twc 
theses which should be kept distinct. The first thesis, 
in DARWIN'S words, is as follows: "The individual 
cells and organs of the whole organism are represented in 
every germ cell and bud by definite material particles. 
These multiply by division, and at cell division pass on 
from the mother cells to the daughter cells." This is the 
essential feature of pangenesis; that is, every germ cell 
(egg or sperm) contains dividing particles .that in cell 
division pass on to every cell produced. This statement 
agrees in general with the facts recognized today under 
a somewhat different terminology, it being only neces- 
sary to call these particle^ chromosomes. This is re- 
markable when it is remembered that the statement was 
made fifty years ago. 

The second thesis is as follows: "All the cells of the 
body .throw off particles at various periods of their 
development. These reach the germ cells and hand 
over to them any characters of the organism which they 
may lack// This is known as the transportation hy- 
pothesis and was an attempt to account for certain facts 
which seemed to indicate^ the ^ inheritance of .acquired 
^characters. It is somewhat surprising to find DARWIN 
explaining heredity on the basis of the inheritance of 
acquired characters, for his theory of the origin of 



6 Plant Genetics 

species by natural selection does not call for the inherit- 
ance of acquired characters. 

The theory of pangenesis proper is quite in keeping 
with the present point of view; but the weakness oi 
the transportation hypothesis was so evident that it 
was soon set aside by biologists. In discarding the 
transportation hypothesis, however, biologists in general 
rejected also the doctrine of pangenesis, a common result 
when a truth is found in combination with the obviously 
false. 

The theory of heredity which was chiefly responsible 
for replacing pangenesis was proposed by WEISMANN^S), 
whose publications appeared during the decade 1880^ 
1890. WEISMANN developed two companion theories: 
one called gerjninal ^c^muity^ which has to do with 



heredity; and the other called germinal selection, 
which is a very imaginative explanation of individual 
variation. Our concern is chiefly with the theory of 
germinal continuity. 

According to DARWIN'S transportation hypothesis, 
any change arising in the organism at any time during 
its life would become- represented by gemmules in the 
reproductive cells. In this way it would be possible for 
acquired characters to be inherited. WEISMANN could 
discover no mechanism in plants or animals which could 
justify such a conception. 

It had been known for some time that germ cells and 
body cells in animals remain separate during their later 
development, but WEISMANN seems to haye been the 
first to point out the significance of this fact. He makes 
Ehe following statement: "The difficulty or the impos- 
sibility of rendering the transmission of acquired char- 



Earlier Theories of Heredity 7 

acters intelligible by an appeal to any known force has 
often been felt, but no one has hitherto cast doubt upon 
the very existence of such a form of heredity." The 
general belief of the time that the inheritance of acquired 
characters was a fact came from two sources: (i) many 
examples of the inheritance of acquired characters 
were being reported; and (2) inheritance of acquired 
characters was thought necessary to explain evolution. 
WEISMANN therefore faced two problems: (i) to explain 
away the reported cases of the inheritance of acquired 
characters; and (2) to provide a theory which would 
make evolution possible without the inheritance of 
acquired characters. 

He took up individually the many reported cases 
of such inheritance and discredited them one after 
another, showing convincingly how they could be 
explained in some other way. He also cited from his 
own experience numerous cases in which the inheritance 
of acquired characters was distinctly absent. As a 
result of his investigation of these cases, he developed 
his theory of germinal continuity, commonly spoken of 
as continuity of the germ plasm, a theory which is in good 
standing today. In the attempt to provide a theory 
which would make evolution possible without the 
inheritance of acquired characters WEISMANN dis- 
tinctly failed. To explain variation, which is the 
basis of evolution, he proposed the theory of germinal 
selection, which is even more imaginative than DAR- 
WIN'S transportation hypothesis. As a consequence 
WEISMAN^S experience was much like that of DARWIN. 
His theory of germinal continuity has fairly well stood 
the test of later investigation and is still current among 



& Plant Genetics 

biologists, while his theory of germinal selection has 
been practically discarded. A brief explanation of these 
two theses is as follows. 

GERMINAL CONTINUITY. A statement of this theory 
in the words of the author is as follows : 

I believe that heredity depends upon the fact that a small 
portion of the effective substance of the germ plasm remains 
unchanged during the development of the egg into an organism, 
and that this part of the germ plasm serves as a foundation 
from which the germ cells of the new organism are produced. 
There is a continuity of germ plasm from one generation to 
another .... hence it follows that the transmission of acquired 
characters is an impossibility, for if the germ plasm is not formed 
anew in each individual, but is derived from that which preceded 
it, its structure, and above all its molecular constitution, cannot 
depend upon the individual in which it happens to occur; but 
such an individual only forms as it were the nutritive soil, at 
the expense of which the germ plasm grows, while the latter 
possessed its characteristic structure from the beginning, namely, 
before the commencement of growth, but the tendencies of 
heredity, of which the germ plasm is the bearer, depend upon its 
molecular structure, and hence only those characters can be 
transmitted through successive generations which have previ- 
ously been inherited, namely, those characters which were poten- 
tially contained in the structure of the germ plasm. It also follows 
that those other characters which have been acquired by the 
influence of special external conditions during the lifetime of the 
parent cannot be transmitted at all. 

This is the theory of germinal continuity and it is in 
general agreement with the results of biological work 
today (see fig. i). 

GERMINAL SELECTION. The purpose of the con- 
ception of germinal selection was to construct a theory 
of variation, and therefore of evolution, which does not 
involve the inheritance of acquired characters. When 




FIG. i. Diagram illustrating WEISM ANN'S theory of germinal 
continuity. Three generations are represented, with cells of germ 
plasm shaded, and those of body plasm unshaded; germ plasm con- 
tinuous from generation to generation, carried over from parent to 
offspring by zygote (Z) ; impossible for body plasm to perpetuate itself 
into a second generation. 



io Plant Genetics 

the theory was being constructed the theory of natural 
selection was widely accepted. It should be understood 
that WEISMANN did not criticize natural selection, but 
he did not believe that the individual variations upon 
which natural selection was based were to be explained as 
the inheritance of acquired characters. The theory of 
germinal selection, therefore, was intended to explain the 
origin of individual variations in some other way. The 
explanation of the theory begins as follows : 

I believe that it is possible to suggest that the origin of heredi- 
tary individual characters takes place in a manner quite different 

from any which has as yet been brought forward In the 

first place, it may be argued that external influences may not 
only act on the mature individual, or during its development, 
but that they may also act at a still earlier period upon the germ 
cell from which it arises. It may be imagined that such influ- 
ence of different kinds might produce corresponding minute 
alterations in the molecular structure of the germ plasm; and as 
the latter is, according to our supposition, transmitted from one 
generation to another, it follows that such changes would be 
hereditary Without altogether denying that such influ- 
ences may directly modify the germ cells, I nevertheless believe 
that they have no share in the production of hereditary individual 
characters Hereditary individual differences must there- 
fore be derived from some other source. I believe that such a 
source is to be looked for in the form of reproduction by which the 
great majority of existing organisms are propagated, sexual 

reproduction The object of sexual reproduction is to 

create those individual differences which form the material out of 
which natural selection produces new species. 

WEISMANN forestalls a criticism which was sure to 
come by saying that the opinion has already been 
expressed that deviations from the specific type are 
rapidly destroyed by the operation of sexual reproduc- 



Earlier Theories of Heredity n 

tion. Such an opinion may be true with regard to 
specific characters, because deviations from the specific 
type occur in such rare cases that they cannot hold their 
ground against the large number of normal individuals, 
but the case is different, as he claims, with those minute 
differences which are characteristic of individuals, 
because every individual possesses them, although 
of different kind and degree. He states that cross- 
breeding between all the individuals of a species is 
impossible, hence the obliterating of individual differ- 
ences is impossible. 

According to WEISMANN, therefore, individual varia- 
tions, and therefore the possibility of natural selection, 
originate in the sex process. Sex reproduction originates 
no absolutely new characters, but merely makes possible 
innumerable combinations of characters. It is evident 
that the pairing individuals must differ to some extent, 
if the result is to be a new combination of characters. 

This raises' the question of the origin of differences 
in the first sexual individuals. Sexual reproduction 
was derived from asexual reproduction, and, according 
to the theory, must have begun with variable material. 
How did these individual differences arise ? WEISMANN 
offers the following explanation. He had stated that 
environment has no modifying influence upon the germ 
plasm in general, but says that "in the lowest one- 
celled organisms the case is entirely different/ 7 In 
this case, as he puts it, "parent and offspring are in a 
certain sense one and the same thing/ 7 and germ plasm 
and body plasm have not been differentiated. In this 
case, therefore, variations induced by external conditions 
are hereditary because the germ plasm is the body plasm, 



12 Plant Genetics 

In this way individual differences arose in one-celled 
organisms; and when the sexual method appeared it 
found varying individuals ready to work upon. The 
mission of sex, therefore, according to WEISMANN, was 
to lay hold of these variations and multiply their com- 
binations, thus providing a wider range of choice for 
natural selection. In this way he explains the origin of 
individual variation without the inheritance of acquired 
characters. 

WEISMANN also devised a mechanism by which 
sexual reproduction results in multiplying combinations 
of individual differences. This mechanism is the basis 
of his theory of germinal selection and is as follows. 

In WEISM ANN'S time chromosomes as such had not 
been recognized, but they were represented in his termi- 
nology by the term idants. He conceived of each idant 
as composed of one or more units called ids. An id 
stood for a complete individual, each id being able to 
control the complete development of an individual. A 
child received both paternal and maternal ids in equal 
numbers; that is, father and mother were each repre- 
sented by a 50 per cent influence on the offspring; 
each grandparent would be represented by 25 per cent 
of the ids in an individual; and so on, indefinitely. As a 
consequence, all existing individuals must now contain 
as many different kinds of ancestral ids as they are 
capable of containing. Sooner or later, owing to the 
accumulation of ids, "sex reproduction can proceed 
only by reduction in the number of ancestral ids, a 
reduction which is represented in every generation." 

This is WEISMANN'S explanation of the grosser 
mechanism of heredity, but it was only preliminary to 



Earlier Theories of Heredity 13 

the theory of germinal selection. In the first place 
he imagined .that each id is composed of determinants, 
which are ultra-microscopic, each of them representing, 
not the whole body structure as does an id, but all the 
characters that belong to a particular kind of cell. 
Each determinant in turn was conceived of as composed 
of still smaller units called biophores, each of which 
represented a single character of a cell. Biophores 
have the capacity to assimilate food, to grow, and to 
reproduce themselves in the germ plasm. This imagi- 
nary structure of the cell is spoken of as WEISMANN'S 
cell architecture, in which biophores are built into deter- 
minants, determinants into ids, ids into idants. In 
fact, this is a remarkable theoretical analysis of the 
structure of a chromosome. 

Selection is conceived of as occurring in the following 
way. After fertilization the biophores are too numerous 
for all of them to obtain expression, and the result is 
a struggle for existence, a sort of ultra-microscopical 
warfare. As a consequence some biophores perish and 
others survive and are perpetuated. This struggle of the 
biophores for nutrition, and the almost infinite combina- 
tions that might enter into the structure of the resulting 
determinants, would open up a wide field for variation. 
This is the theory of germinal selection, a struggle and 
a survival of fortunate biophores, and it may well 
be regarded as fantastic. It has been discarded, not 
only because it is without foundation in fact, but 
chiefly because it is beyond the reach of experimental 
testing. 

What is called Weismannism had a very wide influ- 
ence for a number of years, and the continuity of the 



14 Plant Genetics 

germ plasm, with its result on the doctrine of the inherit- 
ance of acquired characters, is still current. The first 
result of Weismannism was to overshadow DARWIN'S 
theory of pangenesis, which had unjustly shared the 
deserved fate of its companion theory, the transporta- 
tion hypothesis. DE VRIES (2) was the first to recog- 1 
nize this injustice, and in 1889 published his Intracellular 
pangenesis. Without attempting to defend the trans- 
portation hypothesis, DE VRIES showed the real value 
of the doctrine of pangenesis. He claimed that his 
theory of intracellular pangenesis is little more than a 
restatement of the fundamental ideas of DARWIN'S 
pangenesis, but in fact it contains enough new material 
ty justify a definition. 

The material particles which DE VRIES conceived 
ot as the carriers of heredity he called pangenSj the 
equivalent of DARWIN'S gemmules. He claims that in 
the nucleus every kind of pangen of the individual is 
represented, while the cytoplasm in each cell contains 
only those pangens that become active in the cell. He 
concludes that, with the exception of the pangens con- 
cerned in nuclear activities, such as nuclear division, 
all pangens have to leave the nucleus in order to become 
active. But most of the pangens of almost every kind 
are represented in the nucleus, where they multiply, 
partly for the purpose of nuclear division, partly to pass 
into the cytoplasm to engage in other activities. These 
pangens not only pass out into the adjacent cytoplasm, 
but are also carried by the protoplasmic currents into* 
the various organs of the protoplast, where they multiply 
and become active. All protoplasm consists of such 
pangens derived at different times from the nucleus, 



Earlier Theories of Heredity 15 

together with their descendants. Hence the theory 
is called intracellular pangenesis, as contrasted with 
DARWIN'S body pangenesis, that is, distribution of the 
heredity particles in the cell as contrasted with distri- 
bution through the body as a whole. 

DE VRIES formulated this theory before he had 
conducted the breeding experiments that led to his 
theory of mutation, and afterward he applied the theory 
to his experimental results in breeding. 

1. DARWIN, CHARLES, The variation of animals and plants under 
domestication. London. 1890. 

2. DE VRIES, HUGO, Intracellular pangenesis. Chicago. 1910. 

3. POULTON, SHONLAND, SHIPLEY, WEISMANN, on heredity. 
Clarendon Press. 1891. 



CHAPTER III 
THE INHERITANCE OF ACQUIRED CHARACTERS 

The inheritance of acquired characters has been 
mentioned in its relation to WEISMANN'S theory of 
germinal continuity, but it deserves a somewhat fuller 
consideration. The idea was first developed by LAMARCK 
in connection with his theory of evolution, the so-called 
theory of appetency, or the effect of use and disuse. 
FRANCIS GALTON, in 1875, was one of the first to express 
skepticism in regard to such inheritance, but it was 
WEISMANN who was most influential in combating the 
idea. After WEISMANN'S presentation of the situation, 
biologists were divided into two camps in reference 
to the inheritance of acquired characters: (i) neo- 
Lamarckians, who affirm belief in the inheritance of 
acquired characters, and. (2) neo-Darwinians, who deny 
it. Geneticists and embryologists, however, whose work 
brings them most in contact with the problem, seem to 
be fairly well agreed that acquired characters are not 
inherited. 

Much of the lack of agreement in this controversy 
is due to the definition of an acquired character. It 
should be kept in mind that actual characters are not 
inherited, but only the determiners, which regulate the 
way in which the organism reacts to its environment. 
For example, when it is said that a child inherits its 
father's nose, the statement is not meant to be literally 
true; it is meant that just as there was something in 

16 



Inheritance of Acquired Characters 17 

the body of the father that was responsible for the 
development of a particular type of nose, so there was a 
similar something in the child's body that produced a 
similar result. It is merely a matter of convenience to 
speak of the inheritance of characters. 

WEISMANN defined an acquired character as "any 
somatic modification that does not have its origin in 
the germ plasm." This definition is not always easy 
to apply. Examples of acquired characters in the 
Weismann sense are mutilations, effects of environment, 
results of function (as in the use or disuse of certain 
organs), and many diseases that affect the bodily mech- 
anism. WEISMANN gave three reasons for rejecting the 
belief in the inheritance of such characters: (i) there is 
no known mechanism by which somatic characters may 
be transferred to the germ plasm; (2) the evidence that 
such a transfer does occur is inconclusive and unsatis- 
factory; and (3) the theory of. the continuity of the 
germ plasm is sufficient to account for the facts of 
heredity. This last reason has been discussed, but the 
other two should be considered. 

When WEISMANN says that there is no known mech- 
anism by which somatic characters can be transferred 
to the germ plasm, to him it is equivalent to saying that 
it is hard to see how the water that has gone over the 
dam can return and affect the flow of the water upstream. 
He assumes, of course, that the germ plasm is isolated 
from the somatoplasm very early in the development 
of the fertilized egg into an individual, and that when 
it is isolated it takes no active part in the history of the 
body (see fig. i). The somatoplasm is thus merely a 
carrier of the germ plasm and is unable to affect the 



1 8 Plant Genetics 

character of it any more than a rubber hot-water bag, 
although capable of assuming a variety of shapes, can 
affect the character of the water it contains (WALTER 6). 

Opponents of WEISMANN object to any such view 
of the complete isolation of germ plasm. Studies of 
cell lineage in animals have shown that germ cells are 
set apart very early in the development of the individual, 
and that they certainly are not derived from distinctly 
somatic cells. No such statement, however, can be 
made in reference to plants. Germ cells in plants are 
derived from epidermal or hypodermal cells, which 
previously were distinctly somatic; and furthermore 
germ cells have been induced by experiment to form from 
almost any living tissue which is as distinctly somatic 
as any plant tissue can be. 

Another objection to WEISMANN'S view is that every 
organism is a physiological as well as a morphological 
unity, and that cells completely insulated in such a 
unity would be impossible. Cytologists also have come 
to believe that there are protoplasmic connections 
between adjacent cells in practically all plant tissues, and 
in general physiology tends to confirm this. All of this 
means a growing belief that the somatoplasm can affect 
the germ plasm. 

The reply of the Weismannians is that even though 
somatoplasm can affect germ plasm in this general phys- 
iological way, this is a very different thing from the in- 
inheritance of some definite acquired character. To be 
inherited such a character would have to be exactly re- 
developed in the germ plasm, and the influence referred 
to cannot be so specific as that. This of course is a 
theoretical answer and the question can only be decided 



Inheritance of Acquired Characters 19 

by experimental work. A theoretical rejoinder to this 
answer may be suggested. It is like the voice in a tele- 
phone transmitter, which starts vibrations that make 
the receiver repeat the voice. 

WEISMANN'S disposition of the claimed cases of the 
inheritance of acquired characters should be considered. 
The supposed evidence for such characters falls chiefly 
into four categories. 

i. MUTILATIONS. Most of the evidence under this 
head is in relation to animals. It must be the common 
experience that mutilations are not inherited in man and 
the domesticated animals. A . few quotations from 
WALTER (6) suggest the situation: 

It is fortunate that the sons of warriors do not inherit their 
fathers' honorable scars of battle, else we would now be a race of 

cripples The feet of Chinese women of certain classes 

have for centuries been mutilated into deformity by bandages 
without the mutilation in any way becoming an inherited char- 
acter The progressive degeneration or crippling of the 

little toe in man has been explained as the inheritance of the 
cramping effect of shoes upon generations of shoe-wearers; but 
WIEDERSHEIM has pointed out that Egyptian mummies show the 
same crippling of the little toe, and no ancient Egyptian could be 
accused of wearing shoes, or of having had shoe-wearing ancestors. 

Sheep and horses with docked tails, as well as dogs 
with cropped ears, never produce young having the 
parental deformity. WEISMANN'S experiments with 
mice, later verified by other investigators, give addi- 
tional evidence that mutilations are not inherited. He 
bred mice whose tails had been cut off short at birth, and 
continued this performance through twenty-two genera- 
tions, with absolutely no effect on tail length. 



2o Plant Genetics 

Mutilations in plants have received no serious 
consideration; in fact, no one seems to have con- 
sidered the possibility of such inheritance. Cuttings 
for propagation, for example, are usually trimmed to 
prevent excessive transpiration, but no one ever expects 
to find this mutilation perpetuated, even in the plant 
developed from the cutting, much less in the next 
generation developed from seed. In fact, since we 
have begun to learn of the remarkable powers of regenera- 
tion possessed by plants and animals^ we would not 
expect the inheritance of mutilations. 

2. EFFECTS OF ENVIRONMENT. This has long been 
a topic in botany. Trees deformed by prevailing winds, 
like the willows that line the canals in Belgium and 
Holland, or storm-crippled trees along exposed seacoasts, 
are not known to produce progeny showing these char- 
acters when the adverse environmental conditions are 
removed. ZEDERBAUER, on the other hand, found that 
Capsella, which in the course of many years had gradu- 
ally crept along the roadside up into an alpine habitat 
and there acquired alpine characters, retained these 
characters when transplanted to the lowlands. This has 
been accepted as an authentic instance of the inheritance 
of acquired characters; but it is possible that this con- 
quest of an alpine habitat by Capsella can better be 
explained by the gradual natural selection of just those 
germinal variations that best fitted individuals to cope 
with alpine conditions. This would result in the gradual 
establishment of a strain of germ plasm that would 
produce body structures fitted to alpine conditions. 
In other words, this is just the way in which natural 
selection would develop a new elementary species 



Inheritance of Acquired Characters 21 

from the original type. If such a type were established, 
of course its germ plasm would produce alpine plants, 
even in lowland conditions. They might not survive 
long- and natural selection might eliminate them, but 
their structure would be due, not to the inheritance of 
somatic structures, but to the inheritance of an alpine 
germ plasm. 

If corn is planted in poor soil weak individuals 
result. Seed from these weak individuals, when planted 
in good soil, will develop again somewhat weakened 
individuals, and this suggests the inheritance of acquired 
characters. In fact, however, it is merely the direct 
effect of environment continuing through the second 
generation. The weak individuals in the poor soil 
develop small seeds with low nutritive capacity, and 
plants developed from abnormally small seeds are always 
weak, whether the individual that produced the seed grew 
in poor soil or not. 

In 1909 M AYR (4) wrote a notable work on silvicul- 
ture in which he claimed that only species characters are 
inherited in trees, and that the effects of climate are not 
inherited, and therefore that the source of the seed 
makes no difference. In other words, seeds of Scotch 
pine would always produce Scotch pine progeny, 
no matter at what latitude or altitude the ancestors 
had been growing. According to MAYR, therefore, 
there is no inheritance of acquired characters in 
trees. 

In 1912; however, the United States Committee on 
Breeding Nut and Forest Trees (5) came to the con- 
clusion that the source of seed is of great importance. 
This conclusion was based chiefly upon the testimony 



22 Plant Genetics 

of numerous nurserymen, the only significant experi- 
mental work being that of ENGLER of Zurich. ENGLER 
(3) found that in the seedlings in his nursery growth 
in height distinctly decreased as the altitude or latitude 
from which the seed came increased. He also found 
that seeds from pines which had been crippled by 
growing in poor soil conditions gave rise to crippled 
plants when grown in good soil. In many cases trees 
of the third generation still showed the habit " acquired" 
by their grandparents in different habitats. 

These are striking results, but they may be explained 
in either of the two ways mentioned. ENGLER might 
have been dealing with slightly different strains of 
trees, differing in germinal constitution; or it may have 
been another case of the " false inheritance of acquired 
characters " that was explained in connection with corn. 
Seeds from higher latitudes and altitudes might well have 
been smaller, so that we should have expected smaller 
progeny, even when grown in the lowlands. 

A completely satisfactory investigation of the inherit- 
ance of acquired characters in plants still remains to be 
made. Seed from a single parent from a pure strain 
should be planted in diverse conditions and the various 
responses noted. Seed of the second and later genera- 
tions of each lot should then be planted back in the 
original conditions. If there should appear in this 
generation even slight deviations from the original 
type it would be significant, provided the deviations are 
similar to those shown by the immediate parents in 
each case and characteristic of the conditions under 
which the parents grew. The peculiarities of progeny 
due to light-weight seed should be tested by controls 



Inheritance of Acquired Characters 23 

and eliminated from consideration. Certain parts of 
this investigation have been carried on frequently and 
satisfactorily and the whole investigation has been tried, 
but under poorly controlled conditions. It remains, 
therefore, to conduct the entire investigation under 
proper conditions before one can reach any reliable 
conclusion in reference to the inheritance of acquired 
characters by plants. 

Another possible illustration of the inheritance of 
responses to environmental conditions may be obtained 
from the work of BOLLEY (i) on flax. The resistance of 
a plant to a given disease is regarded as a character 
peculiar to certain strains and transmitted as a very 
definite factor in Mendelian inheritance. BOLLEY claims 
that he can get a resistant strain of flax from almost any 
known variety. According to him the resisting ability 
increases from generation to generation, if the crop 
is constantly subjected to disease attack. He took a 
pure-pedigreed strain of flax which had come originally 
from a single non-resisting seed. This was planted in 
slightly "sick" soil, that is, soil infected with the wilt- 
producing organism. Most of the individuals died, but 
"a few scrubs 7 ' survived. He then planted seeds from 
these in slightly " sicker" soil than before, and thus, 
by gradually working his crop into sicker and sicker soil 
in the later generations, he finally obtained a fully 
resistant strain from the pure non-resistant strain with 
which he started. Such a strain he says will not lose its 
resistance if planted progressively in more infected soils. 

BOLLEY also found that if he added to the soil manure, 
alkalies, etc., known to increase the disease by stimulat- 
ing the fungus, he obtained a very few poorly developed 



24 Plant Genetics 

resisting plants, whose seeds, however, the next year 
developed a race with enormous resistance. He gives 
the following theoretical explanation of his results: 

Either (i) the so-called unit character of resistance was present 
in undeveloped form and becomes stronger from year to year 
under conditions of disease; or (2) there never was any character 
present which is entitled to be called a unit character, but it began 
to develop the first year the parent plant came in contact with the 
disease, and the protoplasmic nature of the ancestors of the plants 
which we now have has been such that they accumulated more 
and more the resisting power from year to year, just as they had 
opportunity to develop resistance against a constantly acting 
factor of disease, which, when too powerful, acts as an eliminating 
factor. 

BOLLEY inclines to the second alternative. This 
general conception seems to explain why homegrown 
seed is regularly more resistant than seed from the 
same variety which has had a vacation away from 
home for several years. It has kept in training like a 
football player. BOLLEY says that if these conclusions 
are correct there are probably no unit characters which 
are not fluctuating and there are no fluctuating characters 
which may not readily be fixed. 

These results are striking enough, but their signifi- 
cance depends entirely upon the purity of the strains 
which were used originally and also upon the preserva- 
tion of purity during the experiment. BOLLEY'S phrase 
" elimination factor/' which he uses repeatedly, suggests 
selection from an impufe strain. If his conception is 
true it could be demonstrated by developing a large 
majority of resistant individuals among the non-resistant 
plants which were first subjected to disease attack, 
rather than merely "a few scrubs." In other words, the 



Inheritance of Acquired Characters 25 

whole result resembles the selection of a few resistant 
individuals from an impure strain. 

3. EFFECT OF USE AND DISUSE.- --WEISMANN dis- 
credited this belief, which was the foundation of LA- 
MARCK'S theory of evolution. He was successful in 
practically all the cases that had been presented for 
animals. In plants, of course, it would be hard to find 
anything exactly analogous to the use and disuse of parts 
in animals. One fact, however, may be mentioned which 
is a common experience of botanists. Functionless 
organs gradually become aborted, becoming mere ves- 
tiges, or even suppressed. A study of the organogeny 
of a flower shows that when a floral member is belated 
in its development it is destined sooner or later not to 
appear at all. The Weismannian explanation of this 
situation would probably be as follows. A given species 
has given a nutritive capacity; the less it draws upon 
its nutritive capital for the development of one organ 
the more it can afford to expend upon the development 
of other organs. When an organ becomes functionless 
it no longer has any survival value; survival is then 
dependent upon the relative development of the other 
organs. Certain variations develop the functionless 
organ less than usual and therefore develop the other 
organs more than usual, and under the new conditions 
these variations will survive and the others be eliminated. 

This is a description of natural selection by which 
functionless organs become more and more aborted and 
vigorous organs survive, but such an explanation is 
rather imaginative. 

4. DISEASE TRANSMISSION. Roughly speaking this 
may be grouped as (i) infection by bacteria or fungi, and 



26 Plant Genetics 

(2) some inherent organic weakness. Since the latter 
condition is chiefly serious only in inviting attacks 
by bacteria or fungi, we are concerned chiefly with 
diseases caused by these pathological forms. Realizing 
this, true inhentan^^disease seems to be an impossi- 
bility, for if the p^asite^enters the germ cell it is prac- 
tically sure to destroy it, and there will be no progeny. 
Tn many cases, however, progeny are born diseased, 
but this is due to reinfection of the young embryo from 
the body of the mother. This is not an inheritance but 
i reinfection. In smuts, for example, there is much 
.reinfection by means of the transmission of spores upon 
seeds. This can in no sense be spoken of as inheritance. 

In one respect, however, one may speak of disease 
inheritance. Breeding experiments have shown that 
predisposition to disease and disease resistance, com- 
monly called susceptibility and immunity, are inherited. 
This means that it is of germinal origin, so that it does 
not involve the inheritance of an acquired character. 
On the other hand, as was stated, BOLLEY maintains 
that disease resistance is built up under the influence of 
disease attack, increasing gradually through the genera- 
tions. This would make it an acquired character and one 
that is inherited. 

These are a few of the examples of so-called acquired 
characters and the claims for and against them. Investi- 
gation is not yet in a position to come to any definite 
conclusion in reference to them. The bulk of available 
evidence, however, seems to be against the Jnheritance 
of acquired characters; but there are a number of 
biological facts that seem difficult to explain in any other 
way, In animals the mechanism may seem to make the 



Inheritance of Acquired Characters 27 

inheritance of acquired characters impossible; but the 
situation in plants is distinctly different. 

In closing, we quote the opinion of EAST (2) : 

My confession of faith is, the environment has been an 
immense factor in organic evolution, but its effects are shown 
either so infrequently or after the elapse of so great a time, that 
for the practical purposes of plant breeding we can neglect it as we 
would neglect an infinitesimal in a calculation. 

1. BOLLEY, H. L., The importance of maintaining a constant 
elimination factor in plant breeding. Ann. Rep. Amer. 
Breeders Assoc. 8:508-514. 1912. 

2. EAST, E. M., The bearing of some general biological facts on 
bud-variation. Amer. Nat. 51:129-143. 1917. 

3. ENGLER, ARNOLD, Influence of source of seed. Jour. Heredity 
5:185-186. 1914. 

4. MAYR, H., Waldbau auf naturgesetzJicher Grundlage. Berlin. 
1909. 

5. SUD WORTH, GEO. B., Report of committee on breeding nut and 
forest trees. Ann. Rep. Amer. Breeders Assoc. 8:515-522. 
1912. 

6. WALTER, HERBERT E., Genetics. New York. 1913. 



CHAPTER IV 
MENDEL'S LAW 

Mendel's law is the basis of all work in genetics and 
should be understood from its original statement to its 
somewhat complex development. Before passing to the 
more complex features it is well to recall the points of 
the original thesis. 

In 1865 GREGOR MENDEL (2) published in the pro- 
ceedings of a local scientific society the result of eight 
years of breeding experiments. The publication was so 
obscure that scientific men in general did not see it, and, 
in addition to this, Darwinism was at that time absorbing 
the attention of biologists. For these two reasons 
MENDEL'S work remained unnoticed, and of course 
unappreciated, until it was discovered in 1900 and 
became the great classic of genetics. Its influence, 
therefore, dates from 1900 rather than from the year of 
its publication. 

The substance of MENDEL'S experiments is as fol- 
lows. Wishing to discover the contributions of each 
parent to the make-up of their progeny he chose for his 
work the simple garden pea, which would breed rapidly, 
and exhibited well marked varieties. To magnify his 
results he secured hybrids by crossing distinctly different 
types of peas, and to avoid confusion he considered 
only one character in each experiment. For example, 
he crossed peas which contrasted in character of height, 
of flower color, and of seeds. In every case he obtained 

28 



MendeVs Law 



29 



the same result, so that a single example will suffice. 
Furthermore, he discovered that it made no difference 
whether the staminate parent was a dwarf and the pistil- 
late tall, or vice versa, and so for all the characters used. 
In other words, what are called reciprocal crosses gave 



the same results. 

The progeny of a tall 
parent and a dwarf parent 
were all tall. This genera- 
tion is known as the first 
hybrid generation or the 
F r generation. When this 
generation was inbred the 
progeny was made up of 
tall and dwarf individuals 
in a ratio of 3:1. This 
generation is known as the 
second hybrid generation, 
or the F 2 generation. The 
dwarf forms of the F 2 
generation subsequently 
bred true, producing only 



T X D 



T 
1 
T 



T 

I 



T T T D T T 1 D 



D 
I 
D 



FIG. 2. Diagram illustrating 
visible results of MENDEL'S experi- 
ments. Cross between tall parent 
(T) and dwarf parent (D) gives 
hybrid progeny, which are all tall; 
hybrid progeny inbred gives 3 : i 
ratio in second hybrid generation; 
inbreeding each of these four indi- 
viduals separately gives for third 
hybrid generation results indicated 



in bottom line. 

dwarfs. Of the tall forms 

one-third bred true and two-thirds split up in just such 
a 3 : i ratio as did their immediate parents of the F x 
generation. This may be expressed diagrammatically 
(fig. 2). 

MENDEL'S explanation of this behavior involved 
three theses which at that time were new to biology. 
These theses must be kept distinct from one another. 

i. INDEPENDENT UNIT CHARACTERS. This means 
that an organism, although representing a morphological 



30 Plant Genetics 

and physiological unity, from the standpoint of heredity 
is a complex of a large number of independent heritable 
units. Thus if one pea plant is tall and another one is 
dwarf the behavior of the hybrid produced from them 
with reference to this character will be the same, no 
matter what other characters the parent plants may 
have had. In other words, the characters are independ- 
ent units, unaffected by other characters or units. The 
character of tallness from a tall plant with wrinkled 
seeds or purple flowers will act just the same as from a 
tall plant with smooth seeds or white flowers. Tallness 
is a unit and its behavior in inheritance is independent 
of all other units. 

2. DOMINANCE.- In the germ plasm there are certain 
determiners of unit characters which dominate during 
the development of the body, causing these characters 
to dominate over others and thus become visible. The 
characters dominated over and thus not allowed to 
express themselves are called recessive characters. These 
recessive characters are present in the germ plasm, 
but cannot express themselves and become 'visible as 
long as the dominant characters are present. When a 
dominant character is absent, however, its recessive alter- 
nate is free to express itself and become visible. 

For example, in the case of tall and dwarf peas, tall- 
ness is a dominant character and dwarfness is its alter- 
native recessive. When a dwarf appears, therefore, 
there is present no dominant tallness to suppress it. In 
the Fx generation all the individuals were tall because, 
although they had all received the recessive character 
of dwarfness from one of the parents, they had received 
the dominant character of tallness from the other parent, 



Mendel's Law 31 

and so dwarf ness did not appear in any of them. Such 
pairs of alternative characters are now commonly called 

I ' L '" ~""" ir "'' lff """ r " -^"^''nr.i.mui.tu ^^^^^ ,.*.,. **.Avi^, WwiMw|M , . 

allelomorphs. Thus tallness and dwarfness are allelo- 
morphs in the pea, one dominant over the other, which 
is therefore recessive. 

3. PURITY OF GAMETES. A gamete can contain 
only one of two alternative characters. For example, 
it may contain the character for tallness or for dwarfness, 
but not both. In other words, allelomorphs cannot be 
represented in the same gamete. If the gamete having 
the character for tallness unites with one having the 
character for dwarfness, the resulting zygote will con- 
tain both, but will produce a tall individual because 
tallness is dominant over dwarfness. When this tall 
hybrid produces gametes, however, one-half of them 
will contain the character for tallness and one-half of 
them the character for dwarfness. Thus the alternative 
characters are " segregated " in gamete formation and 
no gamete will have both characters. 

These three theses, independent unit characters, 
dominance, and purity of gametes (better called segre- 
gation) , make up the theoretical explanation of Mendel's 
law. Independent unit characters was of course a 
necessary conception. It was original with MENDEL, 
and has also been original with other investigators, but 
this conception does not represent the essential fea- 
ture of Mendel's law. The idea of dominance had been 
somewhat vaguely proposed before MENDEL'S time. 
In the old literature on animal breeding one meets 
theories of prepotency p , which were proposed again and 
again before the discovery of MENDEL'S work in 1900. 
In any event MENDEL was the first to formulate definitely 



32 Plant Genetics 

the theory of dominance among unit characters. It 
should be realized also that dominance is not an 
essential feature of MENDEL'S theory. Many cases 
are known in which dominance fails, but in other 
regards the Mendelian inheritance is strictly followed. 

The essential feature of MENDEL'S theory is his 
conception of the [ purity ^ gametes , brought about by 
the segregation of alternative characters. The striking 
fact is that this conception, purely theoretical with 
MENDEL, has since been confirmed by cytology. In the 
mechanism of cell division each chromosome is divided 
into two equal parts and each daughter-cell receives 
one of these parts. It is a reasonable inference that 
chromosomes are bearers of hereditary characters u . In 
the production of gametes the number of chromosomes 
characteristic of the organism is reduced one-half. As 
a consequence each gamete carries only one-half the 
characters of the individual that produced it. An 
application of these statements to an explanation of 
MENDEL'S 3 : i ratio will illustrate the situation. 

For convenience we will assume that the nuclei of 
MENDEL'S peas have four chromosomes each (fig. 3). 
In the case of a tall plant two of the four chromosomes 
carry the character for tallness, that is, something that 
determines the production of the tall character in the 
somatoplasm, which is practically the body builder. 
This unknown something is called by various names 
in the literature of genetics, the commonest one 
being determiner. In our illustration, therefore, two 
of the four chromosomes carry the determiner for tall- 
ness (p. 33). At this point two questions may be 
asked. 



Mendel's Law 



33 



Why do just two of the four chromosomes carry 
the determiner for tallness rather than all of them or 
only one of them? Just here it would be difficult to 
explain why no more than two of the four chromosomes 
are represented as carrying the same determiner. This 
will be explained later. It is easy to answer, however, 
why the determiner is being carried by more than one 
chromosome. When gametes are formed the chromo- 




F, Hybrid 



Dwarf Parent 



Gametes 



FIG. 3. Diagram illustrating behavior of chromosomes in MENDEL'S 
cross of tall and dwarf peas. Large rectangular figures, nuclei of 
zygotes or mature individuals; large circles, gametes; small circles 
within zygotes and gametes, chromosomes; letters on chromosomes, 
determiners (T, tallness; D, dwarfness). 

some number is reduced one-half. Since every gamete 
from a pure tall plant carries the determiner for tallness 
there must have been at least two chromosomes carrying 
the determiner before the gametes were formed. 

2. Do these two chromosomes carry any other deter- 
miner than that for tallness? In a tentative way this 
question may be answered in the affirmative, but a 
fuller discussion of the situation must be deferred. 
There is much experimental evidence that indicates 
that more than one determiner is carried on a single 



34 Plant Genetics 

chromosome. In some cases also there are more Men- 
delian determiners than there are chromosomes. 

The situation is represented in fig. 3. This shows 
a somatic cell with the diploid or 2x number of chromo- 
somes. In the formation of gametes this number is 
reduced to the haploid, or x number, which in this case 
is two. The diagram shows that the reduction separates 
(segregates) the two chromosomes carrying the char- 
acter for tallness, so that each gamete contains one. 
This occurs for the other characters as well as for that of 
tallness. From the tall plant, therefore, all the gametes 
will contain the character for tallness, and from a dwarf 
plant all of the gametes would contain the character for 
dwarfness. When these two individuals are crossed the 
zygote will contain both characters, and these two char- 
acters will be transmitted together in the succeeding 
cell generations, "pie individual from such a zygote of 
course would be-all, but at the same time it would be 
carrying a recessive determiner for dwarfness, and this 
fact would be shown by its behavior in breeding. The 
result of inbreeding such hybrids is indicated in the ac- 
companying diagram (fig. 4), which represents the chance 
matings of two kinds of gametes. The obvious results 
are three tall individuals and one dwarf. This is the 
so-called monohybrid ratio, which means the ratio when 
a single pair of allelomorphs is considered. 

Before discussing the further development of Men- 
del's law it will be necessary to explain some of the 
terminology of genetics. When each gamete carries 
the same kind of determiner the zygote is said to 
receive a double dose; when a zygote receives only a 
single such determiner it is said to receive a single dose. 



Mendel's Law 



35 



In fig. 4 one zygote receives a double dose of tallness 
and two others a single dose. These phrases are more 
or less common in the literature of the subject, but the 
more frequent terminology is as follows. When two 
similar gametes unite to form a zygote it is called a homo- 
zygote; when the two pairing gametes are different the 
zygote is called a heterozygotc. Using this terminology 




FIG. 4. Diagram illustrating behavior of first hybrid generation 
(Fi) when inbred. Illustrates meaning of " segregation" and " purity of 
gametes," and how chance matings of Fi gametes result in 3:1 ratio 
in F 2 generation; dwarf individual produced only by zygote in lower 
right-hand comer. 

it is evident that the 3 : i ratio of the F 2 generation is 
really a 1:2:1 ratio, as follows: i homozygote for the 
dominant character, 2 heterozygotes, and i homozygote 
for the recessive character. The 1:2:1 ratio therefore 
is the significant one and appears as a 3 : i ratio only 
because of dominance. 

In the experiment represented in fig. 4 three tall 
individuals appear in the F 2 generation. Superficially 
the individuals look alike, but it is realized that i differs 



36 Plant Genetics 

from the other 2 in germinal constitution, for i will 
produce only one kind of gamete, while the other 2 will 
produce two kinds. . To indicate this situation JOHANN- 
SEN (i) has introduced some appropriate terminology. 
Organisms which seem to be alike, regardless of their 
germinal constitution, are said to be phenotypically 
alike, or to belong to the same phenotype. On the other 
hand, organisms having identical germinal constitution 
are said to be genoty pically alike, or to belong to the same 
genotype. From the standpoint of phenotypes only, 
MENDEL'S F 2 generation shows the 3:1 ratio; but if 
genotypes are considered, it shows the 1:2:1 ratio. In 
other words, this group of forms contains two phenotypes 
but three genotypes. 

Referring again to fig. 4 several things may be 
inferred. It can be seen what will happen in the F 3 
generation when the F 2 individuals are inbred. The 
dominant homozygote will produce only dominant 
homozygotes in the F 3 generation and will continue to 
produce them as long as it is inbred. The two hetero- 
zygotes will split up in the F 3 generation in the same 
1:2:1 ratio as did their hybrid parents of the F x genera- 
tion. The recessive homozygote will produce only 
recessive homozygotes as long as it is kept pure by 
being inbred. 

It is interesting to consider what will happen if a 
heterozygote form is crossed with a homozygous reces- 
sive. It should be obvious that one-half of the progeny 
would be pure recessives, while the other half would be 
heterozygotes, that is, there would be a i : i ratio. A 
similar result would be obtained by crossing a hetero- 
zygote with a dominant homozygote, although all the 



Mendel's Law 37 

immediate progeny would show the dominant character. 
The real situation would be revealed, however, when this 
progeny was inbred, for one-half would be homozygous 
(pure breeders) and the other half would be heterozygous 
(hybrid breeders). 

Thus far we have considered only what is called the 
monohybrid ratio, that is, the ratio obtained from one 
pair of contrasting characters, such as tallness and 
dwarf ness. The next step is to consider the dihybrid 
ratio. MENDEL also used contrasting seed characters, 
finding, for example, that smoothness in seeds is domi- 
nant to a wrinkled condition. Introducing this pair of 
contrasting characters into the situation we have been 
considering, the dihybrid ratio will be the result. Cross- 
ing a tall, smooth-seeded individual with a dwarf, 
wrinkled-seeded individual it is evident that all of the Fi 
or first hybrid generation will be tall, smooth-seeded 
individuals, since both of these characters are dominant. 
In the F 2 generation, however, the following ratio will 
appear: 9 tall smooth, 3 dwarf smooth, 3 tall wrinkled, 
i dwarf wrinkled ; which is a 9 : 3 : 3 : i ratio. This is the 
dihybrid ratio, the explanation of which may be .indi- 
cated in fig. 5. The question may be raised why the 
characters for tallness and smoothness are not repre- 
sented on the same chromosome. If they were, the 
result would be a simple monohybrid ratio, except that 
the tall individuals would always be smooth-seeded as 
well, and dwarfs would be always wrinkled-seeded. 
The possibility of one chromosome carrying two differ- 
ent determiners will be considered later, but at present 
we shall assume that these determiners are on different 
chromosomes. 



38 Plant Genetics 

Fig. 5 shows that we are dealing with two homo- 
zygotes, each producing only one kind of gamete, so 
that all the hybrid progeny will be similar, both geno- 




FIG. 5. Diagram illustrating dihybrid ratio. Upper part shows 
how original parents were crossed to give Fj hybrid; lower part shows 
F x hybrid producing four kinds of gametes; chance ma tings among these 
gametes, when F z is inbred, results as indicated in the large set of squares 
and explains the 9:3:3:1 ratio in the F^ generation. 



MendeVs Law 39 

typically and phenotypically, that is, with the same 
germinal constitution and the same appearance. By in- 
breeding these F x individuals, it will be seen that four 
kinds of gametes are involved. Crossing these four kinds 
of gametes the resulting combinations are indicated in 
fig. 5. The result is four phenotypes, as follows: nos. i, 2, 
3> \> 5? 7> 9? I0 > X 3 are tall smooth individuals; n, 12, 
15 are dwarf smooth; nos. 6, 8, 14 are tall wrinkled; 
no. 1 6 is dwarf wrinkled. This is the 9:3:3:1 ratio. 

It will be noticed that nos. i, 6, n, 16 are homo- 
zygotes and therefore will breed true; but the rest are 
heterozygotes, either for one pair of characters or for 
both, and these would split into various types upon 
further breeding. 

The next step is the trihybrid ratio. MENDEL found 
yellow seeds dominant over green seeds, and if this pair 
of characters is included with those used above the tri- 
hybrid result can be observed. The experiment would 
consist in crossing tall, smooth, yellow individuals 
with dwarf, wrinkled, green individuals; and it is 
obvious that the hybrid progeny would all be tall, 
smooth, yellow, since these three characters are domi- 
nant. Inbreeding the hybrids gives the following 
result in the F 2 generation: 27 tall smooth yellow, 9 tall 
smooth green, 9 tall wrinkled yellow, 9 dwarf smooth 
yellow, 3 tall wrinkled green, 3 dwarf smooth green, 
3 dwarf wrinkled yellow, i dwarf wrinkled green. 
The trihybrid ratio therefore is 27:9:9:9:3:3:3:1. 
This involves 64 individuals and 8 phenotypes. 

1. JOHANNSEN, W., Eiemente der exakten Erblichkeitslehre. 
Jena. 1909. 

2. MENDEL, G., Versuche liber Pflanzen-Hybridcn. Verb. Naturf . 
Vereins in Briinn 4: 1865. 



CHAPTER V 
NEO-MENDELISM 

Thus far we have been considering Mendel's law in 
its simple form and have enlarged but little upon 
MENDEL'S original statement. The value of the law is 
apparent. Upon its republication in 1900 it was taken 
up by biologists and numerous breeders set to work to 
test it. As a consequence data for and against it began 
to accumulate. As might be expected, there was much 
apparent evidence against the law, but as geneticists 
developed a better conception of the mechanism the 
contradictory evidence was explained away. Almost 
every type of inheritance has now been explained 
according to Mendel's law. Some of the explanations 
are very complicated and cannot be included in this 
presentation. A few of the more important cases, how- 
ever, will be^ presented. 

(l) PRESENCE AND ABSENCE HYPOTHESIS 

This may be regarded as a new method of Menclelian 
thought. It was first suggested by CORRENS (3), but 
later was worked out in detail by other geneticists, 
especially HURST, BATESON, SHULL, and EAST. It is 
merely a modification of the mechanism involved. For 
example, in the case of a Hybrid obtained by crossing tall 
and dwarf parents the result had been explained as due 
to the fact that one chromosome bears a determiner for 
tallness and the other one of the pair carries the deter- 

40 



Nco-Menddism 41 

miner for dwarfness. In other words, each one of a pair 
of allelomorphs is represented by a determiner, two 
determiners thus being present. Dwarfness in this 
case would be the result of the interaction of that deter- 
miner and its environment during the development of 
the body; and the same for tall ness. When both were 
present, however, the conception of the situation was as 
follows. The determiner for dwarfness, setting up its 
usual series of reactions, early became paralyzed by the 
determiner for tallness or its products. This result 
was called the dominance of the character for tallness. 
It was as if the determiner for tallness completely pre- 
vented the activity of the determiner for dwarfness. 
This conception was apparently borne out by the facts 
and was the explanation of the mechanism generally 
accepted. 

According to the presence and absence hypothesis, 
however, the situation is looked at from an entirely dif- 
ferent point of view. Tallness is the result of a deter- 
miner, but dwarfness is merely the result of the absence 
of the determiner for tallness. The dominant character 
is produced by an inheritable determiner, but the reces- 
sive character appears only when the dominant deter- 
miner is lacking. This conception has some evident 
advantages and may modify the previous Mendelian 
diagram, as shown in fig. 6. This appears to be a simpler 
mechanism to account for the phenomenon called domi- 
nance. In the case of the dwarf form there is a normal 
course of development; in the case of the tall parent or 
hybrid, however, an additional determiner stimulates cell 
growth, or cell division, or both. It is a simpler and 
more useful conception, so long as it fits the facts. Some 



42 Plant Genetics 

investigators, however, claim that it cannot be applied 
to all the situations that have been discovered. 

This hypothesis introduces some additional termi- 
nology suggested by BATESON. In our illustration the 
tall parent has two determiners for tallness and therefore 
BATESON calls it^dupleg, having a double dose. For the 
same reason the F t individuals, having only one deter- 
miner for tallness, he calls simplex. According to the 




F, Hybrid 



Dwarf Parent 



Gametes 



FIG. 6. Diagram showing how the original scheme must be modi- 
fied to satisfy the presence and absence hypothesis. 

same terminology the dwarf parent is nulliplex with 
respect to its character of tallness. 

Additional advantages of the presence and absence 
hypothesis will appear in connection with a considera- 
tion of blgnding inheritance and of cumulative factors 
in inheritance. Attention, however, should be called 
to the fact that those who accept the presence and ab- 
sence hypothesis do not use the form of notation thus far 
used in explaining Mendelian inheritance. Assume that 
T is used to express the determiner for tallness, the same 
letter (/) is used to express its absence. For example, 
instead of using D for dwarfness, t is used for "lack 



Neo-Mendelism 



43 



of tallness" (fig. 7). It is a matter of convenience 
to have a symbol to represent the recessive, the absence 
of something that is present in another individual. 

In summary, the essential difference between the 
presence and absence hypothesis and that of dominant 
and recessive is that in the former case the recessive 




Dwarf Parent Gametes 

FIG. 7. Diagram showing how presence and absence scheme is 
actually used, with small letter representing " absence." 

determiner has no existence at all, while in the latter 
case it exists, but is in a latent condition when associ- 
ated with the dominant. 

m^ BLENDS 

This type of inheritance when first discovered was 
thought to be in direct conflict with Mendel's law. 
It is a case in which dominance seems to fail, for the 
two alternative characters both express themselves and 
ithe result is an average between them. It is easy to 
explain this situation in accordance with the presence and 
absence hypothesis without any violation of Mendel's 
law. 

The classic example of blending inheritance was 
presented by CORRENS (3) in breeding work upon 



44 



Plant Genetics 



Mirabilis Jalapa, the common four-o'clock. CORRENS 
crossed red and white varieties, and all the hybrid 
progeny had rose pink flowers. This was a color blend, 
distinctly intermediate between the colors of the two 
parents. The F x generation, therefore, seemed to con- 
tradict Mendel's law in that one color character was 
not completely dominant over the other. The real situa- 
tion, however, appeared in the F 2 generation obtained by 




Sperms 




P . k (S 



. 



FIG. 8. Diagram illustrating blending inheritance, discovered 
by CORRENS in Mirabilis Jala pa. 

inbreeding individuals of the F! generation which showed 
the blend. By inbreeding the pink hybrids CORRENS, 
obtained the perfect 1:2:1 ratio, that is, i red like one 
grandparent, 2 pink like the hybrid parent, and i white 
like the other grandparent.) Segregation was evidently 
taking place, the only unusual thing being the appear- 
ance of the Fi individuals, and that was explained im- 
mediately as failure of dominance (see fig. 8). 



Neo-Mendelisnt 45 

The question this introduces, therefore, is that of a 
mechanism which could account for such a result. The 
easiest explanation offered is that the red parent was a 
homozygote for redness (double dose) and the hybrid a 
heterozygote (single dose) ; the inference is that a single 
dose produces pink while a double dose produces red. 
A theoretical explanation of this occasional difference 
in the result of double and single doses is as follows. 
Imagine that the body cells of a plant have a certain 
capacity for expressing hereditary characters. In such a 
case, just as a given quantity of solvent can dissolve 
only a given amount of solute, so the body cells can 
express hereditary characters only to a definite limited 
extent. In the four-o'clock a single dose of redness 
may be thought of as half saturating the body cells, 
while a double close completely saturates them. In 
cases showing complete dominance, however, a single 
dose completely saturates the cells and a double dose 
can do nothing more. This analogy assists in visualizing 
on the one hand the necessary mechanism of blends 
(apparent failure of dominance) and on the other hand 
that for cases of complete dominance. 

Another example of simple blending inheritance is 
the case of Adzuki beans, described by BLAKESLEE 
(i). In this bean the mottling of the seed coat is 
dominant to the lack of mottling. In the hybrid 
condition, however, the mottling is lighter than in 
the * pure or homozygous condition. Heterozygous 
plants, therefore, can be easily distinguished from 
homozygous plants, so that the 1:2:1 ratio is evi- 
dent on external inspection rather than the usual 3 : i 
ratio. 



46 Plant Genetics 

These two cases are illustrations of what may be 
called simple blending inheritance. There are other 
cases of blending which are more complex and cannot 
be explained so easily. 

EAST (4), in crossing starchy and sweet corn, obtained 
all starchy in the F x generation, followed by the usual 
3:1 ratio in the F 2 . He says that starchiness is due to 
the presence of a determiner which enables the corn to 
mature starch grains. When this determiner is absent 
starch grains cannot be formed, so that the carbo- 
hydrates of the endosperm are left in the condition of 
sugar, and the result is a sweet corn. This is a simple 
case of presence and absence, usually accompanied by 
complete dominance. Occasionally, however, the cross 
between pure sweet and pure starchy races gave semi- 
starchy progeny. The hybrid therefore was inter- 
mediate in this regard and would seem to represent a 
case of blending inheritance, which of course means 
.incomplete dominance. This case, however, cannot be 
explained easily by the presence and absence hypothesis 
or any other hypothesis. The difficulty is that inherit- 
ance, which usually shows complete dominance, occasion- 
ally gives way to blending -inheritance. The blend in 
the four-o'clock was a constant behavior; in this 
corn case it is the occasional behavior; so that the 
mechanism proposed to explain the four-o'clock blend 
cannot explain this inconsistent behavior of corn. 
The best that can be said, apparently, is that some- 
thing is interfering with the mechanism; and this 
is about all that EAST concludes; he says " some- 
thing is interfering with dominance; it is only 
partial." 



Neo-Mendelism 47 

There are two other types of blending inheritance that 
may be mentioned, the mechanism involved in which will 
be discussed in another connection. 

After the first announcement by CORRENS of simple 
blending inheritance further investigation revealed 
many similar cases; in fact it was soon regarded to be a 
common phenomenon that the F x should be an inter- 
mediate.' At the same time it was fully expected in such 
cases that the hybrid would split in the F 2 generation 
in a 1:2:1 ratio. Practical breeders were advised that 
Mendel's law was invariable; that a hybrid must split; 
and therefore that the only way to preserve a valuable 
hybrid was by vegetative propagation. Many breeders 
realized the value of this advice, but some reported cer- 
tain surprising results. They claimed that they had 
made numerous crosses and that in many cases the 
intermediate F r individuals continued to breed true to 
the intermediate hybrid characters, even when propa- 
gated from seed. Gardeners today continue to tell of 
certain Begonia hybrids that continue to breed true from 
seed in intermediate condition. BURBANK (2) claims 
many such cases, in which hybrid blends breed true to 
seed, as did their parents. All such claims contradicted 
the experience of scientific breeders and seem to destroy 
their theories. Accordingly, the Mendelians sought to 
explain these claims of pure-breeding hybrids and suc- 
ceeded in doing so remarkably soon. There has been 
much discussion of this situation and there is no need to 
go into the details of it. It will be sufficient to mention 
briefly the principal methods by which pure-breeding 
hybrids have been explained as not contradicting 
Mendel's law. 



48 Plant Genetics 

Inthe first place botanists suggested parthenogenesis 
or vegetative apogamy in the F x , which of course would 
mean vegetative propagation and would not involve the 
segregation of characters and recombinations in fertiliza- 
tion. In such a case, of course, the hybrid characters 
would be continued. This is true, but why should the 
FI generation exhibit apogamy any more than any other 
generation ? An ingenious theoretical answer to this 
pertinent question has been suggested by EAST (5) as 
follows. "May not the difficulty of maturing sexual 
cells in a wide cross sometimes cause apogamous develop- 
ment, and therefore a continued propagation of a con- 
stant and uniform race?' 7 This suggestion seems 
reasonable, for it conforms to two other recognized 
phenomena. 

JEFFREY (7) and his followers have shown that in 
wide crosses the resulting hybrid produces more or less 
sterile pollen. An investigation of wild plants has indi- 
cated that much hybridizing occurs in nature, especially 
in certain families, as indicated by the amount of sterile 
pollen, which is sometimes as much as 100 per cent. 
Such plants, which reveal their hybrid character only 
by the presence of sterile pollen, are called by JEFFREY 
" crypthybrids." This fact has also been brought out by 
many experimental breeders. 

A second significant fact in this connection was 
announced by EAST, who says " apogamy is evidently 
induced by the extraordinary irritation of foreign pol- 
len." The mechanism involved should be considered. 
A study of the development of the ovule and female 
gametophyte shows that stages are passed through before 
pollination, bringing the ovule to a certain stage of 



Neo-Mendelism 49 

maturity. At that point, however, it was claimed that 
development would stop were it not for the stimulus of 
pollination and pollen tube development. This stimulus 
apparently causes the female gametophyte to continue 
its development to the final stage of preparation for 
the reception of the sperm. Sterile pollen might furnish 
this stimulus as well as fertile. If then pollination by 
sterile pollen has taken place and the female gametophyte 
has been stimulated to its complete ante-fertilization 
development, this development may continue in spite of 
the failure of an effective sperm to arrive, and the result 
might be an apogamous embryo. In some such way the 
relation of apogamy to sterile pollen, resulting from 
wide crosses, might be explained. 

This conception meets with many objections, so many 
that they cannot all be discussed. The chief difficulty, 
however, may be stated as follows. Is there any mecha- 
nism for the transmission of a pollen stimulus to the 
developing female gametophyte ? We know of none, 
but neither can we explain the mechanism of the sensitive 
plant in which there is obviously the transmission of a 
stimulus. Furthermore we know from facts in connec- 
tion with parthenocarpy that pollination stimulates 
development as deep within the tissue as the ovule 
itself, and it would be easy to extend this to the develop- 
ment of the female gametophyte as well. 

All this is still a field of speculation, but in any event 
the reason for EAST'S contention that wide crosseg 
may give hybrids that reproduce apogamously is evi- 
dent. Wide crosses give sterile pollen; sterile pollen 
stimulates the development of the female gametophyte 
but cannot effect fertilization; the result is apogamy 



50 Plant Genetics 

so that the hybrid breeds true to its intermediate 
character. 

Another explanation of pure-breeding hybrids devel- 
ops from the mathematical possibilities of chance mating 
when the character in question is due to numerous 
separate factors. Without going into the details it 
may be said that in such a case the conclusion is reached 
that the intermediate hybrids must of mathematical 
necessity go on producing intermediates with but a 
remote chance of giving back either of the extreme parent 
types. 

In these various ways the Mendelians explain away 
pure-breeding hybrids. In doing so they discard a 
great deal of recorded data which seem to them un- 
substantiated. A definite statement of the situation may 
be obtained from a paper by EAST (5), published in 1910. 

It is believed by many that there are kinds of inheritances 
other than Mendclian; that is, inheritance in which no segrega- 
tion occurs. Far be it from me to deny this. / simply slate 
the fact that there are no exact data extant proving other kinds of 
inheritance. Such data may be found, but it is useless to specu- 
late upon other laws without such evidence. There are several 
cases in which either new characters that breed true or blended char- 
acters that breed true appear to have been formed, but they have 
not been studied with sufficient care for analysis of their mode of 
inheritance to be accurate and final. 

In 1913 EMERSON and EAST (6), in a joint paper, 
stated that there are on record only two indisputable 
cases of non-Mendelian characters. These will be 
considered later in their proper connection. Neither of 
them, however, is a case of pure-breeding intermediate 
hybrids. 



Neo-Mendelism 5 1 

Before leaving the general topic of blending inherit- 
ance a statement should be made concerning "garticu- 
late inheritance/' which means that in certain crosses 
something happens causing unblended fragments of 
both parental characters to appear in the hybrid progeny, 
resulting in that patchwork of parental characters called 
a " mosaic." This is not true blending inheritance, but 
has been named by GALTON "participate inheritance." 
A common illustration is that of a variegated Amaran- 
thus, whose leaves show an irregular mosaic of white and 
green. This is produced by crossing pure white and 
pure green strains of Amaranthus. 

1. BLAKESLEE, A. F., and AVERY, B. T., Adzuki beans and Jimson 
weeds. Jour. Heredity 8:125-131. Jigs. 5. 1917. 

2. BURBANK, LUTHER, Another mode of species forming. Ann. 
Report Amer. Breeders Assoc. 5:40-43. 1909. 

3. CORRENS, C., Die neuen Vererbungsgesetze. Berlin. 1912. 

4. COULTER, CASTLE, DAVENPORT, EAST, and TOWER, Heredity 
and Eugenics. Chicago. 1912. 

5. EAST, E. M., The role of hybridization in plant breeding. Pop. 
Sci. Monthly 77^342-355- figs. 12. 1910. 

6. EMERSON, R. A., and EAST, E. M., Inheritance of quantitative 
characters in maize. Bull. Agric. Exper. Sta. Nebr. no. 2. 
pp. 120. figs. 21. 1913. 

7. JEFFREY, E. C., Spore conditions in hybrids and the mutation 
hypothesis of DE VRIES. Bot. Gaz. 58:322-336. pis. 22-25. 
1914. 



CHAPTER VI 

NEO-MENDELISM (Continued) 
(uj) THE FACTOR HYPOTHESIS 

MENDEL concluded that each plant character de- 
pends upon a single determiner. Inheritance, however, 
has proved to be a much more complex phenomenon 
than indicated by MENDEL'S peas. Ratios have ap- 
peared that were puzzling, and geneticists were forced 
to the conclusion that there may be a compound 
cleterminer for a single character. This conception is 
called the factor hypothesis, and the growing complexity 
of genetics has developed in connection with this 
hypothesis. With the consideration of factors instead 
of determiners one passes from elementary to advanced 
genetics. Previously we have used the word determiner, 
implying MENDEL'S idea that a single determiner is 
responsible for the development of a plant character, and 
this has been true of the examples of inheritance previ- 
ously considered. It is understood now, however, 
that a character is frequently determined by the inter- 
action of two or more separately heritable factors, and 
hence the factor hypothesis. The distinction between 
factors and determiners should be made clear. In case 
only one factor is involved in determining a character, 
there is no distinction between factor and determiner; 
and in such a case the term factor should not be used. 

'(D COMPLEMENTARY FACTORS. This is the simplest 
expression of the factor hypothesis and it may be 
illustrated by some of EAST'S work. Crossing red- 

5 2 



Neo-Mendelism 53 

grained and white-grained corn he obtained all red in 
the F| generation. This would suggest that the F 2 
generation would show 3 red to i white; but it showed 
9 reds to 7 whites, which did not suggest Mendelian 
inheritance. It is in accord with Mendel's law, how- 
ever, if we consider that two complementary factors are 
necessary to produce the red character, and that each of 
these factors is inherited separately. Such a situation 
would give a dihybrid ratio, as indicated in fig. 9. It 
will be seen that out of 16 progeny 9 will be red, for they 
alone contain the complementary factors; the other 7 will 
be white. The situation is thus explained by the dihybrid 
ratio, but although only one character is involved that 
character depends upon two complementary factors. 

Another situation is worth noting. No. 6 of the 
diagram is white because it contains only one of the 
necessary factors; no. n is white for the same reason, 
but its germinal constitution is just the opposite. What 
would happen if these two are crossed ? There is only 
one possibility, since each is a homozygotc producing only 
one kind of gamete. The result would be red, and 
thus a cross between two whites would produce only 
reds. What would happen from crossing nos. 6 and 15, 
the former being a homozygote and the latter a hetero- 
zygote ? It is obvious that the resulting progeny would 
be one-half white and one-half red, although both parents 
are white. The same result would be secured in cross- 
ing nos. ii and 14. A cross between nos. 14 and 15, 
both of which are heterozygotes, would result in 3 
whites and i red, the ordinary 3:1 ratio. These illus- 
trations show how differently the same phenotype may 
behave in inheritance. In each case two whites were 



54 



Plant Genetics 



crossed, that is, the same phenotypes, but three different 
ratios were obtained because the genotypes were different. 




FIG. 9. Diagram illustrating behavior of complementary factors 
in cross between red-grained and white-grained corn. R and C must 
both be present to produce red-grained corn. 

The striking feature of this situation is that one can 
cross two whites and get a red. This gives an insight 
into the so-called phenomenon of reversion. For ex- 



Neo-Mendelism 55 

ample, in the course of numerous breeding experiments 
BATESON (i) obtained two strains of white sweet peas, 
each of which when normally "selfed" bred true to the 
white color; but when these two were artificially crossed 
all the progeny had purple flowers, like the wild Sicilian 
ancestors of all cultivated varieties of the sweet pea. 
This appeared to be a typical case of reversion. Further 
breeding, however, showed that this was just such a 
case of complementary factors as we have been con- 
sidering. One of BATESON'S white strains had one of 
the factors for purple and the other strain had the other 
factor. 

Complementary factors have been defined and the 
method of their inheritance described, but is there 
any mechanism to explain the situation ? A suggestion 
may be obtained from plant chemistry (2). The most 
prominent group of pigments in plants is the group of 
anthocyanins, which are produced as follows. Plants 
contain compounds called chromogens, which are color- 
less themselves but which produce pigments when acted 
upon by certain oxidizing enzymes or oxidases. This is 
a sufficient mechanism for the behavior of comple- 
mentary factors. If one of EAST'S white strains of corn 
contained a chromogen capable of producing red but 
lacked the necessary oxidase it would remain colorless. 
If the other white strain contained the oxidase but no 
chromogen it would remain colorless. In crossing them, 
however, chromogen and oxidase would be brought 
together and a red-grained hybrid would be the result. 
Inbreeding such red-grained individuals of course would 
give red and white progeny in a ratio of 9; 7, as explained 
in connection with EAST'S corn. This seems to be the 



56 Plant Genetics 

explanation of the behavior of complementary factors 
in many cases of color inheritance. 

Where other characters are involved the mechanism 
must be somewhat different. In some cases the two fac- 
tors may be the enzyme and the compound the enzyme 
attacks, as in the oxidase and chromogen situation just 
described. Qn_the other hand, we might be dealing 
with two chemical compounds that are inert when occur- 
ring separately but active when brought together, 
active in such a way as to produce a distinctly new 
character. Also two active substances might neutralize 
one another when brought together in a hybrid, and the 
failure in their activity might result either in a new char- 
acter or the failure of some parental character to develop. 
Such are some of the possible mechanisms to explain the 
behavior of complementary factors. 

Hybridizing, therefore, is much like mixing chemicals 
in a test tube. We know that very wide crosses cannot 
be made successfully; but the surprising thing is that 
certain very close crosses are constantly unsuccessful, 
even though both parents may cross freely with closely 
related types. We obtain a glimpse of the possibility 
of such apparently inconsistent behavior when we 
consider the chemical possibilities suggested by the 
behavior of complementary factors. 

The origin of complementary factors is an interesting 
field of speculation. Did they originate together or 
separately? A natural inference would be that they 
originated together, for neither would be of any use with- 
out the other. It should be remembered, however, that 
the idea of use as explaining the occurrence of every- 
thing in a plant is being abandoned; one must think 



Neo-Mendelism 57 

rather of a plant as a complex physico-chemical labora- 
tory. No one claims that all chemical reactions are 
useful; they are simply inevitable ; and plant characters 
are the result of chemical reactions and physical necessi- 
ties. Even though we assume the simultaneous origin 
of two complementary factors, they would have to be put 
on separate chromosomes, for the factors are separately 
inherited. 

The other alternative is to suppose that these factors 
originated independently in the history of a plant. In 
this case, of course, the first one to be produced would 
remain functionless until finally its complement came 
into existence. This might be an explanation of what 
are called latent characters. Also they might have not 
only originated independently but in different varieties 
or species. In this case if natural hybridizing should 
bring them together the result would be the appearance 
of a new character, and this may have been a very im- 
portant factor in the origin of species. 

This may serve as an introduction to the factor 
hypothesis, with complementary factors as an illus- 
tation, simply because it is the simplest situation. There 
are many other kinds of factors recognized, but we shall 
not attempt to list all of the proposed types. A simple 
illustration of the better known types is as follows: 

a) A complementary factor is added to a dissimilar 
factor to produce a particular character. 

b) An inhibitory factor prevents the action of some 
other factor. 

c) A supplementary factor is added to a dissimilar 
factor with the result that the character is modified in 
some way. 



58 Plant Genetics 

d) A cumulative factor, when added to another similar 
factor, affects the degree of development of the character. 

Some examples of these types will make them clear, 
those for complementary factors having been given 
previously. 

INHIBITORY FACTORS. Recalling EAST'S experi- 
ment with red-grained corn it will be remembered that 
when both factors for red were present the grain was reel, 
but when either factor was absent the grain was white. 
Later he crossed these strains with a new white strain, 
and the result was surprising (3). The pure red strain 
produced gametes carrying both the red factors, and 
it would be expected that whatever such a gamete 
mated with would result in red progeny; but when this 
pure red was crossed with the new strain of white the 
progeny were all white, although the hybrids certainly 
contained both factors for red. The explanation which 
first occurred to EAST, and which later experiments 
confirmed, was that the new white strain contained 
an inhibitory factor, which prevented the development 
of red even though both the complementary factors for 
red were present. Fig. 10 illustrates the situation and 
shows why all the individuals of the Fj generation are 
white. It is interesting to note further the possibilities 
of white and red in the F 2 generation. They would be 
numerous, since we are dealing with trihybrid ratios 
(see fig. n). This does not exhaust the possibilities, for 
the cases given were homozygotes, each producing a 
single kind of gamete. There remains for consideration 
the heterozygote situation (see fig. 1 2) . 

The possible mechanism of the inhibitory factor 
is as follows. We have assumed that red is produced 



Neo-Mendelism 



59 




Pure Red Parent 





White Parent with 
Red Inhibitor 



F, White 
FIG. 10. Diagram illustrating behavior of inhibitory factor 




White 

FIG. ii. Diagram showing some possible combinations in F 2 
when F x of fig. 10 is inbred. Individual on left end of upper set red- 
grained, because R and C both present and / absent; other individuals 
in upper set white, because lacking C or R or both; individuals in lower 
set with inhibitory factor and therefore white, whatever other combina- 
tions of factors they may contain. 



60 Plant Genetics 

only when the enzyme is present to oxidize the chromo- 
gen. Enzymes are very sensitive; their activities may 
be affected or completely checked by various agents. 
Suppose that 7 of the diagram be such an agent and the 
necessary mechanism is apparent. When / is present 
R is paralyzed, so that it cannot oxidize C. 

@ SUPPLEMENTARY FACTORS. A supplementary fac- 
tor is one that is added to a dissimilar factor, with the 
result that a character is modified in some way. 

In his work upon red-grained races of corn EAST 
found occasionally a few purple grains. His conception 
of the situation is as follows (3) . The 
pure red plant contains two comple- 
mentary factors, one (C) a chromo- 
gen, and the other (R) an enzyme, 
which when brought together pro- 
(i\ (l) duced the red color. The purple 
^-^ vlx grains, however, must be explained 








F IG . I2 by the presence of still another factor 

(P), the resulting situation being 
represented in fig. 13. Of course when C is absent no 
pigment whatsoever can be produced. As a consequence 
we will assume that the presence of C is constant, and 
that P and R are variables. For a similar reason we 
will assume that the absence of I is constant. The 
figure shows three possibilities, from which the following 
conclusions may be drawn: (i) when P and R are both 
present the result is purple grains; (2) red appears only 
in the absence of P; (3) P although present will not 
develop any color in the absence of R. 

This is a typical case of a supplementary factor, that 
is, one which is added to a dissimilar factor, with the 



Neo-Mendelism 



61 



result that the color character is modified. The mech- 
anism of this situation will make clearer the behavior 
of the supplementary factor. If C is the chromogen 
and R the enzyme, what is P? The suggested answer 
can be obtained from plant chemistry. It is found 
that the purple pigment is produced by the same sub- 
stance as the red, but represents a higher state of oxida- 
tion. The conclusion is obvious. C is oxidized by R 
up to a certain point, where red is produced; but if P 






Purple Red WKite 

FIG. 13. Diagram illustrating action of supplementary factor 



is also present it represents an additional enzyme, which 
attacks the red pigment and oxidizes it still further into 
purple. P is incapable of attacking the original chromo- 
gen, but when R carries the attack to a certain point, 
P can function and carry the oxidation further. As 
a consequence P without R gives white grains, while R 
gives red grains only in the absence of P. 

(Q CUMULATIVE FACTORS. These will be considered 
under the next heading, " Inheritance of quantitative 
characters." 

In addition to the four types of factors given, 
the literature of genetics also contains discussions on 



62 Plant Genetics 

intensifying factors, diluting factors, distribution fac- 
tors, etc. These, however, do not introduce any new 
mechanisms. 

1. BATESON, W., Mendel's laws of heredity. Cambridge. 1909. 

2. CZAPEK, P. and M. E., Biochemie der Pflanzen. Jena. 1913. 

3. EAST, E. M., and HAYES, H. K., Inheritance in maize. Conn. 
Agric. Exper. Sta. Bull. no. 167. pp. 142. pis. 25. 1911. 



CHAPTER VII 

NEO-MENDELISM (Continued) 

(jfr INHERITANCE OF QUANTITATIVE CHARACTERS. 
This phase of the factor hypothesis, if true, is of funda- 
mental importance not only to genetics but to general 
biology. It is based upon the conception of cumula- 
tive factors, and as it is presented it will be realized 
that it throws light not only upon numerous breeding 
experiments but also upon variation in general, which 
means evolution also. A cumulative factor was defined 
as one which, when added to another similar factor, 
affects the degree of development of the character. 

It will be recalled that CORRENS crossed reel and 
white strains of Mirabilis and obtained pink hybrids. 
The suggested explanation of this result was that a 
single dose of the red determiner gives pink while a 
double dose gives red. When CORRENS inbred these 
pink hybrids, he obtained the result presented in fig. 8, 
that js, i red, 2 pink, i white. This result is obvious 
and the mechanism is plain. 

With this diagram in mind we shall consider some of 
the experiments of NILSSON-EHLE (2) at the Swedish 
Experiment Station. He crossed two strains of wheat 
with red and white kernels. The Fj individuals had 
light red kernels, which of course suggests a repetition of 
the situation shown by Mirabilis in the experiment of 
CORRENS. The F 2 generation, however, showed a very 
different result. The reds and. whites appeared in the 

63 



64 Plant Genetics 

ratio of 15:1; but in addition to this, among the 15 reds 
there could be distinguished varying degrees of redness. 
NILSSON-EHLE suspected that 15:1 meant a dihybrid 




FIG. 14. Diagram illustrating NILSSON-EHLE'S explanation of 15: i 
ratio obtained in F 2 generation from cross between red-grained and 
white-grained wheat. 

ratio, 1 6 individuals being necessary to give the ratio, 
so that he constructed the tentative scheme shown in 
fig. 14. 



Neo-Mendelism 65 

This shows a regular dihybrid ratio, except that the 
two factors involved are similar. Applying the single 
dose and double dose conception, as used in the case of 
CORREN'S pink Mirabilis, we reach the following con- 
clusions: no. i only has four doses and therefore it only 
is deep red; nos. 2 7 3, 5, 9 have three doses and are some- 
what lighter red; nos. 4, 6, 7, 10, n, 13 have two doses 
and are still lighter red; nos. 8, 12, 14, 15 have one dose 
and are very light reel; while no. 16 alone has no dose 
and is the only pure white. This accounts for the 15: i 
ratio, and the different shades of red. This is entirely 
in accord with the conceptions that have been presented, 
and only two assumptions are necessary: (i) that domi- 
nance is absent, and two doses have twice the effect of 
one; (2) that the independent similar factors are cumu- 
lative in their operation, and are paired with their 
absence in the hybrid. This was NILSSON-EHLE'S 
conception, and of course he tested it by further experi- 
mental work, the results consistently confirming the 
conception. 

Since it is important to fix this conception clearly in 
mind, another type of diagram may represent the facts 
even more clearly. The proportion of individuals 
showing the various degrees of redness in the F 2 is 
graphically recorded in fig. 15, each dot representing 
one dose of the factors in question. 

Continuing these investigations, NILSSON-EHLE next 
discovered a new strain of red-grained wheat, which, 
when crossed with the pure white strain, yielded Fi 
hybrids of intermediate intensity of red as before. 
The F 2 generation, however, showed a different situation. 
Reds and whites were obtained in the proportion of 



66 



Plant Genetics 



63 : i ; the 63 reds as before falling naturally into differ- 
ent groups on the basis of degree of redness. Applying 
the same conception as before NILSSON-EHLE discovered 
that in this case he was dealing with a trihybrid situation. 



Pure Red 



Grades of Pink 



White 



FIG. 15. Another method of visualizing NILSSON-EHLE'S 15:1 
ratio (see fig. 14). 

Without constructing the usual Mendelian diagram, 
which would have to be extensive enough for 64 indi- 
viduals, the situation as it appeared in the F 2 generation 
may be represented by fig. 16. If the graph is sur- 
mounted by a curve we recognize the regular " proba- 
bility curve/' exactly the kind of curve biometrigians 



Neo-Mendelism 



6 7 



While 



Pure Rcd Intermediate Grades 

FIG. 16. Diagram illustrating NILSSON-EHLE'S 63: i ratio 



68 Plant Genetics 

use to represent the fluctuating individuals about a 
specific type. 

This conception of cumulative factors, therefore, 
has far-reaching significance. For a long time biologists 
have recognized individual variation within the species. 
DARWIN depended upon it as the basis of his theory of 
natural selection as the origin of species; in fact, ever 
since DARWIN'S Origin of species, individual variation 
has been fundamental in our conceptions. To account 
for this universally recognized phenomenon, DARWIN 
proposed his transportation hypothesis as a possible 
explanation, which, as will be recalled, did not long 
survive. WEISMANN offered in explanation his germinal 
selection, which was soon discarded because it was beyond 
the possibility of experimental testing. Aside from 
these two attempts to explain individual variation no 
other comprehensive scheme had been presented. Bi- 
ologists had simply recognized the fact of individual 
variation without any conception of the mechanism. 
They knew that individual variation existed but had 
even stopped asking why it existed. 

The importance of this new theory, therefore, is 
obvious. It is an ingenious explanation of the inherit- 
ance of quantitative characters and of the existence of 
individual variations. Furthermore, the theory has 
not been developed through meditation, but has its 
basis in scientific experiments. It is imaginative to a 
certain extent, of course, as is every other valuable 
theory, but unlike most such theories it has a substantial 
foundation, namely, Mendel's law. 

The importance of the possible role of cumulative 
factors in explaining individual variation, which in 



Neo-Mendelism 69 

turn is the basis of evolution, has been emphasized 
because its importance has perhaps not yet been suffi- 
ciently appreciated. It promises to be one of the most 
important theories of biology, which of course must be 
tested by future investigation. 

The doctrine of cumulative factors was further 
developed by EAST (i) in his work with corn. He was 
able to explain some of the ratios obtained by assuming 
three or four separately inherited cumulative factors, 
just as NILSSON-EHLE had done. He obtained other 
ratios, however, which required more independent 
cumulative factors to explain. Some idea of the extent 
of these investigations by EAST and his associates may 
be obtained by noting the list of the plant characters 
whose inheritance they explained on the basis of cumu- 
lative factors: number of rows, length of ear, diameter 
of ear, weight of seed, breadth of seed, height of plant, 
number of stalks per plant, earliness of flowering. In 
all of these cases breeding gave the same characteristic 
results. A cross between extreme parents gave hybrid 
progeny intermediate as to the character in question; 
and in the Y 2 generation the two extremes reappeared, 
along with all gradations of intermediates. 

NILSSON-EHLE had been able to put his F 2 inter- 
mediates into definite classes, corresponding to the 
number of doses of the determiner each had received. 
EAST, however, could not do this with such exactness. 
His results showed all gradations, but he could not 
distinguish any definite groups; that is, gradation was 
continuous and complete. In other words, he could 
not tell with certainty from outward appearance just 
how many doses of a given determiner an individual 



70 Plant Genetics 

contained. His results, therefore, do not seem so clear 
and striking as those of NILSSON-EHLE, but they are by 
no means vague and uncertain. For example, even if he 
could not say definitely that a certain individual had 
exactly three doses, he could always say approximately 
how many doses it had; and the breeding results always 
confirmed the idea of a number of cumulative factors 
at work. For example, a plant with three doses may 
vary with respect to the character in question. It may 
approach the condition of the plant with four doses or 
it may vary toward the two-dose condition. Such 
variation may be explained by outside influences. Any 
classification of the F 2 individuals on the basis of the 
number of closes is more or less obscured by the influ- 
ence of outside factors which are uncontrollable, or at 
least uncontrolled as yet in breeding work. 

EAST has visualized these outside factors and dis- 
cussed them. In order to explain this discussion, 
however, we must recall a feature of genetics which 
has previously been mentioned. Plant variations in 
the largest sense fall under two categories, due to 
(i) differences in gametic constitution, and (2) responses 
to environment. The first category is the basis of all 
Mendelian conceptions, while the second category in- 
cludes such variations -as are usually thought not to be 
inherited, being, acquired characters. This category 
is now commonly called fluctuating variations. 

An illustration will make these two categories clear. 
Assume that a plant with a determiner for tallness 
usually becomes 6 feet, while one without this determiner 
becomes 3 feet. The 6-foot plant, however, grown in 
good soil becomes 6 . 5 feet, while in poor soil it is 5.5 



Neo-Mendelism 71 

feet. In inheritance of course the 6.5- and 5.5-foot 
plants behave exactly alike; the same is true of 6-foot 
plants. It must be evident, therefore, that a classi- 
fication of F 2 individuals on the basis of the number of 
doses might well be slightly obscured. If outside influ- 
ences were lacking, the F 2 situation could be represented 



FIG. 17. Diagram illustrating distribution of phenotype classes 
in an F 2 population from cross involving cumulative factors. Practically 
same diagram as fig. 16, and interpreted in same way; short rectangle 
at left indicates that very few plants of population contain maximum 
number of doses; short rectangle at right indicates that very few plants 
contain minimum number of doses; plants with intermediate number of 
doses most numerous, as indicated by tall rectangle in middle (see also 
%. 1 8). 

by fig. 17, or better, fig. 18; but when outside influences 
are active, it may be represented by fig. 19. It will be 
seen from this last diagram that not all individuals 
belonging to a particular size class may have the same 
number of doses; that is, conditions for a smaller-dosed 
individual may be so much better than those for a larger- 
dosed individual that they may exchange size classes in 



72 Plant Genetics 

the result. In this way the results of germinal consti- 
tution may be somewhat obscured by the varying 
external conditions of growth. 






FIG. 18. Graphic illustration of situation represented in fig. 17. In 
both diagrams it is evident that two plants appearing in same quanti- 
tative class must have same number of doses; this should always occur 
if environmental influences did not obscure the result. 





FIG. 19. Diagram illustrating how environmental influences may 
obscure phenotype classes of F 2 . Overlapping of phenotype classes 
makes possible that two apparently identical plants might actually have 
a different number of doses; diagram also shows that while breeder 
could not recognize whether a plant had two or three doses, he could 
distinguish between plants of two and four doses, etc.; thus intelligent 
selection could be effective. 



Neo-Mendelism 73 

Another factor that may obscure these results is what 
is called physical correlation. For example, a corn 
plant of small size but with the hereditary capacity for 
producing large ears could not fully express this capacity. 
It could not produce as large ears as if it had been a 
large-sized plant. 

Still another factor that might obscure the result 
may be called gametic correlation, which is a conception 
presented by EAST. The idea is that factors which are 
inherited quite independently and which affect different 
plant characters might still be conceived of as having an 
influence upon one another. As EAST puts it, "A gamete 
may be a mosaic of independent factors, but the plant 
(produced from the gametes) will not be a mosaic of all 
the characters and factors produced, for these factors 
act and react upon one another in complex ways during 
their development." 

Such are some of the conditions or factors that tend 
to obscure results in the F 2 generation and give rise to 
ratios hard to interpret. The weaker the influence of 
these factors the more clearly do the phenomena of 
cumulative factors come out. The total result of this 
phase of EAST'S work, in spite of obscuring conditions 
that have arisen, has been to strengthen greatly the 
conception of cumulative factors. A summary of his 
conclusions may be stated as follows: 

When one is dealing with quantitative characters, that is, 
those produced by cumulative factors, he is confronted by exactly 
the same principles of Mendel ian inheritance as have long been 
known to apply to qualitative characters. With quantitative 
characters, however, the problem is more complex, due chiefly to 



74 Plant Genetics 

factors cumulative in their operation; (2) the significance of the 
breeding results is usually somewhat obscured by the natural 
fluctuations due to response to uncontrollable factors in the 
environment. 

Others have investigated the problem of cumulative 
factors, and many of the results are favorable to the con- 
ception; but much more investigation should be made, 
for the conception deals with exceedingly fundamental 
situations. If it is true it is extremely important; if it 
is not true a knowledge of this fact is just as important. 
It is like a fork of the roads in our biological progress; it 
is important to know which is the right road for future 
progress. 

In concluding the general topic of cumulative factors 
a modifying statement should be made as to the mecha- 
nism involved. Heretofore it has been assumed that we 
were dealing with numerous, separately inherited factors, 
absolutely identical in their nature, cumulative in their 
effect. No doubt one might regard with suspicion such 
a seemingly artificial mechanism. Probably it would 
be easier to believe if it were modified in the follow- 
ing manner. Instead of assuming numerous factors, 
identical in function, we may assume that each of these 
factors has its own peculiar function, but that that 
function plays a part, directly or indirectly, in develop- 
ing the quantitative character in question. For example, 
suppose height is the character. One of the factors 
determines the development of long internodes; another 
results in numerous nodes; another increases the amount 
of chlorophyll; another determines the size and vigor 
of the root system; another brings early germination 
and long growing season. Such factors, then, although 



Neo-Mendelism 7 5 

not identical, will be cumulative in increasing the height 
of the plant. Of course a single dose of one type of 
factor will not bring the same increase in height as would 
a single dose of one of the other types, and therefore the 
mathematics of the situation would be slightly modified. 
The fundamental mathematical system, however, would 
remain the same, and we would have the satisfaction of 
dealing with a natural mechanism rather than an 
artificial one. 

1. EMERSON, R. A., and EAST, E. M., The inheritance of quanti- 
tative characters in maize. Bull. Agric. Exper. Sta. Nebr. 
no. 2. pp. 120. figs. 21. 1013. 

2. NILSSON-EHLE, H., Einige Ergebnisse von Kreuzunger bei 
Hafer und Weizen. Bot. Notiser 1908: 257-294. 

, Kreuzungsuntcrsuchungen an Hafer und Weizen. 

Lunds Univ. Arsskr. N.S. II. 5:1-122. 1909. 



CHAPTER VIII 

NEO-MKNDELISM (Continued) 

() TRUE-BREEDING HYBRIDS. This situation was 
referred to in chapter v, but comes up again in the 
present context. The statement was made that a num- 
ber of practical breeders had reported that by crossing 
distinct races they had not only obtained hybrids inter- 
mediate with regard to one or more characters, but also 
that these hybrids when inbred continued to breed true 
to their intermediate condition and that the original 
parent races never reappeared. 

One explanation of this situation offered by geneti- 
cists was tHat where a great many factors are involved 
it is practically a mathematical impossibility for either 
of the parent races to reappear, for the chances would be 
overwhelmingly against the exact coming together of the 
exact combination of factors. This explanation should 
be considered a little more fully in the light of what has 
been stated concerning cumulative factors. 
_ , JSuppose that CORRENS in crossing the red and white 
races of Mirabilis had been dealing, not with one factor 
determining redness, but with six, the red color being 
determined by six different factors, cumulative in their 
effect but separately inherited. With only two cumu- 
lative factors the F 2 situation is represented by fig. 15, 
where it is evident that one individual of each parent 
type appears in 1 6 of the F 2 individuals. This ratio 

76 



Neo-Mendelism 77 

may be referred to as 1:4:6:4:1. When three cumula- 
tive factors are considered the F 2 ratio would become 
1:6:15:20:15:6:1 (see fig. 16), that is, one of each 
of the original parents out of 64 individuals. In the 
same way the six cumulative factors assumed, which 
is really a conservative number, would give the following 
F 2 ratio : i : 1 2 : 66 : 220 : 495 : 792 : 924 : 792 : 495 : 220 : 66 : 
12:1, that is, one of each of the original parent types 
out of 4096 individuals of the F 2 generation. 

With these facts it is easy to explain the experience of 
breeders and horticulturists. In the first place, practi- 
cal breeders are not as carefully discriminating as are 
geneticists, so that they would put their individuals of 
Mirabilis into three classes, that is, red, pink, and white, 
without considering the various degrees of redness or 
pinkness. Therefore if they were dealing with six 
cumulative factors and grew less than 4096 individuals 
of the F 2 generation, the chances are that they would 
obtain no individuals representing either the red parent 
or the white parent. Naturally they would conclude 
that the original cross produced an intermediate hybrid, 
which when inbred without exception bred true to the 
intermediate hybrid character. The question might be 
raised in reference to the later generations. Would not 
the original parent type reappear in some generation 
later than F 2 ? The mathematics of the situation is too 
complex for presentation, but the result can be visualized 
in a less exact way. It has been seen that the vast 
majority of the F 2 are intermediates of varying degrees, 
that is, when the F x intermediates were inbred the result- 
ing F 2 was made up of practically all intermediates and 
no extremes. Inbreeding F 2 , therefore, would continue 



78 Plant Genetics 

the same situation, so that the chances of a reappear- 
ance of the original parent types would be very slight, 
so slight that if they did appear they would be described 
as " reversions." In this way many of the cases of true- 
breeding hybrids can be explained. Of course we have 
been dealing only with intermediate hybrids which are 
intermediate quantitatively, due to the action of cumu- 
lative factors. The gardeners and horticulturists might 
claim that their hybrid is not merely different in degree 
from either parent but that it is different also in kind; 
that it exhibits an entirely new character, that is, it 
differs qualitatively. If this is true of course the 
explanation suggested does not apply. The explana- 
tion can be modified, however, leaving the mathematical 
possibilities the same. Suppose the original parent 
strains differed, not by* six cumulative factors, but by 
six factors of different kinds. Under these conditions 
it would be possible to bring together in- a cross an 
entirely new combination of factors which might result 
in characters that would seem entirely new. If we are 
dealing with complementary or supplementary factors 
we might get a character in a hybrid that is entirely new. 
If these factors were sufficiently numerous, the chances 
of separating them again and recombining them exactly 
as in the original parents would be exceedingly slight. 

These possibilities have been presented for two rea- 
sons: (i) as explanations of true-breeding hybrids, 
whether quantitatively or qualitatively different from 
their parents; (2) to illustrate the method of using the 
conceptions of the factor hypothesis. 

(^ A PRACTICAL ASPECT OF THE CUMULATIVE FACTOR 

HYPOTHESIS. Assume that a practical breeder crosses 



Neo-Mendelism 79 

two extreme parent types in the hope of obtaining -a 
hybrid combining the desirable characters of the two 
parents. If the material is corn, he might use one 
parent with large grains but few in number, while the 
other parent has many grains but small ones. Such 
quantitative characters as these would be determined 
by cumulative factors, and the hybrid would be inter- 
mediate in respect to both of these characters, that is, 
the grains would be of medium size and medium number. 
No matter how many crosses he made, he would always 
get this result and not the desired combination of large 
grains and many of them. 

Suppose now that these intermediate hybrids are 
inbred in the hope of obtaining the desired combination 
in the F 2 generation. It will be realized that the chances 
of obtaining a plant combining the two extreme char- 
acters of large grains and numerous grains would depend 
upon the number of factors that enter into the make-up 
of these quantitative characters. Assume that there are 
five factors in each case. The mathematics of the situa- 
tion would show that in order to get the desired pure 
type from a cross between two parents, each having 
their desirable character determined by five cumulative 
factors, it would require 100 acres of corn to have an 
even chance of getting one such individual in the F 2 
generation. It is altogether unlikely that any farmer 
would use 100 acres and a corresponding amount of labor 
on such an extreme chance. Even if he did, it would be 
very problematical whether he would be able to select 
the proper solitary individual on his 100 acres. Even 
an agricultural experiment station would not feel justified 
in conducting such an experiment. 



80 Plant Genetics 

The question arises whether there is any way of 
avoiding this impossible situation. The escape is sug- 
gested by the fact that time can take the place of 
numbers. EAST (2) has shown that by growing 1000 
individuals in the F 2 generation, 100 in the F 3 , and 
50 in the F 4 , one stands as much chance of getting the 
desired combination as by growing 250,000 in the F 2 , 
provided an intelligent selection is made in each genera- 
tion. In other words, one who understands the mecha- 
nism of the inheritance of quantitative characters will 
grow only 1000 individuals in his F 2 generation and will 
select for seed only those individuals with the right 
number of factors. In this way, by intelligent selection, 
[actors are piled up in the right direction from year to 
^ear. In a few years the desired result will be reached 
without the necessity of growing a very large number 
Df individuals. Such work is practicable at experiment 
stations, and it is the kind of work that a number of them 
have been doing recently. Even the ordinary farmer 
should be able to do such work. Although his selection 
of individuals should not be quite as intelligent as that 
of a scientific breeder, he would probably be selecting in 
the right direction and making some advance. A little 
more time and a little more acreage would bring him 
to the desired result. 

AJuj^her^application of the factor hypothesis may be 
considered. The practice we have been discussing under 
the name of the inheritance of quantitative characters seems 
to be little more than what has already been called 
artificial selection, which is the oldest of all methods of 
plant breeding. It is a method that was thought to 
be discredited entirely by the work of DE VRIES (i), 



Neo-Mendelism 8 1 

JOHANNSEN, and others when they discovered what 
they called elementary species or pure lines and demon- 
strated that artificial selection could never result in any 
large or permanent improvement. In consequence of 
this, artificial selection, as the most important method 
of securing desirable races, gave place to pedigree 
culture at the most important experiment stations. It 
was not abandoned, for it had its uses, but it seemed to 
many to be a mediaeval method of breeding. The 
artificial selection, however, which we have been de- 
scribing is different from that so long practiced. In 
brief statement the difference is as follows. 

In the first place, the selection proposed is preceded 
by an intelligent hybridizing, and after that genotypes 
rather than phenotypes are selected; that is, the selection 
is made on the basis of germ plasm rather than of body 
plasm. This would be a sufficient reason for the superi- 
ority of the new method of artificial selection as com- 
pared with the old. A little further analysis, however, 
will make the difference clearer. 

In the old method of artificial selection the breeder, 
in the first place, is dealing with such germinal variations 
as happen to appear in his crop; and in the second place 
he is dealing with those fluctuations which appear as 
responses to the environment. When he selects a large 
plant to use for seed that plant may be large on account 
of its germinal constitution; but on the other hand it may 
be large because it is growing in a less crowded place 
or is more heavily fertilized than the others. In that 
case the large plant might not furnish good seed. The 
plant breeder of the old method undoubtedly made such 
unfortunate selections frequently; that is, he selected 



82 Plant Genetics 

on the basis of external appearance, and external appear- 
ance is often a^yery poor index of hereditary capacity. 
Intelligent selection is based on germinal constitu- 
tion only, and therefore its results are quicker and 
surer. 

Another phase of the subject should be considered. 
When a plant breeder is trying to improve his crops by 
selection for quantitative characters, although he uses the 
old method of selection, he is likely to be making some 
gain, as the experience of hundreds of years has shown. 
The germinal constitution of his crop plants is masked 
by fluctuations of course, but this mask is not complete. 
Most of the plants he selects are bound to possess high 
numbers of factors of the right kind, and he probably 
rejects most of the plants with few factors. In any 
event, he has generally succeeded in the long run in 
getting a somewhat improved race. 

A summarized statement of this situation may be 
helpful. Our recently developed knowledge of the in- 
heritance of quantitative characters seems to justify 
artificial selection, but it does not justify the old blind 
method of selection. It emphasizes the need of intelli- 
gent, trained selection and shows how such selection can 
be made. In order to do this one must understand 
the mechanism of the inheritance involved and must 
understand the make-up (race or pedigree) of the plants 
dealt with, being sure that they are of pure race or 
strain, for selection from the ordinary mixed races of crop 
plants is not only tedious at best but often leads to 
chaotic results. 

The situations just considered enable one to under- 
stand two phenomena which have been baffling scientific 



Neo-Mendelism 83 

plant "breeders for some years. The races of plants 
improved by artificial selection have usually reverted 
to type when selection ceases. This fact was recognized 
for a long time, but was first pointed out clearly by DE 
VRIES (i). Since then we have always expected this 
result, that no improvement will maintain itself but 
will run back unless the selection is continuous. When 
a practical breeder announces that he has developed 
by selection a new race which continues to breed true 
without further selection we are inclined to disbelieve 
him, for we know that only elementary species breed 
true. We explain that the practical breeder bases 
his selection on fluctuations, and therefore his new 
race is bound to revert to type. It is obvious now 
that there is a flaw in this argument. The practical 
breeder may be basing his selection on fluctuations, but 
at the same time he may be piling up cumulative fac- 
tors in the right direction. Thus he might eventually 
secure a race containing all the cumulative factors. 
Such a race would be a homozygote and could not help 
breeding true. Most of the claims of artificially im- 
proved races that breed true may be false, but it should 
be remembered that such a thing is possible and may 
be stumbled upon accidentally, even with unscientific 
breeding. 

There is another phenomenon which has been 
much discussed, and which can now be explained in 
the same way. This is the so- called fixation of hybrids. 
For years breeders have made promiscuous crosses and 
then begun artificial selection with the F 2 generation. 
Eventually they have secured a pure-breeding hybrid. 
It will be remembered that it was in this way EAST 



84 Plant Genetics 

worked with the quantitative characters in corn, and 
the explanation is the same. 

In addition to the practical value of the conception 
of cumulative factors, the theoretical value is worth 
considering, for it explains things which have been very 
vaguely understood. The understanding of the fixation 
of hybrids just mentioned, and races produced by artifi- 
cial selection, clears up our practical breeding methods, 
and this is valuable iiif ormation ; but the conception also 
shows that the origin of species by natural selection as 
announced by DARWIN is possible, a method which 
for some time has been thought to be impossible. 

Of course natural selection in a certain sense has 
always been accepted, almost as generally as the 
fact of evolution. The point in dispute is as follows: 
DARWIN used as the basis of natural selection those 
small individual variations which we have come to call 
fluctuations, the same kind of variations the old plant 
breeder used in his artificial selection. DARWIN claimed 
that such variations could be piled up until the result 
would be a new species. It was in 1900 that DE VRIES 
(i) showed in a convincing way that this kind of varia- 
tion never resulted in a new species; at best it could only 
develop a race which approached the boundary of the 
species and never crossed it. Moreover, such a race 
would revert to type rapidly as soon as some slight 
change in conditions set up a new standard for selection. 
This argument, confirmed by experiment, has been 
generally accepted. 

We now know that individual variations are not 
always mere fluctuations or responses, but may be due 
to varying doses of cumulative factors. A selection on 



Neo-Mendelism 85 

this basis may well result in a new race that breeds 
true; and a race that breeds true is DE VRIES' definition 
of a new species. To reestablish DARWIN'S theory of 
natural selection is certainly an important consideration, 
and the situation illustrates how genetics and evolution 
are tied up together, so that neither one of them can 
be appreciated fully without some knowledge of the 
other. 

A few words may be said in reference to the reversion 
of an old race to its original specific type. DE VRIES 
outlined the situation clearly, and his conclusions are 
generally accepted. It is doubtful, however, whether 
it has ever been understood, since no one has ever 
devised a reasonable mechanism for such a reversion. 
The conception of cumulative factors, however, supplies 
this mechanism. A new race, developed by natural or 
artificial selection among individual differences, means 
the piling up of cumulative factors in a given direction. 
Stop the selection and the old plants with the small 
number of factors are allowed to survive; reproduce, 
cross with the new race, and eventually bring back the 
old species to the original average condition. 

1. DE VRIES, H., Species and varieties, their origin by mutation. 
Chicago. 1905. 

2. EMERSON, R. A., and EAST, E. M., The inheritance of quantita- 
tive characters in maize. Bull. Agric. Exper. Sta. Ncbr. no. 2. 
pp. 1 20. figs. 21. 1913. 



CHAPTER IX 

NEO-MENDELISM (Concluded) 

Thus far we have based our considerations on the 
rather simple theoretical mechanism of inheritance which 
was explained in connection with Mendel's law. We 
shall now consider some well established facts of inherit- 
ance which will oblige us to enlarge somewhat our 
theoretical mechanism. 

(I) COUPLED AND ANTAGONISTIC CHARACTERS. The 
classic illustration of coupled characters was brought 
to light by EMERSON (i) during breeding experiments 
with corn. His material included a strain with red 
grains and red cob. This strain, when crossed with 
another having white grains and white cob, gave an Fj 
generation with red grains and red cob. This indicated 
that red was dominant over white in both grain and cob; 
and since the F 2 generation gave the orthodox 3 : i 
ratio (3 reds and i white) the obvious conclusion was 
that redness in both grain and cob is due to a single 
determiner. 

Other breeding experiments, however, gave a differ- 
ent aspect to the situation. It was found that there 
are races of corn with red grains and white cobs, and 
others with white grains and red cobs. It is evident, 
therefore, that redness in grain and cob is due to two 
separate determiners. 

Let us consider a possible theoretical explanation of 
these two apparently contradictory situations. The red 

86 



Neo-Mendelism 



grain and red cob of the first mentioned experiment 
were not produced by a single determiner, but by 
two determiners coupled together in inheritance. It is 
as if the two chromosomes concerned had a bond between 
them which kept them together during the reduction 
division, and made them pass side by side into the 
same gamete. A diagram will illustrate this situation 
(fig. 20). The only unusual thing in this diagram is 
the bond which holds together the chromosome carrying 




Dominant Parent 

with Red Seeds 

and Cobs 





Recessive Parent 

with White Seeds 

and Cobs 



F, (Red Seeds and Cobs) 

FIG. 20. Diagram representing, in a somewhat obvious way, how 
the behavior of "coupled characters" might be visualized. 

the determiner for red seeds and the chromosome 
carrying the determiner for reel cob. A peculiar function 
of the bond, however, appears in the reduction division 
in connection with gamete formation by the F x genera- 
tion. 

In the case of a dihybrid in which two characters 
are involved the situation developed by the reduction 
division will be recalled (fig. 21). It will be noted 
that there are four chances of pairing off by the four 
scattered chromosomes. The chances of getting a 
gamete with the two determiners is no greater than 



88 



Plant Genetics 



getting one with only one of the determiners. There 
would be just 25 per cent of each of the type represented 
in the diagram, the resulting ratio in the F 2 being 
9:3:3:1. 




F, No Coupling 4 Possible Gametes 

FIG. 21. Diagram showing how normal dihybrid segregation, in 
absence of coupling, produces four different gametes. 

In the case we are considering, however, the mecha- 
nism is different. At the reduction division of the Fj 
generation two chromosomes with the dominant de- 
terminers are linked together by a bond. In conse- 
quence of this there are only two types of gamete 





F, Coupling 



Only 2 Gametes 
Possible 



These 2 Gametes 
Impossible, Due to 
Presence of Bond 



FIG. 22. Diagram showing how coupling would limit possible 
kinds of gametes. 

possible, as indicated in the diagram (fig. 22). In the 
terminology of cytology the two chromosomes with 
the determiners are held so firmly together that they 
pass to the same pole of the spindle. This is the phe- 
nomenon of c^j)ledinhjritance and a theoretical 
mechanism that was devised to explain it. 



Neo-Mendelism 89 

EMERSON then crossed strains having red grains and 
white cob with strains having white grains and red cob. 
The F x generation showed all red grains and cobs, as 
would be expected; but in the F 2 generation three differ- 
ent types of individuals appeared as follows: i with red 
grains and white cob; 2 with red grains and red cob; 
i with white grains and red cob. This result in the F 2 
generation differs from any that has been cited, and 
EMERSON explains it as follows. In this case the two 
determiners are not coupled; in fact they are antago- 
nistic, so that no true-breeding types are produced 
having red grains and red cobs. It is as if the two 
chromosomes carrying the determiners are mutually 
repulsive, so that they always pass to different poles of 
the spindle at the reduction division. EMERSON inter- 
prets the results of these experiments, therefore,, as due 
to the existence of two coupled chromosome^ in the 
first case and two antagonistic chromosomes in the 
second case. 

But is there not another way of looking at this 
situation ? Granted that there is such a phenomenon 
as the coupling of chromosomes, the question arises why 
the second situation may not also be explained by coup- 
ling, the difference being that in this case the coupling 
is between different chromosomes. According to this 
suggestion the two situations would be represented by 
the diagram given in fig. 23. Such a scheme explains 
both cases by using the same kind of force, that is, 
coupling, and does not call for attraction to explain 
one situation and repulsion to explain the other. 

This last conception, however, raises the question, 
why should a chromosome which in some strains of corn 



Plant Genetics 



is coupled with a similar one, in other strains become 
coupled with its opposite (its allelomorph) ? This is a 
theoretical difficulty which will be cleared up when we 
consider the next topic. It will be realized that genetics 
as yet represents an accumulating mass of data in 



COUPLED CHARACTERS" 





Only 2 Gametes 

Possible 
F 2 Gives 3 : 1 Ratio 



"ANTAGONISTIC CHARACTERS" 




Parents 



Only 2 Gametes 

Possible 
F 2 Gives 1:2:1 Ratio 



FIG. 23. Diagram showing how coupled and antagonistic char- 
acters might be explained by the same mechanism. 

reference to the facts of inheritance, and also an ac- 
cumulating mass of speculations as to the explanations 
of the facts. The facts are undoubted, while the specu- 
lations suggest further experimental work. 

(g) LlNKAGE_AND CROSSING OVER. EMERSON COn- 

cluded his original work on coupled and antagonistic 



Neo-Mendelism 9 1 

characters with the following statement: "This is an 
example of a feature which is probably very wide- 
spread in the plant world, but of which at present we 
know little. " Long before any further important work 
was done along this line in the plant world, however, 
MORGAN (2) published the results of his very careful 
and intensive breeding experiments with the fruit fly. 
His ideas have had a very profound influence upon 
subsequent work in genetics. He has given us a more 
accurate picture of the hereditary mechanism and one 
that fits the facts better than any previously proposed. 
In simplest terms the picture is this. Each chromosome 
is a rodlike structure and numerous determiners are 
arranged in a line along this rod. 

We cannot discuss here the many ways in which this 
fundamental conception has cast light upon work in 
genetics. Suffice it to say that it has resulted in a 
new "school." of geneticists whose experiments have 
been more intensive, more exact, and perhaps more 
"fundamental" than those of any other school. The 
work so far has been done mainly with the fruit fly 
and is of a rather complicated nature. We will present 
here some of the simplest underlying ideas merely to 
show the nature of MORGAN'S hereditary mechanism. 
It will be seen how such a mechanism may explain such 
phenomena as the coupled and antagonistic characters 
in corn. This general situation, however, will now be 
referred to by the term linkage, which is in more com- 
mon use. 

When first considering Mendel's law the statement 
was made that mpre than one determiner might be 
located on a given chromosome. As yet we have 



92 Plant Genetics 

considered no such case, but linkage involves this situa- 
tion. In connection with some of MENDEL'S original 
crosses fig. 24 will be recalled. In this case a double 
dominant mates with a double recessive, and the result 
is a dihybrid ratio in the F 2 generation. Suppose, 
however, that determiner T and determiner S are on the 
same chromosome, the situation would be as represented 




Dwarf Wrinkled 
Parent 



4 Possible Gametes 
/.F 2 Shows 9:3:3:1 Ratio 



FIG. 24. Diagram showing normal dihybrid behavior when no. 
linkage is involved. 



in fig. 25. This is linkage; that is, with two determiners 
a monohybrid ratio instead of a dihybrid ratio is 
obtained. MORGAN'S definition is as follows: "When 
factors lie in different chromosomes they give the 
Mendelian expectations; but when factors lie in the 
same chromosome they may be said to be linked, and 
they give departures from the Mendelian ratios." It 
will be noted that this conception rests upon the belief 
that the chromosome is the indivisible unit in inheritance, 
a conception that seems to have been justified by most 
of the breeding results and which conforms to Mendelian 
inheritance. 



Neo-Mendelism 



93 



On the other hand, facts have begun to appear which 
seem contrary to this view. When the idea of linkage 
began to be developed the breeding expectations were 
modified in accordance with it. For example, when it 
is discovered that tall individuals always have smooth 
seeds this fact is explained as linkage. The inference is 
that there never can appear a tall plant with wrinkled 



LINKAGE" 





Only 2 Gametes 

Possible 

.'. F 2 Shows 3 : 1 Ratio 
Dwarf Wrinkled (3 Tall Smooth : 1 Dwarf Wrinkled) 

Parent 

FIG. 25. Diagram showing how linkage would limit the possible 
kinds of gametes. This mechanism will explain simply and accurately 
the "coupling " and "antagonism " of the previous diagrams; this mental 
picture of chromosomes, suggested by EMERSON and developed fully by 
MORGAN, is now generally accepted by geneticists. 

seeds or a dwarf plant with smooth seeds, for if there is 
linkage and the chromosome is the indivisible unit ot 
inheritance it would be impossible for tallness and 
smoothness to become separated. 

This was the situation a few years ago, when suddenly 
there appeared what corresponds to tall individuals 
with wrinkled seeds and dwarf individuals with smooth 
seeds. These unusual types appear rather rarely as a 
general rule, although in a few special cases they have 
appeared as frequently as 20 in 100. The work was done 
with Drosophila (the fruit fly) under such conditions 



94 Plant Genetics 

of control that there could have been no experimental 
error. This new fact demanded explanation, for with 
such chromosomes as TS and ts it would be impossible 
to obtain a tall wrinkled individual as long as the 
individuality of the chromosome is maintained. When 
chromosomes were examined with the modern lenses 
they were found to show all kinds of tangled contortions 
during reduction division, and accordingly the scheme 
shown in fig. 26 was devised. These five stages repre- 
sent phases that allelomorph chromosomes may go 
through during reduction division. Two allelomorph 
chromosomes, which normally would be side by side 
(i) may at times come to lie across one another (2). 
In this position the middle regions of the chromosomes 
are in contact and are conceived of as fusing (3). The 
spindle fibers from each pole then lay hold of this com- 
pound chromosome and the pull comes in the direction 
of the arrows shown in the figure. This results in the 
break indicated in 4. Finally, two new chromosomes 
separate from the old compound chromosome, as indi- 
cated in 5. Thus T is linked with s, and later, when 
two such chromosomes are brought together in crossing, 
the result is a tall wrinkled individual. In the same 
way dwarf smooth individuals may appear. 

This is known as crossing over, and the literature in 
reference to it is extensive. It is on the basis of this 
conception that certain geneticists claim to be able not 
only to locate given determiners on their proper chro- 
mosomes, but also to tell exactly on what parts of the 
chromosomes these determiners are located. 

This mechanism readily explains the phenomenon of 
coupled and antagonistic characters without involving 



Neo-Mendelism 



95 



such clumsy bonds and repulsions between the chromo- 
somes as were presented in the previous scheme. 






FIG. 26. Illustrating how crossing over may occur 



g6 Plant Genetics 

As stated before, most of the work on linkage and 
crossing over has been done with the fruit fly. The 
same technique, however, is now being applied among 
plants. In a recent article WHITE (3) discusses four 
linkage groups in Pisum, and considerable data on 
linkage and crossing over in corn are now being assembled 
at Cornell under the able direction of EMERSON. 

Perhaps a warning is needed as to the term correla- 
tion in this connection. Coupling and linkage are 
phenomena of inheritance, involving the reduction 
division, the gametes, and the zygote. Correlation is 
a physiological phenomenon that appears in the develop- 
ing plant. A tall corn plant may produce large ears 
merely because the plant is tall and regardless of the 
determiners for ear size. This is correlation, and it 
appeared, not in the gametes and zygote, but in the 
physiology of the individual after it started to develop. 
A correlation, therefore, may sometimes deceive as to 
inheritance. 

1. EMERSON, R. A., see Heredity and eugenics (COULTER, CASTLE, 
DAVENPORT, EAST, and TOWER). Chicago. 1912 (pp. 101- 
104). 

2. MORGAN, T. H., Heredity and sex. New York. 1914. 

3. WHITE, ORLAND E., Inheritance studies in Pisum. IV. 
Interrelation of the genetic factors in Pisum. Jour. Agric. 
Research n : 167-190. 1917. 



CHAPTER X 
NON-MENDELIAN INHERITANCE 

Up to this point Menclelism and its various modi- 
fications have been considered, and the impression to be 
gained thus far is as follows. Mendel's law as originally 
stated is clear, reasonable, and well established. There 
have appeared certain types of inheritance which have 
seemed at first sight to contradict Mendel's law; but 
practically all of these later have been shown to be con- 
sistent with that law. The conclusion, therefore, may 
be that Mendel's law is firmly established but that its 
expression is not always so simple as was once sup- 
posed. Illustrations have been given showing how 
Mcndelian explanations must be based frequently upon 
numerous factors of various types. The impression, 
therefore, may be that Mendelism is so established and 
its application so universal that it explains every type of 
inheritance that is known or that ever will be known. 
It is true that Mendel's law is dominant with most 
geneticists as explaining most of the known facts of 
inheritance, although there are certain types of inherit- 
ance that have not been explained as yet quite satis- 
factorily in this way. As a consequence, a number 
of prominent geneticists protest against the universality, 
of the law. These unexplained phenomena must now 
be considered under what may be called non-Mendelian 
inheritance. It will be recalled that EMERSON and 
EAST stated that only two indisputable cases of non- 
97 



98 Plant Genetics 

Mendelizing characters arc known, and these cases may 
be used as illustrations of non-Mendelian inheritance. 

In connection with his work on Mirabilis CORRENS (2) 
uncovered the following situation. The ordinary race of 
Mirabilis has pure green leaves, but one was discovered 
with variegated leaves. The leaves were green for the 
most part but showed irregular white patches, an 
examination showing that in the white areas the chloro- 
plasts were more or less bleached out. CORRENS 
named this race the albomaculata type, a name which 
appears frequently in the literature of genetics. 

The behavior in this type of inheritance was as 
follows. When self-fertilized it bred true, but the pollen 
from albomaculata, when used in crosses, behaved as if 
it had come from normal green plants. For example, 
CORRENS took pollen from an albomaculata plant and 
used it to pollinate a normal green plant. The indi- 
viduals of the F! generation were all normal green, 
and the natural Mendelian conclusion would be that 
albomaculata is a recessive condition appearing when 
the determiner for normal green leaves is lacking. But 
when the F x generation was inbred the progeny con- 
tinued to be normal green generation after generation. 
If albomaculata pollen had introduced a recessive 
character into the hybrid, this would have reappeared 
in the later generations, but it did not reappear. ; The 
obvious conclusion was that albomaculata pollen is just 
like the pollen of the normal green plants and that all 
pollen carried the determiner for the development of 
normal green plants. 

This raised the question as to the result from using 
pollen of normal green plants to pollinate albomaculata. 



Non-Mendelian Inheritance 99 

It is obvious that the F! generation should all be normal 
green and albomaculata would appear only in later 
generations. CORRENS made this cross, using normal 
green pollen and albomaculata ovules, and secured 
results that seemed startling. All of the individuals 
of the Fj generation were albomaculata and all the later 
hybrid generations were albomaculata. To CORRENS 
there seemed to be only one conclusion possible, and 
that was that the pollen did not affect anything one 
way or the other, and that inheritance was all on the 
maternal side. When a possible mechanism to explain 
such a phenomenon is considered it is obvious that the 
situation cannot be explained on the basis of Mendel's 
law. The explanation suggested by CORRENS is ingen- 
ious and seems fairly reasonable. 

He suggests that the albomaculata character is due 
to a disease of the cytoplasm which does not affect the 
nucleus. The nuclei in these albomaculata plants are 
supposed to carry factors for chlorophyll formation, just 
like those of the normal green plants, but the cytoplasm 
about the nucleus is diseased. Thus the disease is passed 
on at cell division simply because the cytoplasm is 
divided and passed on. In fertilization, however, no 
cytoplasm enters the egg with the male gamete, but only 
a nucleus; therefore it would be impossible for the dis- 
ease to be transmitted by the pollen, and an albomaculata 
pollen would have no effect in inheritance. The only 
chance to transmit the disease would be through an 
egg with diseased cytoplasm. In inheritance, there-; 
fore, the albomaculata character is handed down 
only by the female parent, and this is maternal 
inheritance. 



ioo Plant Genetics 

This would explain the peculiar breeding results 
obtained by CORRENS. If this explanation is true 
Mendel's law is not contradicted , for such a phenomenon 
is entirely outside the field of that law. In fact, this 
should perhaps not be regarded as inheritance at all 
but as a case of reinfection. The albomaculata condi- 
tion is not a true plant character; it is a pathological 
state, not inherited as such, but the bacteria in the 
cytoplasm of the egg reinfect the next generation. 

The conclusion therefore is that MendePs law is not 
contradicted by this phenomenon, but it introduces 
entirely new possibilities of inheritance quite outside the 
scope of Mendel's law. This conclusion is reached if 
one accepts CORRENS' explanation, but this might well 
be doubted. To state that albomaculata cytoplasm is 
always infected and that the nuclei are always immune 
is a bold assumption. If we assume, with some cytolo- 
gists, that the nucleus consists entirely of chromatin and 
that chromatin is immune to this disease this would 
furnish the proper mechanism, but this is questionable. 
Again, when it is claimed that no bacteria could enter 
the egg with the male nucleus another rash assumption is 
made. After all, the case of albomaculata may still 
be regarded as a genuine case of non-Mendelian inherit- 
ance of the type known as maternal inheritance. 

The other example of non-Mendelian inheritance to 
which EMERSON and EAST referred is much more serious, 
introducing somatic segregation, which is a very far- 
teaching topic. When EMERSON and EAST made their 
statement somatic segregation had been rarely observed, 
there being really only one authentic case on record, 
that is, the case of the common geranium (Pelargonium 



Non-Mendelian Inheritance 101 

zonale) described by BAUER (i). Since that time, 
however, many more cases have been discovered, and 
the fact of somatic segregation has become fairly well 
established. 

An illustration of the situation may be given as 
follows. If a white-leaved plant and a normal green- 
leaved plant are crossed the resulting hybrid illustrates 
what has been mentioned as "particulate inheritance/' 
that is, the hybrid is variegated, showing irregular 
patches of white and green. If one of these white 
patches completely includes a bud there will probably 
be produced by that bud a completely white branch. 
The flowers of this branch, when self-fertilized, give rise 
through their seeds to white individuals only and would 
evidently continue to breed true to the white condition 
if white individuals could be matured. In like manner 
the variegated hybrid may give rise to a pure green 
branch, which would start a line of pure green indi- 
viduals. 

This is an Illustration of what is called " somatic 
segregation." Ordinarily, of course, factors are segre- 
gated only at the reduction division, and only in that 
division does the proper cytological mechanism for seg- 
regation exist. Somatic segregation, however, means 
that segregation takes place in the somatic tissue, quite 
apart from the usual reduction division. Cytologists 
assure us that cell divisions in the somatic tissue are not 
reduction divisions, in this tissue chromosomes being 
regularly reproduced in equal numbers. Each chromo- 
some divides to form two chromosomes, and these two 
are similar to the mother and to one another; therefore 
they carry exactly the same quota of determiners, that is, 



102 Plant Genetics 

there is no segregation of determiners. This situation is 
not merely shown by cytology, but is also in accord 
with most of our breeding results. 

In the case of somatic segregation, therefore, there 
seems to be only one possible conclusion, and that is that 
sometimes reduction^ division occurs in the somatic 
tissue, at least so far as certain chromosomes are con- 
cerned. Such a conclusion is not unreasonable. Crosses 
of certain factors might cause a very unstable condition in 
certain chromosomes, so that they might not conform 
to the ordinary mechanism of cell division. Certain 
chromosomes, or certain determiners on the chromo- 
somes, instead of dividing normally might pass un- 
divided to one pole of the spindle at cell division and 
be entirely lacking at the other pole. 

The only other alternative would be to discard the 
old ideas of the mechanism of cell division and inherit- 
ance, and this we are hardly ready to do. In any 
event the phenomenon of somatic segregation opens 
an important field for some critical cytological research. 

It will be realized that somatic segregation is a 
much more fundamental situation to explain than 
maternal inheritance. There is no reason to suppose, 
however, that it contradicts Mendelian inheritance 
seriously; perhaps it enlarges the scope of Mendelism. 
Mendel's law is violated only in the fact that segregation 
occurs at an unusual place; and that is no reason for 
discarding Mendelism. It complicates the situation 
somewhat because the ordinary reduction division 
furnishes such a complete mechanism for segregation; 
but MENDEL formulated his law before the reduction 
division was discovered in plants. Somatic segrega- 



Non-Mendelian Inheritance 103 

tion may lead to the discovery of an occasional unusual 
kind of reduction division in the somatic tissue. 

1. BAUER, ERWIN, Das Wesen und die Erblichkeitsverhaltnissc cler 
"Varietates almomarginatae hort." Von Pelargonium zonale. 
Zeitschr. Ind. Abstamm. Vererb. 1 1330. 1909. 

2. CORRENS, CARL E., Ubcr Bastardierungsvcrsuche mit Mirabilis- 
Sippen. Bcr. Dcutsch. Bot. Gesell. 20:594. 1902; ibid., 
Vererbungsversuche mit blass (gelb) griinen und bltmtblat- 
terigen Sippcn bei Mirabilis jalapa, Urtica pilulifcra, u. Lunaria 
annua, Zeitschr. Ind. Abstamm. Vererb. 1:291. 1909. 



CHAPTER XI 
MODIFICATION OF UNIT CHARACTERS 

The severest blow at Mendelism is the modification 
of unit characters. The three theses involved in 
Mendel's law are unit characters, dominance, and segre- 
gation. It has been seen how dominance may fail with- 
out affecting the essential feature of Mendel's law. We 
have just considered how segregation might occur else- 
where than at the reduction division and yet not inter- 
fere seriously with Mendel's law. Unit characters, 
however, cannot be treated in this way, for they are 
the very foundation of Mendel's law. In the very 
nature of things they remain as units, separate in 
inheritance ; if they are observed splitting up or varying 
in any way they would no longer be units, and the 
foundation of Mendelism would be weakened. 

Before taking up the real cases of modification of 
unit characters, another unusual phenomenon concern- 
ing unit characters may be considered first, a phenome- 
non which is quite in accord with Mendelism as explained 
today. It is introduced here because at first thought 
it might seem to strike at the essential nature of unit 
characters; in fact, it has been so used. 

CASTLE (i), working with fancy mice, found that on 
rare occasions a unit character might disappear com- 
pletely in cross-breeding. It might be claimed that this 
result is due to the introduction of an inhibiting factor, 
or to the separation of complementary factors. CASTLE 

104 



Modification of Unit Characters 105 

realized these possibilities and tested them. Of course 
if it was a case of complementary factors which had 
become separated he would have been able to recombine 
them in later generations by the proper crosses. Recog- 
nizing the significant crosses and making them he ob- 
tained no result; the character did not reappear. In 
the same way breeding tests for an inhibiting factor 
which could be separated out in later generations yielded 
no results. The unit character was gone beyond recov- 
ery; it had simply dropped out and was lost. 

Such an occurrence may seem surprising at first, but 
it is what should be expected as an occasional occur- 
rence. Mendel's law was republished along with DE 
VRIES' mutation theory, and as a consequence scientific 
breeders, interested in Mendel's law, were also on the 
watch for mutations. DE VRIES had shown that muta- 
tions might involve either the appearance of an entirely 
new character or the dropping out of an old character. 
It is this situation, therefore, that appeared in CASTLE'S 
experiments with mice, namely, the abrupt disappear- 
ance of a unit character is simply the result of mutation, 
and this involves no violation of Mendel's law. 

The real attack upon Mendelism involves a different 
kind of behavior on the part of unit characters. The 
notable example of this unexpected behavior of unit 
characters was discovered in connection with CASTLE'S 
work on hooded rats (2). 

He isolated from his rat populations a certain strain 
which showed a particular black and white coat pattern. 
This type was then inbred for a number of generations 
to insure that it bred true. This fact having been 
established it was next determined that this black and 



io6 Plant Genetics 

white coat pattern behaved as a simple unit character 
in inheritance. Then, starting with a single pair of 
rats of this new pure strain, the following breeding 
experiments were performed. For twelve generations 
selections were made from this new strain without a 
single outcross, that is, every generation was inbred, 
thus insuring the constant purity of the stock. In one 
series selection was made for an increase in the extent 
of the pigmented areas; in the other series selection 
was made for a decrease in the extent of these areas. 
The result was that the areas in the one series steadily 
increased while in the other they steadily decreased. 
Thus far nothing very unusual is involved. CASTLE 
points out, however, the following important facts which 
were developed: (i) with each selection the amount of 
regression (" running back") grew less; that is, the 
effects of selection became more permanent; in other 
words, in each succeeding generation there was a 
decreasing tendency to revert to the original average 
type; (2) advance in the upper limit of variation was 
attended by a like advance of the lower limit. The 
total range of variation, therefore, was not materially 
changed, but there was a progressive change in the 
point about which the variation occurred. In other 
words, it is like a progressive shifting of the center of a 
circle ; the diameter of the circle does not change but the 
position of the circle, determined of course by its center, 
is gradually changing. These were the two important 
facts which CASTLE brought out and they have been 
stated approximately in CASTLE'S words. 

Fig. 27 will help to make the situation plain. The 
average amount of variation in any one generation of 



Modification of Unit Characters 



107 



the pure stock (the diameter of the circle referred to) is 

A 
indicated by < > . Of course, even "pure 

stock" varies somewhat, since no two individuals are 



> " M> ' Pure-Bred 
Generation* 




The Control 



FIG. 2 7. --Diagram illustrating CASTLE'S selection experiment 
with hooded rats. 

exactly alike, biology recognizing what is called " indi- 
viduality/' The point, however, is that the compara- 
tively small variation in a pure stock is not due to 
germinal differences but to responses called out by 
varying external conditions, such as nutrition, light, etc. 



io8 Plant Genetics 

These response variations, usually called fluctuations, 
vary with different individuals, but the hereditary 
capacity of all of them remains the same. A selection on 
the basis of fluctuations within a pure line, therefore, 
should not result in any permanent improvement; in 
fact, it has been demonstrated many times that no such 
improvement can be effected in this way. When selec- 
tion is made, however, among varying doses of cumula- 
tive factors, an entirely different situation is faced, for 
in such a case we are not dealing with a pure line. 

The significance, therefore, of CASTLE'S results may 
be realized. He bred his original pure line for many 
generations and found that it varied only within very 
narrow limits; and these slight variations therefore he 
regarded as mere fluctuations. Furthermore, he found 
that the character of his pure line developed in crossing 
as a simple unit character and that no complex factors 
were involved. With this evidence he should not have 
been able to effect any permanent changes by selection; 
but this is exactly what he did. Selecting in opposite 
directions he developed two new strains, the boundaries 
of the new strains being distinct from one another and 
distinct from the boundaries of the original strain, that 
is, the non-selected pure type. 

CASTLE'S next step was significant. He crossed 
each of his new strains with the same wild race, the 
result being that each of his new strains behaved as a 
simple recessive unit, giving a 3 : i ratio among the 
grandchildren. 

The logical conclusion from this series of experiments 
may be given in CASTLE'S words, as follows: "The 
conclusion seems to me unavoidable that in this case 



Modification of Unit Characters 109 

selection has modified steadily and permanently a 
character unmistakably behaving, as a simple Mendelian 
unit." The importance of this conclusion is evident. 
Mendelism has been based upon the conception that 
unit characters cannot be modified. Mendelians have 
granted only two possible methods for the origin of new 
races: (i) by recombinations of existing characters by 
hybridizing; (2) by the sudden and complete dropping 
out of an existing unit or the equally sudden addition of 
a new unit, both of which possibilities may arise from 
mutation. No Mendelians will grant, however, the 
possibility of modifying an existing unit character, the 
thing which CASTLE claims to have done, and bases his 
claim upon well controlled experimental breeding. If 
CASTLE'S contention is true it must result in the fun- 
damental modification of Mendel's law. The whole 
mechanism will have to be modified or new fields of 
variation not known to exist will have to be taken into 
account. 

The statements of the Mendelians in reference to this 
situation should be considered. Their explanation is 
based in effect upon the situation we have already 
developed in connection with cumulative factors. The 
claim is made that CASTLE started with a peculiar 
character which fluctuates continually and has never 
been brought to as small a variability as have most 
other characters. The question is raised whether 
CASTLE'S assumption that this variability is merely due 
to fluctuation is altogether justified. May not the 
variability be due to varying closes of cumulative 
factors? Suppose for the moment that this were the 
case; it would not be surprising that CASTLE could 



no Plant Genetics 

develop two diverse strains by selection, for selection 
would result in piling up the cumulative factors in one 
direction or another. CASTLE'S rejoinder would be that 
if this is a cumulative factor situation why do none of 
the extremes appear in the non-selected stock, which 
instead breeds approximately true within very narrow 
limits of variability? The answer is made that the 
extremes do not appear in the pure-bred stock merely 
because of mathematical possibility. If we are dealing 
with six cumulative factors, and the so-called pure 
stock has an intermediate number of doses, there 
could not be much chance of getting out the extremes 
in later generations. It will be remembered that it 
would be necessary to secure over 4000 progeny to 
have an even chance of getting one such extreme; or 
at least 50 progeny to get anything that would visi- 
bly approach the extreme. It would seem, therefore, 
that CASTLE'S chances to determine this would be very 
small. Rats certainly do not produce 4000 progeny 
in a single generation; in fact, they produce much less 
than 50; therefore CASTLE'S pure stock goes on in the 
intermediate condition, and only by selection can he 
pile up the factors and reach either extreme. 

Thus far the explanation seems satisfactory. CASTLE 
showed, however, that the coat pattern condition 
behaved in crosses as a simple Mendelian unit; that is, 
it did not split up into complex ratios, but came out as 
a recessive in a regularly 3 : i ratio. This really involves 
no difficulty. Suppose CASTLE crosses one of his pure 
strain rats having the pattern character with another 
strain having an inhibitory factor for the pattern or some 
other character that conceals the pattern. Since the 



Modification of Unit Characters in 

inhibiting factor is simple, the resulting ratio may be a 
monohybrid ratio; that is, in the F 2 generation from 
such a cross the ratio of pattern to non-pattern will be 
1:3, and this is exactly what CASTLE got and what would 
be expected. At the same time the amount of pigmenta- 
tion, determined by numerous cumulative factors, goes 
on in the, same intermediate condition, unaffected by the 
cross. The relation of pattern to non-pattern is merely 
a simple monohybrid system temporarily superimposed 
upon the other more complex system without perma- 
nently affecting it, any more than any inhibitory factor 
permanently affects the factors it inhibits, or a dominant 
permanently affects a recessive. 

It is in this way that Mendelians can explain away 
CASTLE'S results. CASTLE does not admit the justice 
of the explanation, but continues to maintain that he has 
modified a unit character by selection, and some geneti- 
cists agree with him. Whether CASTLE is dealing with 
cumulative factors or not can never be settled by exact 
demonstration, since the progeny of rats is too small. 
It is rather significant, however, that in plants, where 
larger progenies are involved, the cumulative factor 
hypothesis is well established, and no one claims to have 
modified a Mendelian unit in plants. The best that can 
be said concerning CASTLE'S claim is that it cannot be 
proved false, but that it is possible to explain his results 
by the cumulative factor hypothesis. 

The question might be raised, however, why cling 
so strongly to the cumulative factor hypothesis and 
force CASTLE'S results into this conception? Is there 
anything sacred about a unit character that it should 
not be modified just as complex chemical molecules may 



H2 Plant Genetics 

be modified in certain reactions? Why not admit 
that Mendelian factors may be modified, and explain 
CASTLE'S results in this way? The reason is that when 
we begin to admit that unit characters and single 
Mendelian factors may be modified, the whole conception 
of inheritance becomes chaos. The great advantage of 
the factor hypothesis is that it furnishes the clearest 
method of describing breeding results. A statement 
by EAST (3) on this point is pertinent. 

Taking into consideration all the facts, no one can well deny 
that they are well described by terminology which requires 
hypothetical segregating units, as represented by the term 
" factors." What then is the object of having the units vary at 
will ? There is then no value to the unit, the unit itself being only 
an assumption. It is the expressed character that is seen to vary; 
and if one can describe these facts by the use of hypothetical 
units, theoretically fixed, but influenced by the environment and 
by other units, simplicity of description is gained. If, however, 
one creates a hypothetical unit by which to describe phenomena, 
and this unit varies, he really has no basis for description. 

In other words, this statement means that the great 
value of the factor hypothesis is to supply a terminology 
or describing the facts of inheritance rather than a 
means of explaining these facts. According to this, 
therefore, it may be seen that although CASTLE claims 
to have modified a unit character by selection, his 
results can come under the factor hypothesis, and thus 
our descriptions may be kept clear and uniform. 

There is another piece of work, however, that cer- 
tainly cannot be explained (that is, described) in terms 
of the factor hypothesis; on the contrary, it almost forces 
one to admit that unit characters, even though hypo- 
thetical, may vary. This is the work of JENNINGS (4) 



Modification of Unit Characters 113 

on Protozoa. JENNINGS, for example, takes a single rhi- 
zopod (Difflugia) and uses it as the basis of a new strain. 
Every member of this new strain has come from this 
original individual by cell division. In this case sex 
is not involved, and the possibility of new combina- 
tions of factors is eliminated. It is obvious that such 
a strain should breed perfectly true, but JENNINGS 
shows that it does not. Changes of two kinds occur, 
namely, mutations, which of course are a part of the 
Mendelian program, but according to JENNINGS "the 
overwhelming majority of hereditary variations are 
minute gradations. Variation is as continuous as can 
be detected." He gradually piles up these minute 
variations by selection and finally develops a new 
species. There can be no claim of piling up cumulative 
factors here, for no sex is involved nor can these gradual 
changes be mutations. Mutations of course can be 
minute, but they are not continuous and in the same 
direction; mutations are jumps, even if the jumps are 
small, and they occur in every direction. 

The only other possibility is as follows. It will be 
remembered that WEISSMAN claimed that environment 
may directly affect the germ plasm in the simplest micro- 
organisms, just such organisms as JENNINGS deals with; 
but JENNINGS claims that the characters of his rhizopods 
are not modified by environment. How he is assured 
of this is a question, for the factors of environment are 
very numerous and complex and organisms are probably 
very sensitive. So far as circumstances permit, the 
conditions of environment were kept constant, but 
whether they really were constant or not is another 
matter. Leaving out of consideration this question of 



ii4 Plant Genetics 

environment, it must be admitted that JENNINGS' 
work proves that unit characters may be modified by 
selection. 

The evidence just given as to the modification of 
unit characters seems rather conflicting. It will be 
realized, however, that there are at present two leading 
schools of geneticists, one believing that unit characters 
may be modified by selection, and the other believing 
that unit characters are invariable. This latter school 
we have been referring to as Mendelians, but this is 
hardly an accurate designation, since both schools are 
really Mendelians. They are usually distinguished as 
follows: "mutationists/' who believe in the introduction 
of new hereditary units by mutation alone and believe 
that unit characters cannot be modified by selection; 
and " selectionists," who believe that unit - characters 
may be modified by selection. In the previous dis- 
cussions the views of the mutationists have been em- 
phasized for two reasons: (i) mutationists are in the 
majority at present; (2) their hypothesis gives a com- 
prehensive and systematic basis for describing the facts 
of inheritance. JENNINGS has recently tried to reconcile 
the two schools by showing that their claims amount 
to the same thing. Such a reconciliation is hopeful but 
it may prove to be impossible. 

1. CASTLE, W. E., Heredity in relation to evolution and animal 
breeding. New York. 1911. 

2. , The inconstancy of unit characters. Amer. Nat. 

46:352-362. ipi2. 

3. EAST, E. M., The Mendelian notation as descriptive of physio- 
logical facts. Amer. Nat. 46:633-655. 1912. 

4. JENNINGS, H. S., Observed changes in hereditary characters in 
relation to evolution. Jour. Wash. Acad. Sci. 7: 281-301. 1917. 



CHAPTER XII 
PARTHENOGENESIS AND VEGETATIVE APOGAMY 

Parthenogenesis and vegetative apogamy are very 
important to keep in mind in connection with work in 
genetics. Geneticists have paid too little attention to 
these phenomena, and as a consequence their data are 
not always reliable. The distinction between the two 
terms is as follows. Parthenogenesis is the develop- 
ment of an embryo by an unfertilized egg, while vege- 
tative apogamy is the development of an embryo by 
another cell of the female gamctophyte. The term 
" apogamy" includes both, meaning the production of a 
sporophyte by a gametophyte without involving an 
act of fertilization. Parthenogenesis is not peculiar to 
any region of the plant kingdom, and a few illustrations 
will be sufficient. 

In water molds (Saprolegnia) parthenogenesis is very 
common. All stages in the abortion of the male organ 
(antheridium) are found and still eggs apparently 
germinate as freely as if fertilized. In fact, in many 
species of Saprolegnia it is a question whether fertiliza- 
tion ever occurs. 

Ferns are notably apogamous, any cell of the game- 
tophyte (including unfertilized eggs) producing an 
embryo under certain conditions. This miscellaneous 
production of embryos by the gametophyte is an 
impressive illustration of the fact that any vigorous 
protoplast, under appropriate conditions, can produce 



n6 Plant Genetics 

a new individual. This behavior of ferns is particularly 
interesting on account of the chromosome situation 
involved. Since the gametophyte is an x structure and 
the sporophyte a 2x structure, when apogamy was first 
discovered it was taken for granted that in some way 
nuclear fusion had occurred and the 2X number obtained. 
More recent work, however, notably that of YAMA- 
NOUCHI (2), showed that the apogamous sporophyte is 
an x structure, and such a sporophyte produces spores 
without the reduction division. 

Ferns are also aposporous, which means the formation 
of gametophytes by vegetative cells of the sporophyte, 
the gametophyte in such a case being a 2X structure. 
With the phenomena of apogamy and apospory estab- 
lished, the question naturally rises as to their bear- 
ing upon the machinery of Mendelism, involving the 
reduction division and the segregation of hereditary 
factors. 

Seed plants, however, are of particular interest, since 
they have furnished the chief material for' work in 
genetics. Many cases of parthenogenesis and vegetative 
apogamy in seed plants have come to light. It is an 
interesting fact that, although these cases are well dis- 
tributed throughout the groups of seed plants, they seem 
to be especially numerous among the Compositae. In 
many cases, therefore, the embryos of seed plants are 
produced by unfertilized eggs or other cells, as in Anten- 
naria, Hieracium, Taraxacum, Tkalictrum, etc. Some 
of the best known cases were subjected to cytological 
investigation, and it was found that in most cases the 
parthenogenetic egg is diploid (2^), thus containing the 
sporophyte rather than the gametophyte number of 



Parthenogenesis and Vegetative Apogamy 117 

chromosomes. It is evident that for some reason the 
reduction division had failed in connection with spore 
formation, so that the spores were diploid, resulting 
naturally in a diploid gametophyte. In these cases it 
was found that no reduction division occurred during 
the life- history, the diploid number continuing through 
both sporophyte and gametophyte. The interesting 
observation was made also that parthenogenetic eggs 
are slower in starting to germinate than those normally 
fertilized. 

Polyembryony is also common among seed plants, 
and this involves apogamy. Mention has been made 
of the case of an onion seed containing five embryos, 
only one of which could have come from a fertilized egg. 
The four possible sources of embryos are found to be the 
fertilized egg, the unfertilized egg, gametophyte cells, 
and sporophyte cells. 

Another interesting fact concerning parthenogenesis 
in seed plants has been added comparatively recently (i). 
A plant known to be parthenogenetic was shown to have 
2x eggs, but it was also known to be pollinated and its 
sperms were seen discharged into the embryo sac. These 
sperms were apparently functional in every particular, 
and with the x number of chromosomes, and yet, 
although a sperm came into contact with an egg, there 
was no fusion. The inference has been that a diploid 
egg may be incapable of fertilization and is partheno- 
genetic if it functions at all. It does not follow that only 
diploid eggs are parthenogenetic, for parthenogenesis 
in ferns contradicts this conclusion. 

The bearing of these facts upon genetics should be 
considered. Plant geneticists have been using the most 



n8 Plant Genetics 

complex material. Results have been given in experi- 
mental work with peas, four-o'clocks, wheat, and, most 
of all, corn, all of them seed plants. This selection of 
material was natural, for such plants represent the 
region of the plant kingdom in which practical breeding 
occurs. It was natural to make practical breeding 
scientific. 

Not only have geneticists used the most complex 
material but they have selected sexual reproduction as 
the method of reproduction to be investigated, which is 
the most complicated kind of reproduction. This selec- 
tion was also natural, for Mendel's law is based upon 
sexual reproduction, and it is this law that geneticists 
have been testing. 

The further difficulties of the situation should also 
be considered in order that one may be in a position to 
estimate the value of results. Students of morphology 
are familiar with the fact that the sexual structures of 
seed plants are peculiarly involved with other structures. 
They do not stand out distinctly, as in the algae, for 
example. Not only are the sexual structures (eggs and 
sperms) beyond the reach of observation, and therefore 
of experimental control, but there is the alternation of 
generations to consider, inheritance being carried through 
one generation to express itself in the next. A sporo- 
phyte does not beget a sporophyte but a gametophyte, 
and this in turn begets the embryo sporophyte. 

Add to this complex situation the possibility of 
parthenogenesis and vegetative apogamy and it is ob- 
vious that the origin of embryos found in seeds is not 
assured. If two embryos appear it is evident that at 
least one of them holds no relation to the pollen parent ; 



Parthenogenesis and Vegetative Apogamy 119 

and even when only one appears, as is usually the case, 
how can it be certain that it has arisen from an act 
of fertilization? One can assume for the most part 
that embryos are the result of fertilization, but the 
increasing number of cases of apogamy introduces a 
serious element of uncertainty. If two individuals 
have been crossed, and the expected results in the F r 
or F 2 generations do not appear, how can it be told 
whether the pollen parent has entered into the result 
at all? Doubtless puzzling results in genetics have 
arisen more than once through apogamy; the crossing 
has been controlled, but the results are taking place out 
of sight and of course out of control. The crossing is 
only the gross part of the performance. If instead of 
applying pollen to stigma one could apply sperm to egg, 
there would be far more control. The program of events 
between pollination and fertilization should be kept in 
mind, and also the program between fertilization and 
the escape of the embryo from the seed. Not a single 
stage of these performances is under observation, much 
less under control. Pollination is effected, and then the 
plant emerging from the seed is observed. One must 
take for granted that all of the numerous events that 
lie between have been completely orthodox. Usually 
they are, but sometimes they are not; and then the 
unexpected result is forced into the orthodox program. 
It is really surprising, with so much of our experi- 
mental* work out of sight and beyond control, that so 
many constant results are obtained. It is an impres- 
sive lesson as to the wonderful uniformity of conditions 
between pollination and fertilization, and then between 
fertilization and embryo escape. This gives assurance 



I2o Plant Genetics 

that, in spite of the complexity of material and in spite 
of lack of control, the facts of inheritance are being 
uncovered when the consistent results are considered. 
It also assures us that an unexpected result may occur 
now and then and that it is not necessary to explain it 
by trying to make it consistent with other results. A 
single illustration will make this clear. \ Suppose that a 
species is persistently parthenogenetic, as some are 
known to be, and still there is pollination. If one were 
conducting breeding experiments with such a plant he 
would discover that inheritance was of a sort called 
maternal inheritance, that is, inheritance determined 
by the ovule parent and not affected by the pollen 
parent., In explaining this situation it would not be 
necessary to have recourse to CORRENS' idea of diseased 
cytoplasm ; one would suspect parthenogenesis and then 
look for it. 

Self-sterility, which is an important and growing 
subject, to be developed later, is related in one way or 
another to apogamy. Mention has been made of the 
stimulating effect of the mere presence of a pollen tube 
in inducing an unfertilized egg to germinate without 
sperms being discharged or even formed. An increas- 
ing number of plants are found to behave in this way, 
especially in certain families. Notably in some culti- 
vated races of apples and grapes self-sterility has been 
developed, that is, the pollen grains are impotent upon 
their own stigmas. In such cases the fruit develops 
just as if fertilization had taken place; in many cases 
seeds develop within the fruit and even embryos in the 
seeds. Strangely enough if " foreign" pollen is used 
it may not be impotent but results in fertilization. All 



Parthenogenesis and Vegetative Apogamy 121 

of this indicates that the origin of embryos cannot always 
be assured, and apogamy is a factor that may at any 
time upset any expectation based on Mendel's law 
without violating that law. If Mendel's law is based 
on the act of fertilization, and that act does not occur, 
the result has nothing to do with the law. 

In view of what has been said, the recent movement 
of some plant geneticists to include the lower plants in 
their work will be understood. This results in the 
following opportunities. In the study of sexual repro- 
duction the simpler plants (such as algae) have sexual 
structures that are not involved with other structures, 
and the whole performance of fertilization and embryo 
development is in sight and capable of control. The 
difference between a sex act and embryo development 
under cover and in the open, when observation and 
control are desired, is obvious. Furthermore, back of 
inheritance through the sex act is inheritance through 
spores. Such inheritance needs investigation in con- 
nection with inheritance through sex. There are cer- 
tain things that all forms of inheritance have in 
common, and these things should be kept distinct from 
those which are peculiar to sex inheritance. Only in 
this way can any knowledge be reached of the special 
role of sex in inheritance and the peculiar features it 
has added. 

A practical plant breeder may be interested only in 
the fact that he can obtain a new individual from a seed 
the pedigree of whose embryo in the nature of things 
cannot be demonstrated; but a scientific plant breeder, 
that is, a geneticist, must be interested in the conditions 
that determine inheritance, and these will be discovered 



122 Plant Genetics 

by investigating and controlling all kinds of inheritance. 
No more favorable material for determining the funda- 
mental facts of inheritance can be found among plants 
than the spores of the simpler forms. They are accessible 
and therefore capable of control; a succession of spore- 
produced individuals represents a line whose purity can- 
not be questioned; the so-called " modification of the 
germ plasm" can be accomplished with a precision that 
is impossible in the case of an egg nucleus within an 
ovule and an ovary. In short, free from all the entangle- 
ments of sex, the possibilities of variations in pure lines 
can be determined and the possibilities of inheritance 
of such variations. Such observations will establish the 
facts common to all inheritance and will enable us to 
recognize the contribution of sex to inheritance. 

This is no criticism of the work that has been done 
with seed plants. That work was the natural beginning 
and the results have been remarkable. It is simply 
calling attention to the necessary limitations of experi- 
mental work in genetics with seed plants and the desira- 
bility of extending the field of genetics so as to include 
the lower forms that will permit certain desirable 
things to be done with greater exactness. 

1. PACE, LULA, Apogamy in Atamosco. Bot. Gaz. 56:376-394. 
pis. ij, 14. 1913. 

2. YAMANOUCHI, SHIGEO, Apogamy in Nephrodium. Bot. Gaz, 
45:289-318. pis. Q, 10. 1908. 



CHAPTER XIII 
INHERITANCE IN GAMETOPHYTES 

Thus far the discussions have dealt with inheritance 
in sporophytes; in fact, genetics practically never con- 
siders gametophytes, through which inheritance must 
pass from one sporophyte to the next. The reasons 
for this neglect are obvious. Practically all of our land 
vegetation is made up of sporophytes and therefore 
practically all of our experimental material has been 
sporophytes. Furthermore, gametophytes are incon- 
spicuous (out of sight in seed plants), hard to get at, 
hard to work with, and apparently of no economic 
importance. Besides, in animals, as is well known, the 
generation equivalent to the gametophyte of plants is 
represented only by a few cell divisions in the matura- 
tion of gametes. In other words, the gametophyte 
has no significance in the animal kingdom; and since 
inheritance in plants is of interest to most investigators 
chiefly because it throws some light upon inheritance in 
animals, there has been no demand for any knowledge 
of inheritance in gametophytes. 

This means that this subject has not been studied, but 
it may be considered here briefly. It may be admitted 
that our ideas of genetics have been fairly well developed 
without including the gametophyte in inheritance, but 
the question is whether they cannot be developed better 
and perhaps more quickly by including the gameto- 
phyte. There are two reasons that may be suggested: 

123 



124 Plant Genetics 

(i) Such a study will test the theories of inheritance 
derived from the study of the sporophyte; (2) it deals 
with less confusing material. 

In the first place, the genetics of gametophytes may 
furnish a good test of the theories of inheritance derived 
from the study of sporophytes. The chromosome 
equipment of a gametophyte is exactly the same as the 
chromosome equipment of a gamete, in both cases the 
chromosome number being haploid. This results from 
the fact that in plants the reduction division occurs in 
connection with spore formation; it is only rarely that 
it occurs in connection with gamete formation, as in 
animals, and then only when spores are cut out of the 
life-history. Fig. 28 represents the typical life-history 
of one of the higher plants, so far as the chromosome 
situation is concerned. At the reduction division, 
whose Mendelian significance has been considered, the 
chromosome number is reduced from 8 to 4; and as a 
result the spore, the gametophyte, and the gamete 
all contain 4 chromosomes. At fertilization two sets 
of 4 chromosomes are brought together by the male and 
female gametes, and therefore the zygote contains 8, 
and the resulting sporophyte continues the 8 chromo- 
somes. At a certain season or, better, under certain 
conditions cells of the sporophyte are set apart to pro- 
duce spores (spore mother-cells, each with 8 chromo- 
somes), and the reduction division follows, each spore 
receiving 4 chromosomes. This familiar picture of the 
life-cycle may now be applied to genetics. 

The fact that the chromosomes are reduced in num- 
ber from 8 to 4 is not the significant fact; the significant 
fact is how this reduction takes place, that is, not the 



Inheritance in Gametophytes 



superficial result but the process. The process may be 
recalled for a moment. Suppose that each of the 8 
chromosomes in a spore mother- cell contains a single 
determiner ; in such a case a nucleus can be represented 



SPORE 

MOTHER-CELL 
8 CHROMOSOMES 



(REDUCTION; 
1 DIVISION 1 




SPOROPHYTE 

INDIVIDUAL 

8 CHROMOSOMES 



GAMETOPHYTE 

INDIVIDUAL 
4 CHROMOSOMES 



ZYGOTE 

8 
CHROMOSOMES 



<- \ FERTILIZATION! 




FIG. 28. Diagram representing life-history of a higher plant as 
regards chromosome number. 

by fig. 29. When the reduction division occurs it 
means that 4 of these chromosomes will go to each of 
the two resulting nuclei. When this happens, are the 
chromosomes chosen indiscriminately? It has been 
learned that there is a certain limitation in the choice, 



126 Plant Genetics 

a limitation expressed by the statement that at the 
reduction division allelomorphs never go to the same 
nucleus, that is, the reduction division would never 
produce a spore whose nucleus would contain both A 
and a, for A and a are allelomorphs. All of this is 
evident in connection with the Mendelian diagrams. 
It should be remembered, however, that when it is said 
that a spore or a gamete contains only half as many 
chromosomes as does a sporophyte the whole truth has 
not been stated. In fact, such a statement does not 
include the most significant fact from the Mendelian 
point of view; that is, that a spore 
or a gamete never contains both of a 
pair of allelomorphs in the nucleus. 
It should be realized that 4 and 8 
chromosomes are used only as con- 
venient representative numbers, for 

in plants there is a wide variation as 

FIG. 29 L 

to these numbers, and in the vast 
majority of cases the sporophyte number is consider- 
ably more than 8. In angiosperms, for example, the 
haploid number ranges from 3 in Crepis to 45 in 
Chrysanthemum. The statement that the essential 
feature of the reduction division is to separate allelo- 
morphs into different nuclei is simply a statement of 
the theory of segregation, which is the fundamental 
feature of Mendel's law. If this statement is not 
true Mendel's law is not true, and all of our ideas of 
inheritance will have to be reconstructed. 

With this in mind, it may be considered how a knowl- 
edge of inheritance in gametophytes may be of service. 
For convenience we will consider first a single pair of 




Inheritance in Gametophytes 127 

allelomorphs (A and a). It will be recalled that such a 
pair of allelomorphs may behave in two different ways in 
hybrid sporophytes. In case of complete dominance, 
A alone will be expressed in the hybrid A a. In case of 
lack of dominance or of only partial dominance, the 
hybrid A a will partake of the nature of both A and a; or, 
in other words, it will be a blend, intermediate between 
the two pure parents A A and aa. 

A similar situation may be considered in the game- 
tophyte generation. In a gametophyte, A can be 
present or a, but not both. Here at least is a case of 
presence and absence about which there is no doubt. 
In this case there would be invariably pure dominance, 
or what corresponds to it, but there could not be a blend. 
It is interesting to consider the results of this situation. 
If in a study of inheritance in gametophytes cases of 
pure dominance only are discovered the result is quite 
consistent with our theories. If, on the other hand, 
cases of blending inheritance are discovered what are to 
be the conclusions ? The only possible conclusion would 
be that our theoretical mechanism is wrong; that 
chromosomes are not the bearers of hereditary characters, 
or at least not the only bearers; or else that the behavior 
of the chromosomes at the reduction division is different 
from that which cytologists and breeders have long 
supposed. In other words, we have here a chance to 
test the doctrine of segregation through the reduction 
division. 

The case of complementary factors may also be 
considered. Recalling the behavior of complementary 
factors in the sporophyte generation the question arises 
about their behavior in the gametophyte generation. 



128 



Plant Genetics 




Of course complementary 
factors can exist together in 
a gametophyte. A gameto- 
phyte could not contain A a 
(allelomorphs), but it could 
contain both A and B, 
which may be comple- 
mentary factors. 

It is evident also that 
supplementary and inhibi- 
tory factors are also possible 
in gametophytes, just as are 
complementary factors, for 
they are not segregated by 
the reduction division. Of 
course a gametophyte show- 
ing an entirely new char- 
acter produced by the 
interaction of two comple- 
mentary factors would have 
to be a gametophyte grown 
from a hybrid sporophyte 
in which these two factors 
had been brought together; 
and the same is true in 
reference to supplementary 
and inhibitory factors. 
With these facts in mind the 
validity of our theoretical 
mechanism could be tested. 

Finally, the case of 
cumulative factors could be 



Inheritance in Gametophytes 



129 



considered. These could also coexist in a gametophyte 
but not to the same degree as in a sporophyte. If three 
cumulative factors were active in a sporophyte, it is 
evident that there could be gradations as indicated 
by fig. 30. If, on the other hand, three cumulative 
factors were active in a gametophyte there could only 
be the gradations indicated by fig. 31. Thus the 
behavior of cumulative factors in sporophytes and 
gametophytes would differ quantitatively; there would 



A 

* s 

S N 

A' 







2 Doses 







I Dose 







Dose 



GAMETOPHYTE 

FIG. 31. Diagram showing how the cumulative factors of fig. 30 
could produce only four different quantitative classes in the gameto- 
phyte generation. 

not be so many gradations in gametophytes for the 
same number of factors involved. 

As a general conclusion, therefore, it may be said that 
a knowledge of inheritance in gametophytes would test 
our theoretical mechanism for inheritance in sporo- 
phytes. 

There is another important value to be secured from 
a knowledge of genetics in gametophytes. In the study 
of genetics in the sporophytes of the higher plants we 
are dealing with very complex organisms. Attention 
has been called to the possibilities of parthenogenesis 



130 Plant Genetics 

and vegetative apogamy, and to the fact that in such a 
case any theoretical scheme of factors constructed to 
explain breeding results would be without any signifi- 
cance. 

Again, in such a complex organism as the sporophyte 
it is hard to say at all times just what things are being 
inherited and what things are merely responses. Finally, 
there always remains the confusing problem connected 
with germ plasm and body plasm. Are these two plasms 
as distinct in the higher plants as they seem to be in 
animals ? In any event, can the body plasm affect the 
germ plasm in any significant way, and is there any 
mechanism to justify the inheritance of acquired char- 
acters? These questions are simply samples of the 
many complexities that confront us when dealing with 
such organisms as the higher plants. The suggestion 
has been made already that simpler plants be studied 
which show fewer complexities. Algae, and even bry- 
ophytes, have much to teach in the field of genetics, 
for in such plants germ plasm and body plasm are not 
clearly differentiated; frequently they are almost one 
and the same thing.. Also such a phenomenon as 
parthenogenesis is more easily studied in such plants, for 
the structures involved are superficial and therefore 
easily observed and controlled. This suggests another 
advantage, therefore, in the study of inheritance in 
gametophytes, for in algae practically everything is 
gametophyte, while in bryophytes the gametophyte 
generation is the dominant one and therefore the one 
providing the best experimental material. 

There is one disadvantage in breeding work among 
these lower forms that should be realized, and that is 



Inheritance in Gametophytes 131 

the question of technique. It might be asked how one 
could breed such a form as Ulothrix, in which the pro- 
ductive cells are microscopic and are practically impos- 
sible to identify or even to isolate successfully. One 
could begin with such a form as Spirogyra, however, 
in which the zygotes are rather large cells, so large 
that they can easily be detected without a microscope. 
Two biologists, BARBER (i) and KITE (3), by using fine 
pipettes, have been able to isolate bacteria successfully 
and even to drag individual chromosomes out of nuclei. 
It would be far simpler to isolate the zygotes of Spiro- 
gyra. Incidentally, it may be mentioned that the 
zygotes of Spirogyra are sometimes produced by the 
union of three sexual cells instead of two, and breeding 
results in such a case should be very interesting. 

For the following two reasons, therefore, a study of 
inheritance in gametophytes is demanded and should 
be undertaken: (i) to test our hypothetical mechanism 
with its segregation of many kinds of factors; (2) to 
study the lower plants where breeding phenomena are 
not obscured by so many complexities. 

In a secondary way, also, the gametophytc generation 
becomes significant in genetics. Under the topics 
of self-sterility, xenia, and hybrid vigor, inheritance 
in gametophytes will be touched upon again. The 
preceding discussion has been merely a suggestion of 
the perspective of an entirely new territory for explora- 
tion, a territory which should be made a part of that 
which is already occupied. 

In conclusion, the following quotation from EAST (2) 
is pertinent: " Modern discoveries tend more and more| 
to show that the sole function of the gametophyte of j 



132 Plant Genetics 

the angiosperms is to produce sporophytes. The 
characters which they carry appear to be wholly sporo- 
phytic, the factors which they carry functioning only 
after fertilization." This is a logical inference from most 
of the data on genetics. It does not mean, however, 
that we should leave all. gametophytes out of considera- 
tion, for active investigation of their behavior in inher- 
itance should reveal facts of considerable theoretical 
value. 

1. BARBER, M. A., Univ. of Kansas Sci. Bull. no. 4. 1907 (p. 3). 

2. EAST, E. M., and PARK, J. B., Studies on self-sterility. I. 
The behavior of self-sterile plants. Genetics 2: 505-609. 1917. 

3. KITE, G. S., The physical properties of the protoplasm of cer- 
tain animal and plant cells. Am. Jour. Physiol. 22:146-173. 



CHAPTER XIV 
SELF-STERILITY 

It has been discovered that many of the races of 
cultivated plants, notably apples, do not develop viable 
seed when self-pollinated. The practice adopted by 
horticulturists, therefore, to meet this situation is to 
make mixed plantings of several races. In this way 
foreign pollen is always present, and this results in 
viable seed where " own " pollen fails. This phenomenon 
is known as self-sterility and seems to be well established 
in many cases. 

In connection with this phenomenon, the following 
problem confronts us. Is self-sterility of any advantage 
to plants in nature, or is it merely a necessary evil that 
has arisen as a result of culture; or is it merely mechan- 
ical or chemical necessity that has arisen in response to 
certain conditions and is of no significance one way or 
the other in the economy of plants ? Opinion is divided 
on this point. Most biologists, however, claim that 
self-sterility is of some ^advantage to the plant. This 
opinion is related of course to belief in the Darwinian 
doctrine of the survival of the fittest, survival being a 
proof of usefulness. For this reason it has become 
customary to regard a plant character as useful until 
the contrary is proved. This point of view has been of 
service in the past and is still useful, subject to certain 
limitations. 

133 



134 Plant Genetics 

If self-sterility is useful to the plant the n&tyuce of its 
usefulness should be considered. The situation may 
be outlined as follows. The first plants used only 
asexual reproduction, and this has proved to be a very 
simple and effective method. Why then was sex ever 
developed? It will be remembered that WEISMANN 
was the first to answer this question clearly, claiming 
that sex was developed to secure greater variation. 
Greater variation is desirable of course to secure adapta- 
tions to diverse conditions of living. It is well known 
that asexual reproduction is the surest way to duplicate 
the parent form and would result in uniform plants 
generation after generation. If a species remains uni- 
form within certain narrow limits it will be impossible 
to meet changing conditions. This will result in two 
evils: (i) if conditions are changed where the uniform 
species is growing the species will be exterminated; 
(2) even if the local conditions remain the same the 
species will be unable to spread into new localities where 
the conditions are different. If, on the other hand, a 
species shows wider variations, certain of these might 
fit the new conditions. Such variations will survive 
when conditions change and would always solve the 
problem of migrating into new habitats. For these 
reasons variation is necessary to the general success of 
the species. 

This increase in the range of variations, therefore, is 
the significance of the introduction of sex, for the fusion 
of two germ plasms in the sex act greatly increases 
chances for variation. WEISMANN thought that sex 
secured the small individual variations which are so 
common in any species, but now most such variations 



Self -Sterility 135 

are regarded merely as fluctuations or responses. Such 
variations are not inherited, and therefore can be of no 
phylogenetic significance; that is, of no value to the 
future of the species. Our recent knowledge of the 
factor hypothesis, however, indicates that many of these 
variations are more than fluctuations. They may 
represent germinal differences, such as varying "doses," 
and such differences are inherited and may be of some 
value therefore in the future of the species. 

It will be realized, however, that it is possible to 
have a homozygous race. If a race were strictly 
homozygous in respect to every heritable factor the 
sex act would result in no more variation than would 
asexual reproduction, provided of course our ideas of the 
mechanism of inheritance in sexual plants are correct. 
The fact is that a perfectly homozygous plant is probably 
rare. Supposing, however, that it were common, we 
would have to resort to some other mechanism than the 
sex act to secure the desirable variation. If sex does 
not secure variation what can ? 

The answer is cross-pollination. Although races 
were all homozygous, cross-pollination would make 
possible the crossing of different races, which would 
inevitably result in great variation in the progeny. 
Even if homozygous races are few it is certainly true that 
in any event cross-pollination results in greater variation 
than self-pollination. This is the biological basis of 
the many devices in plants to secure cross-pollination; 
in fact, this is carried to such an extreme in many 
cases that self-pollination is absolutely prevented. Sta- 
mens and carpels may mature at different times; and 
the great number of dioecious plants must all be 



136 Plant Genetics 

cross-pollinated. DARWIN uncovered this situation and 
concluded that there is a general tendency among 
plants to secure cross-pollination. The significance of 
this universal tendency seems to be to secure variation. 

Some ascribe another value to cross-pollination. 
Hybrid vigor is a well established phenomenon, which 
will be discussed later. Continued inbreeding fre- 
quently results in degeneracy, such inbreeding races 
being spoken of as " running out." It is a well known 
fact that to cross an inbred race with some other race 
results in a kind of rejuvenescence, a regaining of the 
original vigor that was present before inbreeding began. 
Even in the case of two normally vigorous races a cross 
will often produce a hybrid more vigorous than either 
parent. Of course, one might regard hybrid vigor 
itself as a device to secure crossing. 

In any event, it seems to be safe to conclude that 
cross-pollination is of advantage to plants, and of all 
mechanisms to secure cross-pollination, self-sterility 
may be regarded as the culmination. If "own" pollen 
cannot produce viable seed in a plant cross-pollination 
becomes a necessity. Inbreeding cannot continue in a 
race that has become or is becoming self-sterile. 

This raises the question why is "own" pollen not 
effective where foreign pollen is? There are well 
known cases in which pollen is not effective on stigmas 
of the same race but quite effective on those of another 
race. Although the biological advantage of such be- 
havior is evident, this does not explain the mechanism 
involved. 

The theories offered in the explanation of self-sterility 
are very numerous. Some years ago an attempt was 



Self-Sterility 137 

made to explain self-sterility as a response. It was 
demonstrated that when a race of plants was grown 
under conditions of unusually high humidity its pollen 
grains would burst before they had a chance to function. 
This explanation was obviously true in a few simple cases, 
but the phenomenon of self-sterility usually involves 
something much more perplexing than merely the non- 
functioning of pollen. The real problem is to explain 
why pollen will not function on "own" stigmas and 
will function on the stigmas of other closely related races, 
while it requires foreign pollen to function on "own" 
stigmas. Such a complex situation could hardly be 
explained as a response; more probably it is involved 
with inheritance. Morphologists, cytologists, ecologists, 
physiologists, and practical horticulturists have all 
contributed explanations; in fact, it is not assured as yet 
to which of these fields the phenomenon belongs. 
In the meantime, what concerns us is whether self- 
sterility can be explained by the principles of genetics. 

Such a theory has been proposed. Several investi- 
gators have suggested that there is a heritable factor for 
self-sterility, and some investigations of this possibility 
have been undertaken, but as yet most of the results 
have been negative. It must be realized, however, that 
if self-sterility is ever to be explained on the basis of 
hereditary factors the mechanism involved will be 
rather unusual. Although most of the explanations 
proposed by geneticists have proved inadequate there is 
at least one that deserves consideration. It is a typical 
Mendelian explanation and very ingenious. It fitted the 
facts perfectly when first proposed and, still better, it 
has continued to fit the facts that have been uncovered 



138 Plant Genetics 

by subsequent breeding work. Furthermore, it is the 
first true Mendelian explanation that has been based 
on the germinal constitution in the gametophyte as a 
deciding factor. 

In 1914 BELLING (i) published a paper entitled "A 
study of semi-sterility." Apparently this is not true 
self-sterility, but it is a closely related phenomenon and 
may be considered in this connection. BELLING was 
conducting breeding experiments with two races of 
beans, both of which were completely fertile. When 
he crossed them, however, the resulting hybrids were 
semi-sterile. Uniformly just one-half of the pollen 
grains appeared empty and collapsed, while one-half 
of the ovules had no embryo sacs. The sterile pollen 
and ovules appeared in random distribution with the 
fertile. 

Inbreeding the semi-sterile hybrids, BELLING ob- 
tained an F 2 generation which showed the following 
features: one-half the plants had perfect pollen, the 
other one-half had a mixture of equal numbers of good 
and bad pollen grains in all their flowers. The plants 
which had perfect pollen also had perfect ovules, while 
the plants with 50 per cent sterile pollen also had 50 
per cent sterile ovules. In the F 3 generation all the 
descendants from the fertile plants had perfectly good 
pollen and ovules; but the progeny of the semi-sterile 
plants again split up into two classes (fertile and semi- 
sterile) as before. 

BELLING states his general conclusion as follows: 
"The explanation of the random abortion of one-half 
of the pollen and one-half of the embryo sacs must 
apparently be by the segregation of Mendelian factors 



Self-Sterility 139 

among pollen grains and embryo sacs individually, and 
not by the action of these factors on the zygotes." 

To make this suggestive situation clear the diagram 
(fig. 32) may be considered. It enlarges a little upon 
SELLING'S original ideas as he stated them, and empha- 
sizes the sporophyte-gametophyte relationship. Pollen 
grains and embryo sacs are gametophytes in the sense 
that they inclose the male and female gametophytes, 
so that when the diagram shows sterile gametophytes it 
is the same as saying that both pollen grains and embryo 
sacs are sterile. This of course is just what BELLING 
found; whenever one-half the pollen grains in random 
distribution were sterile one-half the embryo sacs in 
random distribution were also sterile. 

It should be remembered that BELLING started with 
two completely fertile races. Suppose that the parent 
race A had a factor (X) whose absence brought steril- 
ity in the gametophytes (pollen grains or embryo 
sacs). Species B has a different factor (Y), with a 
similar effect, but inherited independently. When 
BELLING crossed these races, all of the resulting F x 
hybrids were semi-sterile. In other words, in every 
F! plant one-half the gametophytes were sterile. It is 
easy to see why XO and OY are fertile, also why OO 
is sterile (lacking both factors); but why should XY 
be sterile when it has both factors ? BELLING explains 
it by saying that gametophytes are unlike sporophytes 
in that normally they have single factors instead of 
double factors. The germinal capacity of a game to- 
phyte is just one-half that of a sporophyte. It is as 
if a gametophyte were " supersaturated " by a double 
factor. Such a situation is abnormal for a gametophyte 



140 



Plant Genetics 



PARENT) 
RACE SPOROPHYTE: 



PARENT 

RACE 

B 



SPOROPHYTE: 



F,i SPOROPHYTE: 




FERTILE) 



FERTILE) 



TILE) 



'(FERTILE) 



TILE) 



'(FERTILE) 



'(STERILE) 



(STERILE) 



FIG. 32. Diagram illustrating Balling's explanation of semi- 
sterility. 



Self -Sterility 141 

and brings abnormal results. Therefore the gameto- 
phyte having the abnormal double dose (XY) is just as 
sterile as the gametophyte with no dose (OO). 

In developing the F 2 ratios of course only the fertile 
gametophytes function. XY and OO are eliminated, 
so far as posterity is concerned, so that we have to 
deal only with the chance matings among the fertile 
gametophytes (XO and OY). According to mathe- 
matics there are four possible matings between these 
two gametophytes (fig. 33). Out of these four result- 
ing F 2 sporophytes two would evidently produce only 
fertile gametophytes and would remain fertile as long as 
they are inbred. The other two are exactly like the 
original F r hybrid and therefore semi-sterile, giving 
one-half sterile gametophytes. The whole dynasty 
may be represented as follows: 

P Fertile X Fertile 

> 
FI Semi-sterile 

5% 



, . ^ . 

F 2 Fertile Semi-sterile 



50% 50% 

^ ^ 

Fertile Fertile Semi-sterile 



v t y 

F 4 Fertile Fertile Fertile Semi-sterile 

This is a very ingenious scheme and, like others, 
should be tested by further experiments. To a certain 
extent it has already met this test, for BELLING (2) 
has subsequently reported a few more generations 
in which the breeding results were entirely consistent 



142 



Plant Genetics 







with those of the earlier generations. Also he has 
discovered two new pure races of beans which give 
similar results. It remains to be seen whether this 
explanation will fit other cases of semi-sterility. 

This theory of semi-sterility of course may have 
some value in throwing light upon the problem of com- 
plete self-sterility. It seems to be of greater theoretical 

interest, however, in 
being the first attempt 
to include inheritance in 
gametophytes in an ex- 
planation of Mendelian 
results. BELLING him- 
self does not stress this 
gametophyte relation- 
ship; in fact, he does not 
use the word "gameto- 
phyte"; and yet it is 
probably this gameto- 
phyte relationship that 
represents the most 
significant feature of his 
work and will prove to be the most 'suggestive feature 
in future investigations. 

Attractive as this explanation may be, vte must 
realize, however, that as yet it merely explains semi- 
sterility and does not clear up the fundamental com- 
plexity of the more general topic of self-sterility, that is, 
why pollen fails on "own" stigmas and functions on 
foreign stigmas. 

Very recently an explanation of the more general 
problem of self-sterility has been proposed by EAST 



@@ 



FIG. 33. Diagram showing how 
the F 2 would be produced according 
to SELLING'S idea of semi-sterility. 



Self -Sterility 143 

and PARK (3). This explanation seems to fit the facts 
correctly but has certain theoretical disadvantages. 
In any event it involves such complex Mendelian con- 
ceptions that we shall not try to explain it here. One 
statement, however, which was considered in the pre- 
vious chapter again commands our attention. EAST 
and PARK state "that modern discoveries tend more 
and more to show that the sole function of the gameto- 
phytes of the angiosperms is to produce sporophytes. 
The characters which they possess appear to be wholly 
sporophytic, the factors which they carry functioning 
only after fertilization/' This statement would seem to 
throw out of court all consideration of the gametophyte 
generation and would regard as impossible BELLING'S 
theory, which is based on the direct influence of the 
germinal equipment of gametophytes upon gameto- 
phytes themselves. We will not enter into any general 
discussion of this point, but merely suggest that the 
self-sterility in the two cases might have been involved 
with distinctly different phenomena, since SELLING'S 
material showed degeneration and sometimes complete 
abortion in pollen and embryo sacs, while the material 
(Nicotiana spp.) of EAST and PARK was self-sterile 
merely because of the failure of pollen tubes. The 
hereditary mechanism of the two cases must be quite 
different. 

i. BELLING, JOHN, A study of semi-sterility. Jour. Heredity 

5^65-75. figs. 7. 1914- 
2> ? A hypothesis of semi-sterility confirmed. Jour. 

Heredity 7:552. 1916. 
3. EAST, E. M., and PARK, J. B., Studies on self-sterility. I. The 

behavior of self-sterile plants. Genetics 2:405-609. 1917. 



CHAPTER XV 
THE ENDOSPERM IN INHERITANCE 

We have dealt chiefly with inheritance in the sporo- 
phyte, in connection with which most work in plant 
genetics has been done. Brief mention has been made of 
inheritance in thej;ametophyte, in connection with which 
there has been very little work. It is natural now to 
consider inheritance in the endosperm. This classifica- 
tion raises the question as to the nature of the endo- 
sperm. In general it has been regarded as belonging 
to the gametophyte generation, but since the discovery 
of " double fertilization " in 1898 some have claimed 
that it belongs to the sporophyte generation. On the 
basis of chromosome numbers it is neither, so that there 
is also the claim that the endosperm is neither sporophyte 
nor gametophyte; at least we are justified in considering 
inheritance in endosperm as a separate topic. As might 
be inferred, endosperm shows some features characteristic 
of a gametophyte, others characteristic of a sporophyte, 
and still others peculiar to itself. Judgment as to its 
nature, therefore, will depend upon which of these 
features are emphasized. 

It is generally believed that angiosperms have been 
derived from gymnosperms, and it is natural therefore 
to explain angiosperm structures by the corresponding 
structures of gymnosperms. The gymnosperm and 
angiosperm ovules are contrasted in fig. 34, which will 
assist in the following discussion. In gymnosperms the 

144 



The Endosperm in Inheritance 



situation is clear. After the germination of the mega- 
spore, everything within the old megaspore wall is 




Gymnosperni 



Archegonium 

Egg 

Old Megaspore 

Wall, Inclosing 

emale Gametophyte' 




Angiosperm 



Egg 

Fusion Nucleus 



Old Megaspore 

Wall, Inclosing 

Female Gametophyte 



OVULE 




Gymnosperm 



Embrya 
Megaspore Wall 

Endosperm 




SEED 



Angiosperm 



Embryo 
Megaspore Wall 

Endosperm 



FIG. 34. Diagrams contrasting young ovules and mature seeds of 
gymnosperms and angiosperms. 

gametophyte tissue; fertilization^ affects the egg only, 
resulting in a sporophyte embryo. In the seed, there- 
fore, the embryo is imbedded in nutritive tissue which is 



146 Plant Genetics 

evidently the vegetative body of the female gameto- 
phyte, and this tissue is called the endosperm. 

In angiosperms, however, a new situation intro- 
duces doubt. It can be said as before that after the 
germination of the megaspore everything within the 
megaspore wall is a female gametophyte tissue, but 
it cannot be said that fertilization affects the egg only, 
for one of the sperms fuses as regularly with the fusion 
nucleus as does the other sperm with the egg. It will 
be remembered that the fusion nucleus is formed by 
two nuclei, which have migrated from each end of the 
sac, so that when the sperm enters into the fusion there 
is a triple fusion. After fertilization the fertilized egg 
of course forms the embryo sporophyte, but usually 
every nucleus of the old gametophyte disappears except 
the fertilized fusion nucleus, which then forms the 
endosperm in which the young sporophyte is imbedded. 
For this reason the fertilized fusion nucleus is usually 
called the ^jidosgerm ^nucleus. 

A comparison of angiosperm and gymnosperm seeds 
reveals the f ollowing^contrast (see fig. 34) . In the appear- 
ance of their essential structures they are exactly alike, 
and for this reason some botanists claim that the endo- 
sperm of angiosperms is the same as that of gymno- 
sperms, that is, gametophyte tissue. The opposing claim 
is that, although the gymnosperm endosperm is gameto- 
phyte tissue, the situation in angiosperms is essentially 
different. In angiosperms the endosperm does not arise 
from morphologically unmodified gametophyte tissue, as 
in gymnosperms, but entirely from the endosperm nu- 
cleus, and this nucleus is clearly the product of the fusion 
of male and female nuclei. With such an origin the endo- 



The Endosperm in Inheritance 147 

sperm nucleus is comparable with the zygote, and the 
endosperm tissue is sister to the embryo sporophyte. 
In other words, in angiosperms, embryo and endosperm 
are twins. This means that the endosperm of angio- 
sperms belongs to the sporophyte generation, although 
of course it is a distinct individual which produces no 
progeny. The embryo sporophyte is a parasite upon its 
twin and destroys it. 

It will be recognized that there is some reason for 
each of these claims. Is there any way of testing the 
claims, that is, of distinguishing between sporophyte and 
gametophyte tissue ? The cytological distinction is that 
the sporophyte is 2X tissue and the gametophyte x 
tissue. Applying this test it is found that endosperm 
tissue is neither x nor ix, but 3% as might be expected 
from the triple fusion. The conclusion involves several 
possibilities, as follows: $x is evidently nearer 2X than x, 
and therefore endosperm tissue is more like sporophyte 
tissue than gametophyte tissue; but on the other hand 
two of the #'s have come from the female gametophyte, 
and therefore two-thirds of the endosperm nucleus is 
female gametophyte tissue. On the basis of predomi- 
nance, therefore, endosperm tissue is more like female 
gametophyte tissue than anything else. Finally, there 
is a third alternative, and that is that the 3^ condition 
deserves to be set apart in a category by itself, which 
would mean that the endosperm is neither gametophyti 
nor sporophyte. 

These are the claims and the evidence as to the na- 
ture of angiosperm endosperm. Opinion is not settled, 
but the facts are clear. This prepares for a consid- 
eration of the bearing of this situation upon genetics. 



148 Plant Genetics 

The geneticist is not much concerned about the exact 
morphological or cytological nature of endosperm, but he 
is much concerned about its behavior in inheritance, and 
perhaps the phenomena of endosperm inheritance may 
help to decide whether endosperm is gametophyte or 
sporophyte or neither. 

Certain races of corn have yellow endosperm, while 
in other races it is white (colorless). If a cross is made 
with pollen from the yellow endosperm race - on the 
silks of the white endosperm race, what would the infer- 
ence be as to the result ? We could assume that yellow is 
dominant over white, since yellow is probably due to the 
presence of a factor which is lacking in the white. In 
making such a cross, therefore, we would expect a hybrid 
embryo to be formed which would show the dominant 
character of yellow endosperm when this embryo 
becomes a plant bearing ears the next season. On the 
contrary, we find that the dominant yellow character 
appears, but it appears that same year. The cross 
of course puts the yellow endosperm factor in the young 
hybrid embryo, but we cannot imagine that this embryo 
passed the character out into the endosperm that sur- 
rounds it. The real mechanism is as follows. 

Some time after this phenomenon was discovered in 
1872, it was named xtmia (in 1881), the definition of the 
term being the direct effect of foreign pollen upon the 
endosperm. At the time of its discovery the mechanism 
involved in xenia was not understood. Later, double 
fertilization was discovered, and this furnished a complete 
mechanism. A pollen grain from the yellow endosperm 
race contains two male gametes, and each gamete con- 
tains the factor for yellow endosperm. One of the 



The Endosperm in Inheritance 149 

gametes fertilizes the egg and produces a hybrid embryo, 
which, in the next generation, behaves as a heterozygote 
for yellow endosperm. ^The other male gamete fertilizes 
the fusion nucleus and produces the endosperm nucleus, 
which therefore contains the factor for yellow endosperm, 
the result being that the endosperm is yellow, although 
the ovule belongs to the white race. Xenia means, 
therefore, that the endosperm is a hybrid as well as 
the embryo, and "triple fusion" involves the trans- 
mission of hereditary characters. Fertilization of the 
fusion nucleus is just as essential as fertilization of the 
egg, and so far as inheritance is concerned the endosperm 
and embryo are sister sporophytes. 

Xenia has caused much discussion in genetics. It 
throws light upon the nature of endosperm and suggests 
that it belongs to the sporophyte generation because it is 
a product of an act of fertilization. Because of its 
behavior in inheritance geneticists would naturally 
regard the endosperm as a sporophyte, an abnormal 
sister to the embryo. 

Cases of xenia are not limited to yellow endosperm. 
Xenia appears also in the crosses between sweet and 
starchy corn. Crosses of red corn and purple corn also 
show xenia, but in this case additional details appear. 
A section of a grain of corn is shown in fig. 35. There is 
first the pericarp or "seed coat," which is the ovary 
wall, belonging to the old sporophyte, and therefore 
does not concern us. Within this is a thin aleurone 
layer, which is the outer layer of endosperm, while the 
bulk of the seed consists of the starchy endosperm. 
Since aleurone is endosperm, colors peculiar to it would 
show xenia in inheritance. This was shown in the case 



Plant Genetics 



Pericarp 



Endosperm 



of EAST'S red and purple corn, the colors being located 
in the aleurone. 

There is another phase of ibis situation to which 
attention should be called. By pollinating the silks 
of a white-grained individual with pollen from a red- 
grained individual xenia is secured, the resulting grains 
being red like those of the pollen parent. In the recip- 
rocal cross, however, that is, pollinating silks on a red- 
grained individual with 
pollen from a white- 
grained individual, a differ- 
ent result is obtained. 
The resulting grains are 
not white like those of the 
pollen parent, but red like 
those of the ovule parent. 
There is no xenia, there- 
fore, for the pollen has no 
immediate effect upon the 
developing endosperm. 
This seeming difficulty, 
however, is easily explained. When the pollen parent 
is white and the ovule parent is red, the endosperm gets 
its characters from both parents, and since red is domi- 
nant over white the resulting endosperm will be red 
because the female nuclei that entered into the triple 
fusion carried the factor for red endosperm; and there- 
fore the pollen from the white parent seemed to have 
no effect. The mechanism works in all cases, but owing 
to dominance, xenia appears only in certain cases. 
There is no need to discuss all of the Mendelian situations 
in which xenia may occur. An understanding of the 




Embryo 



Cross-Section of 
Corn-Seed 



FIG. 35. Diagram of corn seed 



The Endosperm in Inheritance 151 

fundamental situation as described should enable us 
to analyze such cases and reach a conclusion as to the 
expected results. 

A law which EAST (i) has formulated in reference to 
xenia is pertinent: "When two races differ in a single 
visible endosperm character, in which dominance is 
complete, xenia occurs only when the dominant parent 
is male (pollen parent). When the two races differ in 
a single endosperm character, in which dominance is 
incomplete, or when they differ in two characters, both 
of which are necessary for the development of the visible 
differences, in both of these cases xenia occurs when 
either parent is male/' This may be called "normal" 
xenia. What may be called "abnormal" xenia must 
now be considered. 

WEBBER (3) in 1900, in experimenting upon xenia in 
corn, uncovered some interesting abnormalities. Pollen 
from a red-grained race, applied to silks of a white- 
grained race, should result in solid red grains if xenia is 
normal. In most cases WEBBER obtained this result; 
but sometimes there appeared grains which showed 
blotches of red and white in a kind of irregular mosaic. 
For these cases he constructed an ingenious and reason- 
able explanation. 

Normally the second male nucleus fuses with the 
fusion nucleus, and the result is a solid red grain. In 
some cases, however, the second male nucleus does not 
join with the two other nuclei, but all three divide 
separately without having fused, and in such a case the 
mature grain is white with red streaks. In still other 
cases the two polar nuclei fuse, but the second male 
nucleus does not enter into the complex, and in this case 



152 Plant Genetics 

the fusion nucleus and male nucleus divide separately 
to form endosperm. WEBBER concludes that in this 
last case there will be very mucj^ white and very little 
red in the mature grain; for the fusion of the polars 
to form tKe fusion nucleus has resulted in the usual 
growth stimulus that is evident in all fusions. In the 
previous case in which all three divided separately, if 
more white than red develops, it is simply because there 
are two female polars (white-producing) to one male 
gamete (red-producing). In the latter case, however, 
the fusion of the two female polars stimulates them to 
more rapid division, so that they greatly preponderate 
over the single unstimulated male gamete, the result 
being that much white and little red appears in the 
mature grain. 

The appearance of red and white blotches rather than 
a more regular distribution of colors is explained by the 
usual method of endosperm formation. Endosperm 
formation begins with free nuclear division, the resulting 
nuclei being free in the cytoplasm of the sac. The 
cell walls, which limit them, are not formed for some 
time; sometimes not until all the nuclei have been 
formed. Before a large number of free nuclei have 
appeared they move from the central region of the sac 
and usually become placed near the wall, where free 
nuclear division continues. When walls begin to appear, 
separating the nuclei, wall formation begins at the 
periphery of the sac and extends toward the center, 
in what is called centripetal growth. This program, 
which is common in seed plants and is known to occur 
in wheat, is doubtless the program in corn. If then the 
second male gamete fails to unite with the fusion nucleus 



The Endosperm in Inheritance 153 

and each divides separately, when their progeny nuclei 
move out to the periphery of the sac the nuclei of male 
and female origin doujbtless become more or less inter- 
spersed. In their further division there would be groups 
of cells of male origin interspersed among groups of 
female origin. The result would be reel and white areas 
on the mature grain, intermingled as irregular blotches. 

Later, EAST (i) met the same phenomenon in his 
experiments. He found, however, that sometimes 
half the grain was red and half white, with a definite 
boundary line between the two areas. Such a situa- 
tion evidently did not agree altogether with WEBBER'S 
conception of the mechanism; in fact, WEBBER himself 
found similar cases and explains them as follows. When 
the migration of the nuclei from the center of the embryo 
sac to the periphery occurs, if the nuclei from the male 
gamete have remained grouped together, as might well 
occur, and the nuclei from the fusion nucleus have 
remained grouped, it is probable that in their migra- 
tion to the periphery those of one group would come 
to occupy one portion of the periphery and those of the 
other group the other portion. In other words, the 
two groups would migrate en bloc to different regions of 
the sac wall. This would lead to the production of 
grains in which approximately half the endosperm would 
resemble one parent and the other half the other parent. 

We have called these cases abnormal xenia. The 
explanation is ingenious, based upon facts known to be 
true in other seed plants. Whether these phenomena 
occur in corn or not remains to be proved, but the 
angiosperm embryo sac program is so uniform that 
we can hardly doubt it. Inasmuch as corn is probably 



154 Plant Genetics 

the most important material in plant genetics today it is 
very desirable that the histology of the sac should be 
investigated in detail. It will be realized that xenia 
supplies a very convenient index for breeders. Corn 
breeders especially must realize fully the fact of xenia, 
and they are able to work more intelligently if they under- 
stand the mechanism, since the principal work in corn 
breeding has to do with the character of the endosperm. 

Having introduced the phenomenon of double fertili- 
zation in connection with xenia, its use in explaining other 
things than the mere fact of xenia may be considered. 
It will be recalled that EAST (2) has done a large amount 
of experimental work with sweet and starchy corn. 
Careful examination enabled him to distinguish two 
distinct races of starchy corn. Both races of course had 
starchy grains, but the starch seemed to be laid clown 
in different ways in the two races. In one race the 
starch occurred in a loose powdery or floury condition, 
while in the other race it was compacted into a hard, 
flinty, or so-called corneous condition. The two races, 
therefore, may be spoken of as floury and corneous races 
of starchy corn. 

EAST made various crosses between these two races 
to discover the method of inheritance of the two endo- 
sperm characters. Naturally such characters would 
be expected to show xenia. In the following descrip- 
tion, therefore, when the F x generation is referred to, 
both the hybrid embryo and the hybrid endosperm 
surrounding it will be included. 

When EAST used the floury race as the pollen parent 
and the corneous race as the ovule parent, the F x genera- 
tion was all corneous. When he made the reciprocal 



The Endosperm in Inheritance 155 

cross (corneous pollen and floury ovule), the F t genera- 
tion was all floury. This result certainly suggests 
maternal inheritance, for in both cases it is the character 
of the ovule parent that is transmitted, while the pollen 
parent seems to have no effect. If it is assumed that 
this is a case of maternal inheritance, similar to the 
four-o'clocks of CORRENS, with their diseased cytoplasm, 
two problems are encountered: (i) to prove that this 
behavior is not due merely to parthenogenesis; (2) to 
discover the mechanism to explain maternal inheritance 
in this case. 

In the first place EAST established the fact that there 
was no possibility of parthenogenesis. Continuing his 
investigation he inbred the F x generation in each case 
and examined the F 2 progeny. If we are dealing with a 
case of maternal inheritance, what should the F 2 genera- 
tion show? It should be exactly the same as the F r 
generation, for in true maternal inheritance a race will 
go on forever breeding true to the maternal character, 
whether it is self -pollinated or cross-pollinated. If this 
had been a case of true maternal inheritance EAST 
should have obtained the following situation: 

Floury X Corneous Corneous X Floury 

N|/ ^ 

FI Corneous Floury 

i i 

F 2 Corneous Floury 

I * 

etc. etc. 

Actually, however, he obtained the following result: 

Floury X Corneous Corneous X Floury 

^ I 

F! Corneous Floury 

F 2 ^ Floury - Corneous \ Corneous | Floury 



156 Plant Genetics 

The conclusion is therefore that this is not a true case 
of maternal inheritance. EAST offers a very reasonable 
explanation of these results, based upon double fertiliza- 
tion plus the cumulative factor idea. These characters 
appear superficially to be maternal for the following 
reasons. The endosperm nuclei are 3$, 2X from the 
female and ix from the male. In the characters under 
discussion the presence of two factors always dominates 
the presence of one factor; thus corneous female crossed 
with floury male produces progeny which are all pheno- 
typically (in appearance) corneous, while floury female 
crossed with corneous male for the same reason produces 
progeny which are all phenotypically floury. The 
cumulative factor idea will be recognized. The mother 
always determines the character of the hybrid endo- 
sperm because there are always two female nuclei to 
predominate over the single male nucleus. In the 
embryo, however, this predominance does not occur, 
for there only a single female nucleus has fused with 
a single male nucleus. When this hybrid embryo 
matures, therefore, it is evident that it will produce 
gametes of two sorts, 50 per cent corneous and 50 per 
cent floury. It is evident that the female is really the 
only decisive factor, so far as the appearance of endo- 
sperm is concerned, so that the ratios appearing among 
the female gametes in the F x generation will be the 
ratios that will appear also in the F 2 endosperms. In 
other words, 50 per cent of the F 2 endosperms will be 
corneous and 50 per cent floury, no matter what may 
be the source of the pollen. It is obvious that the 
explanation of this peculiar form of apparently maternal 
inheritance depends entirely upon a clear conception of 



The Endosperm in Inheritance 157 

the phenomenon of triple fusion. On the other hand, 
this type of inheritance indicates that the triple fusion, 
so far from being an unimportant cytological process, 
is really significant. 

It may be questioned why the other endosperm 
characters are not inherited in this way, as for example 
aleurone colors, just as well as floury and corneous 
characters. The question cannot be answered satis- 
factorily, any more than it can be explained why some- 
times there is complete dominance and sometimes 
only partial dominance. All that can be said is that 
corneous and floury characters appear to be inherited in 
a different way from endosperm colors, where the 
presence of one color factor is sufficient to cause the 
complete determination of color. 

1. EAST, E. M., and HAYES, H. K., Inheritance in maize. Conn. 
Agric. Exper. Sta. Bull. no. 167. pp. 142. pis. 25. 1911. 

2. HAYES, H. K., and EAST, E. M., Further experiments on 
inheritance in maize. Conn. Agric. Exper. Sta. Bull. no. 188. 
pp. 31. pis. 7. 1915. 

3. WEBBER, HERBERT J., Xenia, or the immediate effect of 
pollen in maize. U.S. Dept. Agric. Bull. no. 22. pp. 44. 
pis. 4. 1900. 



CHAPTER XVI 
HYBRID VIGOR 

The phenomenon of hybrid vigor has been mentioned 
in a general way, but its relation to genetics remains to be 
considered. Geneticists have proposed various explana- 
tions of hybrid vigor, and some of these will be presented. 

The first record of observations on hybrid vigor is 
that of JK.OLREUTHR in 1776, who states that crossing 
results in an increase in general vegetative luxuriance 
and an increase in the facility of vegetative propagation 
and viability. Later GARTNER, discussed the same 
phenomenon but gave no important new ideas. Finally, 
hybrid vigor attracted the attention of DARWIN, who 
states that crossing hastens the time of flowering and 
maturing and increases the size of the individual. He 
adds the very important fact that it is not mere crossing 
that gives the stimulus, but crossing forms that differ 
in the constitution of their sex elements; in other words, 
crossing between individual flowers on the same plant 
gives no advantage. It is DARWIN who is responsible 
for bringing hybrid vigor to the attention of botanists, 
although the modern popular impression would be that 
BURBANK deserves this credit because of his experience 
in producing some remarkably fast growing, large, and 
vigorous hybrids. Among the modern investigators of 
hybrid vigor, the work of SHULL and EAST will be con- 
sidered, both of whom have worked upon the subject 
during the last ten years. 

158 



Hybrid Vigor 159 

SKULL'S (6) conclusions up to the year 1910 may be 
summarized as follows. His work was entirely with 
corn, and the conclusions contained some very significant 
points. 

1. "The progeny of every self-fertilized corn plant is 
of inferior size, vigor, and productiveness, as compared 
with the progeny of a normally cross-bred plant derived 
from the same source." In general this conclusion 
would be admitted by everyone, but it raised one 
question. It was known that when two races have been 
inbred for many generations they frequently "run out/' 
gradually losing their vigor. In such a case a cross 
between the two races tends to restore the original vigor. 
The remaining question, however, is whether the same 
thing may be effected by a cross between two inbred 
races which have not run out but remain in normal vigor. 
SHULL answers that hybrid vigor is exhibited when both 
parents are above the average condition as well as when 
they are below it. 

2. Another question which naturally arises is as fol- 
lows. When these crosses are made it is of course the 
F! generation that shows the hybrid vigor. If the F x 
generation is inbred, what is the status of the F 2 gen- 
eration with reference to vigor? SHULL answers this 
question in the following general way. "The decrease 
in size and vigor which accompanies self-fertilization is 
greatest in the first generation, and becomes less and less 
in each succeeding generation, until a condition is reached 
in which there is (presumably) no more loss of vigor." 
The facts involved in this statement may be represented 
in fig. 36. In this figure it can be seen clearly how 
the great loss of vigor comes immediately after self- 



i6o 



Plant Genetics 



fertilization again begins. After that self-fertilization 
brings additional loss in vigor, but this loss is less with 
each succeeding generation. It is as though a very 
definite limit were being approached and each genera- 
tion goes down one-half of the remaining distance 
toward that limit. Just why and in what way this limit 
is approached will be considered later in connection with 
the work of EAST. 



Parent 
Races 



F, 



F 7 F a etc. 



FIG. 36. Illustrating status of hybrid vigor in F x and later genera- 
tions. Vigor represented by height of rectangles. 

3. "A cross between_sibs (sister and brother) within 
a self-fertilized family shows little or no improvement 
over self-fertilization in the same family/' This, it will 
be noticed, is simply carrying a little farther the point 
that DARWFN originally discovered. DARWIN found 
that crosses between flowers on the same plant did not 
result in hybrid vigor. SHULL now finds that crosses 
between different individual plants in the same race is 
of no effect. Wejrealize that an inbred race should be 
homozygous; therefore all the individuals involved 
would have the same germinal constitution. A cross 
between any two such individuals certainly could not 



Hybrid Vigor 161 

produce a hybrid in any sense, so that it would not be 
surprising that such a cross fails to bring hybrid^vigor 

4. "A cross between plants belonging to two self- 
fertilized families results in a progeny of as great vigor 
size, and productiveness as are possessed by families 
which have never been self- fertilized/' The conclusior 
from this is that inbreeding results in no permanent 
loss of vigor. A race may "run out" if inbred con- 
tinuously, but when crossed with another race it im- 
mediately seems to regain all the original vigor. It 
is as though all germ plasm contains the potentiality 
of developing vigorous individuals. This potentiality, 
however, cannot express itself until the proper combina- 
tion of conditions arises, and this proper combination 
seems to be connected in some way with hybridizing. 

5. " Reciprocal crosses between two distinct self- 
fertilized families are equal" in producing hybrid vigor. 
When reciprocal crosses are equal it immediately sug- 
gests Mendelian segregation. Is it possible that hybrid 
vigor may be explained in terms of Mendelism ? These 
are the five "laws" of hybrid vigor developed by SHULL 
in 1910. It should be noted that they are not hypotheses 
but observed facts. The hypotheses were developed 
later when more of the facts were in. 

A practical suggestion made by SHULL in connection 
with hybrid vigor is of interest. Granted that hybrid 
vigor is an established fact, the question of its practical 
use in connection with crop plants should be taken into 
account. If a farmer after years of work has finally 
developed a desirable new strain of corn by selection, or 
isolation, or both, he is not likely to favor hybridizing 
with some other strain in any wholesale way. He must 



l62 



Plant Genetics 



preserve his pure strain at all costs. SHULL has sug- 
gested the following solution of this practical problem, as 
indicated in fig. 37. Two desirable strains (A and B) 
are developed. One small plot (I) is planted entirely 
with A, and at some distance another small plot (II) is 
planted with A and B in alternating rows. Plot I is 
used only to perpetuate strain A in pure condition. In 
plot II all the A plants are detasseled. The silks of 
these A plants, therefore, are pollinated by B pollen only, 

A B A B A B 



PLOT I PLOT 

FIG. 37. SHULL'S scheme of planting for making practical use of 
hybrid vigor in corn. 

and the resulting grains in the A rows are all bound to be 
hybrids. Using these grains as seed for the crop, hybrid 
vigor will be obtained. At the same time both A and B 
are perpetuated in the pure condition, since the B rows 
in plot II are always self -pollinated, as there is no 
other pollen in that neighborhood. This is a very 
simple solution of the problem, without necessitating 
laborious hand-pollination. 

The investigations and conclusions of EAST (2) may 
next be considered. SHULL did his work entirely with 
corn, but EAST investigated the problem in a more 
wholesale way. After assembling an extensive collec- 



Hybrid Vigor 163 

tion of data, he gives the summarizing statement that 
59 out of 85 angiosperm crosses showed a noticeable 
increase in vigor. EAST of course did not continue to 
investigate all of these 85 types, but concentrated upon 
two representatives chiefly. Corn was selected as rep- 
resenting species normally cross-fertilized in nature, 
while tobacco was used as representing those species 
generally self-fertilized in nature. EAST'S results with 
corn need not be discussed, for they confirmed in every 
point SHULL'S results. One very valuable addition, 
however, was made; namely, that some crosses resulted 
in relatively less hybrid vigor than others. For this re- 
sult EAST developed a very significant explanation, which 
has revolutionized ideas on hybrid vigor, and that is that 
hybrid vigor is proportional to the number of factors in 
which parents differ. This situation may be visualized 
from the diagram, in connection with which certain 
situations may be developed. 

Parents Fi 

AABBCCDDXAABBCCdd -AABBCCDd = little hybrid vigor 
AABBCCDDXAABBccdd = AABBCcDd = more hybrid vigor 
AABBCCDDXAAbbccdd = AABbCcDd = still more hybrid 

vigor 
AABB CCDD X aabbccdd = AaBbCcDd = most hybrid vigor 

It is the Fj generation of course that shows the vigor, 
but what index can be obtained from the germinal 
formula of the F x generation as to the amount of hybrid 
vigor that it will show ? It is evident that the index is 
that hybrid vigor is proportional to the number of 
factors in the F x generation in the heterozygous con- 
dition. Thus in the first case shown in the diagram 
there is only a single heterozygous set (Dd), and the 



164 Plant Genetics 

result is little hybrid vigor. Following down the dia- 
gram it will be noted that 2, 3, and 4 of these hetero- 
zygous sets show an increasing amount of hybrid vigor. 
These are the facts which lie at the basis of EAST'S theory 
which he calls heterozygosis. This term should not be 
confused with "heterosis," which is commonly used to 
express merely the fact of hybrid vigor. 

We shall now consider how this conception of hetero- 
zygosis explained the phenomena that SHULL had previ- 
ously discovered in reference to hybrid vigor. 

1. The fact of hybrid vigor. Heterozygosis claims 
that hybrids are vigorous because of the heterozygous 
sets. 

2. The decrease in vigor after self-fertilization begins 
again. The greatest loss in vigor comes between the F r 
and the F 2 generations. Thereafter the loss becomes 
gradually less each generation, approaching a definite 
limit when no more loss in vigor occurs. Heterozygosis 
explains this as follows: 

AABBCCDDXaabbccdd 

I 
AaBbCcDd 

In this case the F r generation is 100 per cent hetero- 
zygous, all four sets being heterozygous, and therefore it 
is very vigorous. In later generations, as is well known, 
more or less homozygous sets will be split off. Intro- 
ducing homozygous sets into some individuals will reduce 
the aggregate heterozygous condition of the whole crop 
to something less than 100 per cent; there will there- 
fore be a corresponding loss in vigor. Mathematics 
will show why this loss is greatest between the F x and the 
2 generations. Thereafter the loss gradually approaches 



Hybrid Vigor 165 

the limit when the perfectly homozygous condition is 
reached for the whole population, and then there can 
be no more loss of vigor. 

3. A cross between sister and brother effects nothing. 
-This is evident, for it introduces no heterozygosity. 

4. "A cross between plants belonging to two self- 
fertilized families results in a progeny of as great vigor, 
size, and productiveness as are possessed by families 
which have never been self -fertilized/' Heterozygosis 
explains this by showing that a cross between two pure 
lines may bring into the hybrid a maximum number 
of heterozygous sets, quite as many as are normally 
present in normally cross-fertilized families. 

5. Reciprocal crosses are equivalent. This obviously 
follows from the theory of heterozygosis. 

In connection with this work EAST was under- 
taking to discover the nature of hybrid vigor. In what 
respects are such hybrids vigorous? In reply to this 
question EAST offers the following analysis of hybrid 




it is an increase and acceleration of cell 

. 

division; in other words an increase in the power of 
assimilation. One can early observe a slight increase 
in the size of the cotyledons. The more rapid growth 
and earlier maturity of the seedlings is quite noticeable. 
Then one sees a distinct increase in the size of the roots. 
In the stem there is no increase in the number of nodes, 
but the internodal development is striking. Usually the 
stem growth is greater than the leaf growth, but the 
increase of the latter can be definitely traced. The size 
of the flower is usually not affected, nor is there any 
change in the size of small fruits, such as tobacco. In 



1 66 Plant Genetics 

fleshy fruits, such as tomato and eggplant, there is a 
marked increase. On the individual plant there are 
distinctly more flowers and fruits, and in some cases 
separate inflorescences are longer, such as the ears of 
corn. 

EAST next studied tobacco as representing those 
species which are generally self -fertilized in nature. 
It is the common impression that tobacco is a striking 
exception in the matter of hybrid vigor. In tobacco 
crosses the hybrid progeny, instead of being more 
vigorous, are frequently less vigorous than either parent. 
EAST admits that there are certain cases of this kind, 
but points out a number of other cases which are quite 
normal. As yet there is no very satisfactory explana- 
tion of the tobacco situation, and we must be satisfied 
at present with the tentative conclusion that in tobacco 
there appear certain unknown limitations to hybrid 
vigor. In general it may be said that hybrid vigor 
appears prominently in species normally cross-pollinated 
in nature and less prominently in species generally self- 
fertilized in nature. 

In this connection it should be stated that hybrid 
vigor is also manifested in some regions of the animal 
kingdom. It seems, however, that it is not so general 
a phenomenon among animals as among plants. In 
fact, many zoologists refuse to recognize in hybrid 
vigor any general law, pointing out the many cases 
among animals in which hybridizing apparently results 
in loss of vigor. It should be noted, therefore, that the 
present discussion of hybrid vigor applies primarily to 
plants and should not be extended in any general way 
to the animal kingdom. 



Hybrid Vigor 167 

Having considered the salient facts of heterozygosis, 
a statement by EAST on the "Value of heterozygosis in 
evolution" is pertinent. 

It can hardly be doubted that heterozygosis does aid in the 
development of the mechanisms whereby flowers are cross- 
fertilized. Variations must have appeared that favored cross- 
fertilization. Those plants producing a cross-fertilized progeny 
would have had more vigor than their self-fertilized relatives. 
The crossing mechanism could then have become homozygous and 
fixed, while the advantage due to cross-fertilization continued. 
But was this new mechanism an advantage ? It must have been 
often an advantage to the species as a whole. In competition 
with other species the general vigor of those which were cross- 
fertilized would aid in their survival. But the mechanism may 
not have been useful in evolving real vigor in the species, because 
of the survival of weak strains in combination. In self -fertilized 
species, new characters that weakened the individual would have 
been immediately eliminated. Only strains that stood by them- 
selves, that survived on their own merits, would have been 
retained. On the other hand, weak genotypes in cross-fertilized 
species were retained through the vigor that they exhibited when 
crossed with other genotypes. The result is, therefore, that self- 
fertilized strains that have survived competition are inherently 
stronger than cross-fertilized strains. On this account weak geno- 
types may often be isolated from a cross-fertilized species that as 
a whole is strong and hardy. 

Some recent investigations have furnished striking 
confirmation of the theory of heterozygosis. The work 
was done originally by COLLINS and KEMPTON (i), and 
later confirmed and extended by JONES (3). In brief it 
is as follows. If corn sporophytes exhibit heterozygosis, 
will the endosperm also show the same phenomenon ? 
Endosperms, as has been stated, are genetically equiva- 
lent to sporophytes in several ways. If crossing increases 



168 Plant Genetic, 

vigor and size of sporophytes, therefore, it might be 
expected to increase the size of endosperms also. 
Furthermore, endosperms have considerable advantage 
over sporophytes as material for such investigation. We 
say that hybrid sporophytes are more vigorous than 
pure-bred sporophytes, but just how much more vigor- 
ous cannot be stated with exactness. In order to 
demonstrate this clearly it would be necessary to have 
the hybrid and the pure-bred stock growing side by side 
in exactly the same conditions, but such conditions 
cannot be controlled with exactness. The environ- 
mental factors affecting the size and vigor of a corn plant 
are numerous, complex, and to a large extent uncon- 
trollable. Thus two different plants, growing side by 
side, might be in a distinctly different environment 
without the fact being recognized. It cannot be 
said, therefore, with much certainty that one plant 
shows hybrid vigor and one does not when there are 
so many other unknown factors that might affect size 
and vigor. On the other hand, if it is claimed that the 
endosperm of one grain shows hybrid vigor while the 
endosperm of the grain next to it upon the same ear 
does not, the statement would be more exact, for the 
two endosperms have developed under conditions which 
are unquestionably much more constant than the condi- 
tions surrounding the different sporophytes in a corn- 
field. 

JONES selected a plant with white endosperm and 
pollinated it with a mixture of its own pollen and pollen 
from a yellow endosperm race. In the mature ear, 
therefore, he had a mixture of yellow endosperm grains 
and white endosperm grains. The former grains of 



Hybrid Vigor 169 

course were hybrid, since the yellow factor was intro- 
duced by the foreign pollen, while the white endosperm 
grains must have resulted from "own" pollen and were 
homozygous. In this way JONES obtained side by side 
in the same ear endosperms obviously hybrid and en- 
dosperms obviously homozygous. When he weighed 
these two types he found that the hybrids exceeded the 
homozygotes in weight by from 5 to 35 per cent, or an 
average of 20 per cent. 

He then made the reciprocal cross, using a similar 
mixture of yellow and white pollen on silks of the yellow 
race. Of course all the resulting endosperms were 
yellow, but the hybrids which had the yellow factor only 
from the female side were distinctly lighter yellow than 
the homozygotes which had the yellow factor from both 
male and female sides. Weighing these two types JONES 
obtained the same results as before, the hybrids exceeding 
the others in weight by an average of 20 per cent. This 
is the clearest demonstration of heterozygosis that has 
yet been given, for the conditions of the experiment 
were ideally constant. 

The theory of heterozygosis accounting for the 
phenomenon of hybrid vigor is the one most generally 
accepted. Its claim is that hybrid vigor appears in 
proportion to the number of factors in which the parents 
of the cross differed. 

This claim may be considered briefly. Is hetero- 
zygosis really an explanation of the phenomenon of 
hybrid vigor? It seems obvious that it is not. It 
was known that hybrids are vigorous because they are 
hybrids. Heterozygosis states that hybrids are vigorous 
to the degree that their parents differed in hereditary 



170 Plant Genetics 

factors; in other words, this is merely a statement that 
hybrids are vigorous because they are hybrids, with 
the addition that the more hybrid a hybrid is the more 
vigorous it is. It follows therefore that heterozygosis is 
not an explanation of hybrid vigor, but merely a restate- 
ment of the phenomenon in Mendelian terms, with the 
additional idea that there may be various degrees of 
hybrid vigor. It is not the intention to discredit 
heterozygosis as a valuable conception, but merely to 
point out that it is not a real explanation, merely a more 
intelligent statement of the facts. Restatements should 
not be confused with explanations. 

A suggested explanation may be considered. In 
nature a " struggle for existence " occurs among species 
and individuals. There occurs also a struggle for ex- 
istence among unit characters. If a unit character 
is undesirable it is .eliminated, for the individual or 
species that carries it is eliminated. The unit charac- 
ters, therefore, that have survived and appear in the 
plants of today are for the most part desirable ones, 
although some undesirable ones also may have survived, 
having been carried through in association with the 
desirable characters. The majority of unit characters 
today, however, may certainly be regarded as desirable 
ones, and a majority is sufficient for our present con- 
sideration. 

The question may be raised as to what constitutes a 
desirable character. It nlay be any one of a number of 
things, but is there not some feature common to all 
desirable characters ? The common feature of all de- 
sirable characters would seem to be vigor. Each desir- 
able character must add somewhat to the vigor of 



Hybrid Vigor 171 

the plant that contains it, and if vigor is increased such 
things as size and productiveness will also be increased. 
Those plants, therefore, will be most vigorous which have 
in combination the greatest number of desirable char- 
acters, and it is obvious that the plants which have the 
greatest combination of such characters are hybrids. 

A diagram similar to that which was used to explain 
heterozygosis may be considered: 

p. P. F 

AABBCCDDXAABBCCdd =AABBCCDd -little hybrid vigor 
AABBCCDDXAAbbccdd =AABbCcDd~ = still more hybrid 

vigor 

In that explanation it was stated that the first case 
showed little hybrid vigor because it had only one 
heterozygous set (Del), while the other case showed 
more hybrid vigor because it had three such hetero- 
zygous sets. Hybrid vigor, therefore, appeared in 
proportion to the number of heterozygous sets in the 
hybrid. This diagram served the purpose in explaining 
heterozygosis, but it will now be discarded because 
it does not represent the most important result when 
two races are crossed. The important result is repre- 
sented in the following diagram: 

Px-AABBCCddeeff *' , i -, 

=more hybnd Vlgor 



P,-AABBccddeeff \ ATJ , , , ., . 

Pa-aabbCCDDeeff JAaBbCcDdeeff = less hybrid vigor 

The thought is that in each of these two cases the 
hybrid is more vigorous than either parent, not because 
it contains more heterozygous sets but because it 
contains more positive, dominant factors. For example, 



172 Plant Genetics 

in the first case each parent contains three factors, the 
small letters representing merely the absence of factors. 
The F! generation, therefore, contains six factors, and 
for this reason is more vigorous than either parent. It 
is stated in the diagram that in the first case there is 
"more hybrid vigor " and in the second case "less 
hybrid vigor," simply because hybrid vigor is a relative 
term. It represents merely how much more vigorous 
the hybrid is than either parent. In the first case the 
parents have three factors and the hybrid six, and the 
increase is three, which measures the amount of hybrid 
vigor. In the second case each parent has two and the 
hybrid four; the increase therefore is only two, and for 
this reason there is less hybrid vigor in the second 
case than in the first. 

Assuming that the majority of factors are desirable, 
and the desirable factors make for general vigor, it would 
follow that the most vigorous plant will be the one 
containing the greatest number of positive factors. 
It has been shown that plants that contain the greatest 
number of factors are hybrids, and for this reason 
hybrids are more vigorous. 

The following question may be raised. JTf it is 
granted that all desirable factors tend somewhat to 
increase the general vigor, do they all do this to the 
same degree? The obvious answer is in the negative, 
and this has no bearing upon the validity of the explana- 
tion. On the other hand, if heterozygosis be accepted 
for an explanation the question would present a diffi- 
culty. Heterozygosis would claim that Aa induces 
vigor, not because of any particular factor that it repre- 
sents, but because it is a heterozygous set. It seems to 



Hybrid Vigor 173 

be more reasonable and accurate to suppose that certain 
factors induce more vigor than others. EAST recognizes 
this in the following statement: "This stimulus to 
development is cumulative up to a limiting point, and 
varies directly with the number of heterozygous factors 
in the organism, although some factors may have 
more powerful action than others." 

It is obvious that the suggestion made above is that 
of a real explanation of hybrid vigor and not merely a 
restatement. It is rather an obvious explanation that 
has probably occurred to a number of geneticists. 
KEEBLE and PELLEW (5) suggested it in 1910, and 
since that time it has been somewhat discussed in the 
literature, being referred to as "the hypothesis of 
dominance (accounting for hybrid vigor).' 7 At first 
statement the theory seems sound, but actually it does 
not fit the facts. The two chief objections to this 
theory of dominance may be found in the publications 
of SHULL, EMERSON, and EAST (2). 

, f>. If hybrid vigor were due to dominance it would be 
possible in generations subsequent to the F 2 to recom- 
bine in one race all of the dominant factors. Thus 
there could be isolated a race that was " 100 per cent 
vigorous," and since it would be homozygous its vigor 
would not be lost by inbreeding. Actually, though, 
hybrid vigor cannot be fixed in this way; "all maize 
varieties lose vigor when inbred." 

2.) Experience assures us that the distribution of 
individuals in the F 2 generation with reference to hybrid 
vigor is represented graphically by a symmetrical curve, 
similar to the normal probabilities curve; the class 
containing the greatest number of individuals is that 



174 Plant Genetics 

which shows the medium amount of hybrid vigor, 
while on either side of this class the fall in the curve is 
regular, reaching its lowest points in the two small 
extreme classes which show respectively greatest hybrid 
vigor and least hybrid vigor. According to the domi- 
nance hypothesis the largest class of the F 2 individuals 
would be that showing greatest hybrid vigor, while 
the smallest class would be that showing least hybrid 
vigor. The curve representing such a situation would 
be unsymmetrical and strikingly different from that 
which actually occurs. 

For these two reasons the dominance hypothesis 
seems to have been discarded. Although it is theoreti- 
cally attractive its failure to satisfy these two important 
details of the hybrid vigor situation has condemned it. 

Very recently JONES (4) has ingeniously modified the 
dominance hypothesis so as to avoid these difficulties. 
At first consideration his theory seems to be clearly the 
most reasonable explanation of hybrid vigor that has 
yet been presented, although in time it may suffer 
from destructive criticism. The argument is essentially 
the same as that for the old dominance hypothesis, with 
the following important modification. Under the old 
hypothesis it was stated that in the cross A Abb XaaBB = 
AaBb the hybrid showed vigor because it combined 
the two dominant determiners AB. This met with the 
practical objections mentioned above. JONES visualizes 
the situation as represented in fig. 38. In this case it is 
a question of linkage. The vigor of one parent is due 
to the two dominant determiners A and D, while that 
of the other parent is due to the two dominant deter- 
miners C and B. The hybrid is more vigorous than 



Hybrid Vigor 



175 



either because it combines all four dominant determiners. 
The attractiveness of this scheme is that it escapes the 
objections that were made to the older dominance 
hypothesis. 

1. The fact that 100 per cent hybrid vigor cannot be 
fixed is quite in accordance with JONES'S scheme, for 
it is obviously impossible to isolate a homozygous race 
with all four factors A, B, C, and D. 

2. A simple mathematical demonstration will show 
that the distribution of F 2 individuals is quite what it 




FIG. 38. Diagram to aid in visualizing JONES'S explanation of 
hybrid vigor by dominance of linked factors. 

should be, represented by a symmetrical curve similar 
to the curve of probabilities. 

In fact this new theory, "the dominance of linked 
factors/ 7 seems altogether sound. We should reason- 
ably expect that each chromosome would contain one or 
more dominant determiners (conducive to vigor) linked 
with one or more recessives. 

The discussion has left the impression that hybrid 
vigor is to be explained by dominance (and linkage). In 
this day of factors and determiners such a hypothesis is 
quite appropriate. It may be, however, that in the 



176 Flant Genetics 

future such a phenomenon as hybrid vigor may be 
explained in terms of the stabilities and reactivities of 
the constituents of specific protoplasts. 

In conclusion, attention should be called to the danger 
of confusing phenomena of hybrid vigor with those of 
cumulative factors. The two situations are somewhat 
similar, but careful consideration will discover distinct 
differences. It is perhaps unsatisfactory to assume 
absolute lack of dominance in the case of cumulative 
factors and the essential presence of dominance (partial 
at least) in explaining hybrid vigor. Such assumptions, 
however, are not necessarily contradictory. The two 
things should be recognized as distinct phenomena, 
although they are at work simultaneously and probably 
interact to give complex results. 

i. COLLINS, G. N., and KEMPTON, J. H., Effects of cross-pollination 
on the size of seed in maize. U.S. Dept. Agric. Circular 124. 

r ^3- 

7 EAST, E. M., and HAYES, H. K., Heterozygosis in evolution and 
in plant breeding. U.S. Dept. Agric., Bur. PL Ind. Bull. 243. 
pp. 68. pis. 8. 1912. 

3. JONES, D. F. ; Bearing of heterosis upon double fertilization. 
Bot. Gaz. 65:324-333. figs. j. 1918. 

4. , Dominance of linked factors as a means of accounting 

for heterosis. Genetics 2:466-479. 1917. 

5. KEEBLE, F., and PELLEW, C., The mode of inheritance of 
stature and of time of flowering in peas (Pisum sativum). Ge- 
netics 1:47-56. 1910. 

6. SHULL, G. H., Hybridization methods in corn breeding. Amer. 
Breeders Mag. 1:98-107. 1910. 



CHAPTER XVII 

SEX DETERMINATION 

The subject of sex determination must be included in 
any consideration of genetics, but it is extensive enough 
as measured by the amount of investigation to form the 
subject of a book. In consequence in this connection it 
can only.be given brief consideration. The current the- 
ories of sex determination fall naturally into two general 
categories : 

1. Many biologists believe that sex is predetermined 
by the chromosome equipment that enters into the 
zygote. 

2. Other biologists believe that sex may be deter- 
mined otherwise than by chromosomes; that it may be 
determined at fertilization, or before, or even after- 
ward; in any event, that it is not the chromosomes 
that determine sex but some general physiological 
condition. 

These are the two schools in regard to sex determina- 
tion; in fact, each school includes a number of theories, 
but the fundamental idea in each group is the same. 
These two general views will be referred to as the 
Chromosome theories and the physiological theories. 
Representative examples of each will be given first, and 
since sex determination has been investigated more 
thoroughly and for a much longer time in animals than in 
plants the illustrations will be taken from animals. 

177 



178 



Plant Genetics 



CHROMOSOME THEORIES. A classic example of the 
simplest kind is found in the nematode worms. Fig. 39 
will indicate how sex is determined in this case. The 
mechanism is obvious. Both male and female have ten 



MALE 



FEMALE 



MATURE INDIVIDUALS 



ooo o 

> O 

ooo o 




Zygotes 



oo oo 

> > 

oooo 



(This type of zygote 
produces a male) 



(This type of zygote 
produces a female) 

FIG. 39. Illustrating behavior of sex chromosomes 



chromosomes to determine most of their somatic char- 
acters; but in addition there are extra chromosomes that 
determine sex, known variously as sex chromosomes, 
^chromosomes , and heterochromosomes. In this case the 
male contains only one sex chromosome while the female 
contains twg. At the reduction division, connected 
with gamete formation, the chromosome equipment 



Sex Determination 179 

is reduced one-half. It is obvious that in the female 
each egg receives one sex chromosome, and therefore 
all eggs are alike in this feature. In the male, however, 
with one sex chromosome, at the reduction division 
the solitary sex chromosome goes to one pole, leaving 
the other pole without such a chromosome. __A a 
result there are two kinds of sperms, one half containing 
a sex chromosome, the other half containing none. At 
fertilization, if an egg mates with a sperm having a sex 
chromosome the zygote ^contains two, and this results 
obviously in a female, for females are characterized by 
two sex chromosomes. -With a sperm of the other type, 
the zygote receives only one sex chromosome and must 
produce a male individual. As a result males and 
females are produced in equal numbers, and sex is 
determined by the type of sperm that enters into the 
sex fusion. 

In such cases females are homozygous for sex and 
males are heterozygous, for in the male the sex chromo- 
some is paired with its absence. This is the commonest 
situation, although it would be theoretically possible to 
have females heterozygous for sex, having the single 
chromosome, while the male had a pair. In this case 
sex would be determined by the type of egg that entered 
into the sex fusion. In addition, there are cases where 
one sex has one sex chromosome and the other sex has 
none. Finally, there are casjes in which one sex has a 
pair of large chromosomes, while the other sex has one 
large and one small sex chromosome. In all of these 
cases, however, the fundamental mechanism is the same. 
Either male or female (commonly the former) is hetero- 
zygous for sex, producing two kinds of gametes, as 



180 Plant Genetics 

shown by the chromosome situation. Sex, therefore, 
is_predetermined by the kind of gamete that enters into 
the sex fusion. 

This statement represents the chromosome theories 
of sex determination, according to which it seems evident 
that sex is absolutely predetermined. Outside conditions 
can effect no_ change, and therefore sex cannot be con- 
trolled. It results, therefore, with mathematical pre- 
cision in 50 per cent males and 50 per cent females, just 
as a Mendelian ratio comes out 3:1. , t 
{ PHYSIOLOGICAL THEORIES. In 1906 RICHARD HERT- 
WEG (5) performed some sex determination experiments 
with frogs. The eggs are laid free in the water before 
fertilization, so that they furnish unusually good material 
for such experiments. Normally the eggs are fertilized 
very soon after they are laid, with the result that the 
progeny consists of approximately 50 per cent males and 
50 per cent females. HERTWEG took some of these 
eggs and allowed them to overripen before fertilization 
took place ; that is, he put aside some eggs as soon as they 
were laid and allowed them to remain unfertilized for an 
unusually long period. While these eggs were standing 
in the water he found that they absorbed an unusual 
amount of water, and the obvious conclusion was that 
very ripe eggs show higlj water content. He then 
allowed these overripe eggs to be fertilized, and the 
resulting progeny wei^ iQo,.per ^ c^t_males. His con- 
clusion was that sex was not determined by the chromo- 
some equipment, but by the physiological conditions in 
the egg, high water content resulting in males. 

This theory was confirmed in a striking way in 
1912 by Miss KING (7), who performed the converse of 



Sex Determination 181 

HERTWEG'S experiments, using toads' eggs. Taking some 
newly laid eggs she withdrew water from them, then 
allowed them to be fertilized, and the resulting progeny 
were 90 per cent females. The obvious conclusion is that 
eggs with low water content produce females. 

Finally, there is the remarkable work of PJJDDLE (n) 
with pigeons. HERTWEG and Miss KING had found 
that sex is determined by the physiological factor of 
water content. RIDDLE has investigated a little more 
critically, analyzing the physiological conditions of both 
male and female, and gives the following contrast: 

Male Female 

High percentage of water Low percentage of water 

Low percentage of fat High percentage of fat 

Low percentage of phosphorus High percentage of phosphorus 

High metabolism Low metabolism 

It appears from this that high water, low fat, and 
low phosphorus are male attributes or conditions, 
while the female attributes are the reverse. The main 
feature of difference, however, to which the other con- 
trasting conditions are subordinate is that the male 
shows high metabolism and the female low metabolism. 
The idea is that any physiological conditions that 
affect water content or fat content or phosphorous con- 
tent in -the egg will affect the sex in the resulting 
progeny. 

Following these ideas, RIDDLE was able to^control 
sex by various means; furthermore, he makes the 
somewhat startling statement that sex is a quantitative 
phenomenon; that is, the difference between male and 
female is a difference in degree only. A diagram 
(fig. 40) will illustrate the situation. It represents a 



i8 2 Plant jpenetics 

graduated scale based on relative water, fat, and phos- 
phorous content. The egg may be at any point on the 
scale, and the sex of the individual produced by the 
egg will depend upon its position on the scale. An 
egg in any position to the left of the middle results in a 
male and to the right in a female. It should be noted, 
however, that if the egg is near one of the extremes the 
progeny will be either a very masculine male or a very 
feminine female; while if the egg lies near the middle 
point, on one side or the- other, the progeny will be a 

High H 2 I.ow H,0 

|^ w p a{ Very Male Some Female Some Male Very female jj i p flt 





Males with Females with 




Very Male 


Some Female Some Male 


Very Female 


Males 


Characteristics Characteristics 


Females 


V 


\/ 


^ 



Low P ^_ ^i* ^ ^_Jt- " lgh P 

High MALES FEMALES Low 

Metabolism Metabolism! 

FIG. 40. Illustrating RIDDLK'S idea of sex. Sexes differ only 
quantitatively, and it is possible to find various degrees of maleness 
and femaleness at different points along the scale. 

male with some female characteristics or a female with 
some male characteristics; in other words, a feminine 
male or a masculine female. In fact, RIDDLE is actually 
able to bring this about, obtaining at will males with all 
degrees of maleness, etc. He even suggested at one 
time that when we learn more about this method of sex 
control we might be able to eliminate from human 
society such undesirables as the effeminate man or the 
masculine woman. 

There are three possible conclusions in reference to 
these contradictory theories: i) an acceptance of the 
one and rejection of the other; (2) the claim that both 
amount to the same thing, that is, that they express the 



Sex Determination 183 

same fundamental facts in different terms or by the use of 
different indices; (3) the claim that both are true but 
cover different territories; that one of them explains 
certain types of cases and the other explains other types 
of cases. At present the third alternative seems to be 
the most reasonable and least difficult of explanation. 

In view of the evidence on both sides one can hardly 
say that either the chromosome theory or the physio- 
logical theory is absolutely wrong. Again, to interpret 
either theory in terms of the other is theoretically very 
difficult. The cytologists certainly present a striking 
demonstration of the chromosome theory, showing 
the sex chromosomes and the constant occurrence of the 
proper numbers in a given sex. They also show the 
machine at work with its inevitable sex product of 50-50; 
and of course they are logical in concluding that sex 
cannot be controlled. 

On the Bother hand, the physiologists show just as 
clearly that sex can be controlled artificially; by chan- 
ging the chemistry or physics of the situation they change 
the sex. This of course is cytologically unorthodox; 
and as a consequence some cytologists have attempted 
to clear up the situation by the following claim. Granted 
that certain chemical changes may change the sex, 
the result is clue to the fact that the mitotic figures are 
disturbed and the sex chromosome situation changed. 
It is always the sex chromosomes that determine sex, 
but they may rearrange themselves in consequence of 
abnormal chemical stimuli. Such an explanation seems 
like the last resort. One might imagine that the nucleus 
might cast out a sex chromosome and thus change an 
expected female into .a male ; but to make the change in 



184 Plant Genetics 

the other direction the nucleus would have to manu- 
facture an additional sex chromosome, which is difficult 
to imagine. It seems safe to conclude, therefore, that 
physiological conditions determine sex in some cases and 
sex chromosomes determine sex in other cases. 

With this background we Will consider briefly such 
work as has been done with plants. In the first place, 
the sex chromosome has not been established for 
plants. SYKES, DARLING, STRASBURGER, and others 
have searched for it diligently but with ^negative results. 
It is true that certain forms show odd chromosomes, 
such as Ginkgo, Galtonia, Melandrium, Fagopyrum, and 
Houstonia, but they have never been satisfactorily 
connected with sex. The most hopeful work has been 
done by^ALLEN (i) with Sphaerocarpus, which will be 
mentioned later. It must not be concluded, however, 
that the chromosome theory of sex determination does 
not hold for plants. Many cytologists fully expect 
sex chromosomes to be demonstrated in plants, and at 
present they point out certain indirect indications of 
the chromosome situation, such as the even separation 
of sexes at the reduction division. The theory therefore 
is firmly believed by many plant cytologists, even 
though the sex chromosome situation has been estab- 
lished in no plant. Attention may be called to the 
fact, however, that plants differ from animals in ways 
that present obstacles to the chromosome theories of 
sex. Without discussing these obstacles fully atten- 
tion may be called to two of them. 

In the first place, hermaphrpditism is very common 
in plants, and this is difficult to explain in terms of the 
sex chromosome theory. In the second place, all the 



Sex Determination i8c 

higher plants exhibit alternation of generations, a phe- 
nomenon practically unrepresented in animals. In at- 
tempting to apply the sex chromosome theory througt 
the life-history of such a plant, with its alternation ol 
generations and heterospory, great theoretical difficulties 
are encountered. In general, therefore, we reach th( 
conclusion that the sex chromosome theory has not been 
established in plants and that certain obstacles seem 
to stand in the way of its establishment. 

On the other hand, no very general physiological 
theory has been established; in short, sex determination 
in plants is not yet well understood. A few suggestive 
facts are known, however, and when more facts are 
available some general theories doubtless will be devel- 
oped. A general survey of the plant kingdom will show 
the suggestive facts that are available. 

ALGAE. No conclusive work has been done with this 
group, but some of the experimental work of KLEBS (8) 
suggests that sex may be controlled in algae. 

FUNGI. Mention will be made only of the work 
of BLAKESLEE and BURGEFF. In his experimental 
work with Mucor, BLAKESLEE (2) found in general 
three different sexual types of mycelia, two of which he 
called plus and minus strains. Although they looked 
alike in every particular, he concluded that they were 
sexually different for the following^reason. Neither 
strain by itself is capable of producing zygotes, but 
when plus and minus strains are brought together sexual 
branches from the one meet sexual branches from the 
other and produce abundant zygotes. The natural 
conclusion is that BLAKESLEE 's plus and minus strairis 
represent the male and female conditions, although the 



186 Plant Genetics 

sex cannot be distinguished. The third type of my- 
celium he called the neutral strain. Although this 
cannot produce zygotes by itself it does produce them 
when mated with either a plus or minus strain. Evi- 
dently, therefore, the sex branches from the neutral 
strain contain both male and female potentialities. 

The answer to the question as to where and how sex 
is determined in these forms involves a number of 
possibilities. When a zygote germinates, one or more 
sporangia are produced jyory early, and this raises the 
question as to the sexes developed. In Mucor itself 
the segregation of sex is evidently completed before the 
formation of the spores in the sporangium, for all the 
spores in a given sporangium are of a given strain, 
producing either male or female mycelia. The sporan- 
gium as a whole, therefore, is either male or female. In 
Phycomyces, however, a different behavior is regular. 
Tife zygote produces a sporangium, but the sporangium 
is not completely of one sex. It produces three types of 
spores: spores producing the plus strain, spores pro- 
ducing the minus strain, and spores producing the 
neutral strain. The plus strain then perpetuates 
only plus strains through its spores, which means that 
the sex is fixed in this case. The minus strain behaves 
in the same way. The neutral strain, however, pro- 
duces spores of all three types, an interesting situation, 
for it suggests the Mendelian segregation. The Mucor 
group deserves to be studied further for suggestions as 
to sex determination. 

The work of BURGEFF (3) is brief but striking. He 
took BLAKESLEE'S plus and minus strains and found 
that they always bred true to their plus and minus 



Sex Determination 187 

conditions. Then by using some remarkable technique 
he grafted parts of one strain on to the other, and the 
result was a neutral strain. This seems surprising for the 
moment, but when it is remembered that Mucor is coeno- 
cytic the result is what might be expected, for the two 
sets of nuclei can mingle freely. 

^LIVERWORTS. A great deal of experimental work 
has been clone with this group, but only one conclusive 
result has been obtained. In dioecious liverworts, as 
Marchantia, the sexes appear in about equal numbers. 
It is natural for cytologists to conclude from this that 
sex is determined at the reduction division in the forma- 
tion of spores. Thus when a mother-cell produces a 
tetrad, two of the spores would produce male gameto- 
phytes and the other two female gametophytes. This 
of course was an assumption, for no sex chromosome was 
disco vereel, nor was it known that the spores of a tetrad 
always divide in this 50-50 way as to sex. 

The assumption was strengthened by some work by 
NOLL (10). He found that gemmae from male game- 
tophytes always produce male gametophytes, while 
gemmae from female gametophytes always produce 
females. This established the fact that in the mature 
gametophyte sex has been fixed, but it did not indicate 
where it had been fixed. 

It remained for STRASBURGER (13) to answer this 
question. He selected Sphaerocarpus, which is pecul- 
iarly favorable material for such work. It is^dioecious, 
like many liverworts, but a remarkable feature is that 
the spores hang together in the tetrael. Orelinarily 
when spores are mature the tetrads are no longer dis- 
tinguishable. Sowing such free spores one may get the 



1 88 Plant Genetics 

50-50 ratio of males and females, but it cannot be 
inferred that the sexes are evenly divided in every 
tetrad; it may have been only an equal division in 
the capsule as a whole. If, for example, the upper 
half of a capsule produced all females and the lower 
half all males, sex would not have been determined at 
the reduction division, where, according to the sex 
chromosome theory, it is determined. S phaerocar pus , 
therefore, gave STRAS BURGER an opportunity to deter- 
mine this situation, for he was able to isolate mature 
individual tetrads, the four spores hanging together. 
He sowed these tetrads in separate pots, obtaining four 
gametophytes in each case. The experiment was con- 
ducted upon rather a large scale, and the results were 
strikingly constant. Two male and two female game- 
tophytes were always produced from a tetrad. This 
certainly established the fact that in this case sexes 
were evenly separated at the reduction division, and 
indicated strongly that the sex chromosome theory 
applies in this group. What these facts established for 
Sphaerocarpus has been taken for granted for the other 
dioecious liverworts. 

Recently J^LLEW (i) has claimed to have demon- 
strated the x chromosome in Sphaerocarpus. He says 
that, one large chromosome, exceeding in length and 
thickness the other chromosomes, characterizes the cells 
of the female gametophyte, while the cells of the male 
gametophyte are characterized by one very small 
chromosome. His investigation seemed to show that in 
spore formation two of the spores of the tetrad receive 
the large chromosome, and the other two the small 
chromosome. These chromosomes have not been defi- 



Sex Determination 189 

nitely connected with sex determination, but the situa- 
tion is most suggestive. 

The sex chromosome theory cannot apply, of course, 
to the monoecious liverworts, where the sexes must be 
separated at a much later period than the reduction 
division. In these cases obviously every spore contains 
the potentialities for both sexes, and the two are sepa- 
rated in connection with some of the vegetative divisions 
of the bisexual gametophyte. In some species of 
Riccia we know that a developing gametophyte pro- 
duces only antheridia for a time and at a later period 
produces archegonia only. The structural basis of this 
change is not known nor the exact point at which it 
occurs. 

MOSSES. A single very significant piece of work 
by MARCHAL (9) will be mentioned. Fjtnaria is a 
dioecious moss, and hence it may be assumed that the 
sexes are separated at the reduction division in the 
formation of spores. Each spore carries the potentiali- 
ties for one sex only; but of course the sporophyte as 
a whole before the reduction division must carry the 
potentialities for both sexes. MARCHAL, by a peculiar 
technique of his own, clipped a fragment from a young 
sporophyte and induced it to reproduce aposporously; 
that is, the sporophyte fragment produced a gameto- 
phyte directly. The fragment must have contained the 
potentialities for both sexes, since it consisted of tissue 
in which reduction division had not yet occurred. 
Logically the gametophyte should be bisexual, pro- 
ducing both antheridia and archegonia, and this was the 
result obtained. It is quite in accord with the sex 
chromosome theory and is a striking confirmation of it. 



190 Plant Genetics 

PTERIDOPHYTES. In this group we are confronted 
by a new complexity. In the preceding groups it was 
necessary to consider separately monoecious and dioe- 
cious forms, assuming that in the latter the sexes are 
separated at the reduction division. In the higher 
Pteridophytes, however, heterospory appears, and 
establishes once for all the dioecious condition in the 
gametophyte generation. In heterosporous forms there- 
fore the gametophytes are always dioecious, but it can- 
not be said that the sexes are separated at the reduction 
division. Obviously no such claim can be made, for it 
would have to assume that each tetrad consists of two 
megaspores and two microspores, a thing which prob- 
ably never occurs, unless possibly in Equisetum, which 
should be investigated. It is safe to conclude therefore 
that sex in heterosporous forms is differentiated before 
the reduction division. A megasporangium produces 
only megaspores, although in doing so the reduction 
division occurs. In Selaginella, which is perhaps the 
best example, one can distinguish a megasporangium 
from a microsporangium as soon as the mother-cells 
are formed. Clearly the sexes are separated before 
the reduction division. In attempting to explain this 
situation cytologists claim that the sexes are separated 
at one of the vegetative divisions of the sporophyte 
previous to the formation of mother- cells. This is 
merely an assumption and an extremely unsatisfactory 
one, for only in connection with the reduction division 
is there a proper mechanism for the separation of sexes 
according to the sex chromosome theory. Furthermore, 
at present there is just as good reason to believe that sex 
in Selaginella is determined by physiological conditions. 



Sex Determination 191 

Before leaving Pteridophytes the homosporous 
forms, notably the true ferns, should be considered 
briefly. In the ordinary bisexual fern prothallium, 
antheridia and archegonia appear, as was described for 
Riccia. A young prothallium can produce antheridia 
only, while a mature prothallium produces archegonia 
only. These facts are interpreted in two ways. From 
the physiological standpoint it is claimed that more 
nutrition is required for the production of archegonia 
and eggs than for the production of antheridia and 
sperms. Young prothallia, therefore, have not the 
nutritive capacity to produce archegonia and can pro- 
duce antheridia only. Mature prothallia, of course, have 
a greater nutritive capacity and can produce archegonia. 
This nutrition theory has now considerable vogue and is 
applied not only to ferns but also to other plant groups. 

Another interpretation is given by cytologists. 
They claim that the sexes are separated by some sex 
chromosome mechanism at some one of the vegetative 
divisions midway in the life of the prothallium. This 
claim is difficult to accept on account of what it involves. 
In accordance with this view a cell division occurs, so 
that thereafter only archegonia will be produced; while 
before this division only antheridia were produced. 
It is hard to understand how a cell division can affect 
the sex of its ancestors, the cells that came . before. 
This difficulty is avoided by saying that a sex chromo- 
some whose presence determines maleness drops out 
of existence at this intermediate cell division, so that 
only archegonia could be produced thereafter. The 
physiological claim certainly seems to be the more 
reasonable one in this instance. 



192 Plant Genetics 

There is an apparently easy experiment which should 
test these claims and decide between them, but as yet 
it has not been performed. Fern prothallia are known 
at times to produce proliferations which reproduce the 
parent prothallium vegetatively. If a proliferation 
could be produced on the archegonium-bearing region of 
a mature prothallium its sexual behavior should throw 
much light upon this problem. According to the physio- 
logical claim, this proliferation should first produce 
antheridia, for it will pass through a young stage of low 
nutritive capacity. According to the cytological claim 
the cells from which the proliferation arose have female 
potentialities only, and therefore the proliferation should 
never produce antheridia but only archegonia. 

SEED. PLANTS. Most of the work on sex determina- 
tion has been carried on with seed plants. Experiments 
have been performed to show the effect of manuring, 
light, temperature, and other environmental factors 
on the sex ratio. The results have been both positive 
and negative. Some conclude that sex is rigidly pre- 
determined in the seed, while others claim that it is 
determined by the later environmental conditions. 
The general results are not conclusive, so that they may 
be passed by with this brief statement. 

There are certain theories of sex determination, 
however, with which every botanist should be familiar, 
for they are an important part of botanical literature. 
They may be stated briefly. 

ORRENS (4) thought that sex was determined at the 
reduction division of the pollen mother-cell. Two pol- 
len grains are produced having female tendencies and 
two having male tendencies. The sex of the progeny, 



Sex Determination 193 

therefore, depends upon which kind of pollen grain 
functions. Such an idea, of course, would be quite in 
keeping with a sex chromosome theory, where the male 
is the heterozygote for sex. 

STRASBURGER (13) took exception to this view. He 
thought that the phylogenetic development of the seed 
plants had completely separated the male and female 
tendencies through heterospory. As a consequence, he 
would regard CORRENS' idea that half the pollen grains 
have female tendencies as impossible. STRASBURGER'S 
idea is that the pollen mother-cell develops pollen 
grains with stronger and weaker male tendencies, while 
the megaspore mother-cell develops eggs with stronger 
and weaker female tendencies. It is therefore the 
algebraic sum of the two as they meet in fertilization 
that determines the sex of the progeny. If a pollen 
grain with strong male tendencies fuses with an egg 
with weak female tendencies the progeny will be male, 
and similarly for the other combination. This flavors 
of RIDDLE'S work. Just what would happen if two 
strong male and female tendencies should meet would 
be interesting. 

STRASBURGER'S strongest evidence for this theory is 
his classic experiment with Mercurialis annua, with 
which every botanist should be familiar. Mercurialis 
may be called imperfectly dioecious. Certain plants are 
prevailingly female but bear a few weak male flowers. 
It would seem as though the female tendencies of such a 
plant are stronger than the male. When such a plant is 
inbred, using pollen from the weak male flowers on the 
stigmas of the strong female flowers, the resulting 
progeny is 100 per cent female, which is in accordance 



194 Plant Genetics 

with the theory. Other plants are prevailingly male but 
bear a few weak female flowers, and inbreeding in this 
case results in TOO per cent males. Finally, there are oc- 
casional plants which are evenly monoecious; that is, half 
of their flowers are strong males and the other half strong 
females. From inbreeding such a plant, the resulting 
progeny is 50 per cent male and 50 per cent female. It 
is obvious that from such results STRASBURGER would be 
convinced of his theory of male and female tendencies. 

Such a theory involves too much of RIDDLE'S quanti- 
tative idea to be acceptable to cytologists. They seem 
confident that sex determiners will be demonstrated 
eventually as distinct physical entities and that such 
phrases as "male and female tendencies" will disappear. 

It will be interesting to apply STRASBURGER'S theory 
to cases of parthenogenesis and vegetative apogamy and 
discover the difficulties encountered. In the first place, 
in monoecious forms it is found that a parthenogenetic 
egg produces a monoecious plant. Should it not produce 
a strictly female plant, because eggs contain only 
female tendencies ? STRASBURGER avoids this difficulty 
by saying that these parthenogenetic eggs are diploid; 
hence they are not truly eggs but merely sporophyte 
cells. The process of producing the new plant, therefore, 
is merely vegetative multiplication, which would natu- 
rally perpetuate the tendencies of the parent body. The 
parent was monoecious, and quite properly the progeny 
of a diploid egg should be monoecious. 

STRASBURGER used this same situation to confirm 
his theory. In dioecious forms parthenogenetic eggs 
should produce only female plants. As a matter of fact 
this is true, as STRASBURGER shows by a number of 



Sex Determination 195 

examples. The dioecious Thalictrum, however, behaves 
differently in cases of parthenogenesis. The progeny 
are sometimes female, but just as often they are male 
This situation deserves further investigation. There 
is no question but that STRASBURGER'S theory is in- 
genious, reasonable, and promising. 

A striking bit of work on sex determination was done 
by CIESIELSKY (14) of Lemberg, who worked with 
Cannabis sativa, the Indian hemp. When he used pollen 
fresh from the anthers the progeny were 85 per cent 
males. When he took pollen from the anthers and 
allowed it to stand for twelve hours or more before 
using it the resulting progeny were 92 per cent females. 
The conclusion is obvious that fresh pollen results in 
males and stale pollen results in females. This can be 
combineel with STRASBURGER'S idea, for one might 
expect fresh pollen to show stronger male tenelencies 
than stale pollen. CJESIELSKY'S work deserves to be 
repeated upon a more extensive scale. 

Probably the most unorthodox behavior of sex is 
that which was rather recently described in the Papaya 
(6). A few cases were describeel in which male Papaya 
trees had been cut back. The whole head of the tree 
had been cut off, eliminating all of the male flowers. 
When new branches put up from the cut trunk they 
bore in some cases hermaphrodite anel in other cases 
prevailingly female flowers. That sex could be so com- 
pletely changed by such an operation was incredible. 
Further examination showed that this result was not 
always obtained and that in fact there were only a few 
such cases on record. Most botanists today doubt the 
validity of this reporteel behavior. 



196 Plant Genetics 

We have a suggestion of the same sort of behavior in 
the case of begonias. Gardeners claim that in Begonia 
one can secure female flowers by pinching off the male 
inflorescences, female inflorescences developing later in 
the same place. One gardener, at least, has been prac- 
ticing this remarkable process for some years. 

In concluding this brief survey of sex determination 
in plants it is obvious that some of the evidence points 
to the sex chromosome theory, while other evidence 
points to some physiological theory. It is perhaps best 
at this time to conclude that one theory holds for 
certain cases and the other theory for other cases. 

This topic should not be left without mention of the 
classic experiments of CORRENS and SHULL. CORRENS 
(4) crossed the dioecious Bryonia dioica with the her- 
maphroditic B. alba, while SHULL (12) crossed Lychnis 
dioica with hermaphroditic mutants from the same. 
The sexual behavior of the progenies in the two cases 
was not identical, but both suggested a sex chromosome 
theory, with the male heterozygous for sex. The theo- 
retical explanations are so complex and dubious that they 
cannot be conveniently discussed here, but the experi- 
ments themselves represent a very significant type of 
work. 

1. ALLEN, CHARLES E., A chromosome difference correlated with 
sex differences. Science 46:466-467. 1917. 

2. BLAKESLEE, A. F., Sexual reproduction in the Mucorineae. 
Proc. Amer. Acad. 40:205-319. 1904. 

, Differentiation of sex in thallus gametophyte and 

sporophyte. Bot. Gaz. 42:161-178. 1906. 

3. BURGEFF, H., Uber Sexualitat, Variabilitat, und Verebung 
bei Phycomyces nitens. Ber. Deutsch. Bot. Gesell. 30:679- 
685. 1912. 



Sex Determination 197 

4. CORRENS, C., and GOLDSCHMIDT, R., Die Verebung u. Be- 
stimmung des Geschlcchts. Berlin. 1913. 

5. HERTWEG, R., Verhandl. Deutsch. Zool. Gesell. 1906; see 
also Biol. Centralbl. 32:1. 1912. 

6. HIGGINS, J. E., Growing melons on trees. Jour. Heredity 7: 
208-220. figs. 7. 1916. 

7. KING, H. D., Jour. Exp. Zool. 12:19. 1912. 

8. KLEBS, G., Uber Probleme der Entwicklung. Biol. Centralbl. 
24:8-9, 14-15? 257-267, 289-305, 444-614. 1904. 

9. MARCHAL, EL. et EM., Aposporie et sexualite chez les Mousses. 
I, II, III. Bull. Acad. Roy. Belgique. Cl. Sci. 1907. 765-789; 
1909. 1249-1288; 1911. 750-778. 

10. NOLL, FR., Vorlaufiger Abschluss der Versuche liber die 
Bestimmung des Geschlechts bei diocischen Pflanzen. Sitz- 
ungsber. Niederrh. Gesell. Bonn. Naturwiss. Abt. 1907. 
S. 68. 

11. RIDDLE, OSCAR, The control of the sex ratio. Jour. Wash 
Acad. Sci. 7:319-356. 1917. 

12. SHULL, G. H., Reversible sex mutants in Lychnis dioica. Bot. 
Gaz. 52:329-368. 1911. 

13. STRASBURGER, E., Versuche mit diocischen Pflanzen in 
Riichsicht auf Geschlechtsverteilung. Biol. Centralbl. 20: 
657. 1900. 

, tJber geschlechtbestimmende Ursachem. Jahrb. 

Wiss. Bot. 48:427-520. 1910. 

14. WESTER, P. J., The determination of sex. Jour. Heredity 
5:207-208. 1914. 



CHAPTER XVIII 
THE BEARERS OF HEREDITARY CHARACTERS 

The conclusion reached in the preceding chapter was 
that _sex may be determined by chromosomes, but that it 
also may be determined by physiological conditions. 
To the geneticists this suggests the following question: 
If the sex character may be determined by physiological 
conditions, why may not other plant characters be 
determined in the same way ? In other words, this 
opens up the general question of the chromosome 
hypothesis in reference to inheritance. It has been 
taken for granted in the preceding discussion that 
chromosomes are the bearers of hereditary characters, 
but is this position altogether justifiable? It will be 
well to consider some of the arguments for and against 
the claim that chromosomes are the bearers of what are 
called hereditary characters. 

ARGUMENTS IN FAVOR OF THE CHROMOSOME HYPOTHESIS 

This evidence will be considered under two heads, 
namely, that from cytology and that from breeding. 

CYTOLOGY.- (T) The program involved in the 
ordinary mitotic division is very suggestive evidence. 
We discover a very exact mechanism worked out, the 
result of which is to divide the chromosomes evenly, so 
that each daughter-nucleus receives an exactly similar 
set of chromosomes, a set exactly similar to the original 
one in the mother-nucleus. These chromosomes are 

198 



Bearers of Hereditary Characters 199 

thus carefully divided and evidently perpetuated without 
alteration. Furthermore, nothing else can be discovered 
in the cell which is so carefully divided and perpetuated 
without alteration. It is natural to conclude, therefore, 
that chromosomes are the bearers of hereditary char- 
acters, for if these bearers were anything that is not so 
carefully divided inheritance would be chaos instead 
of the remarkable cosmos it is. 

(2) The behavior of chromosomes in the reduction 
division is a remarkable piece of evidence. The fun- 
damental features of this division are so familiar that 
they need not be repeated. It is sufficient to say that 
reduction division supplies a remarkable mechanism 
which explains perfectly MENDEL'S theory of segrega- 
tion. This remarkable coincidence of the observed 
cytological facts with the preconceived theoretical 
explanation of inheritance is probably the strongest 
argument in favor of the chromosome hypothesis. 

(3) An argument may also be obtained from the 
cytology of fertilization. In the higher plants the 
contributions of male and female to the structure of 
the zygote are as follows. The egg contributes a 
nucleus plus a portion of the cytoplasm in the embryo 
sac, which may well contain many types of cell con- 
stituents. The male, however, contributes only the 
nucleus stripped of its cytoplasm. It is safe to infer 
that the male contributes quite as much to the off- 
spring as the female, at least in the matter of he r 'Ktary 
characters, so that the conclusion may be that * 
characters are carried entirely in the 

cytologists claim that the nucleus is entir 
chromatin, and since chromatin is * 



200 Plant Genetics 

which chromosomes are composed the male contributes 
only chromosomes to the progeny, and judging from 
breeding results the female contributes nothing more. 
Thus by elimination the conclusion is reached that 
chromosomes are the bearers of hereditary characters. 

(4) In addition to the foregoing arguments there 
are some minor cytological evidences that confirm the 
chromosome theory. Tetraploid sporophytes, that is, 
sporophytes with twice the normal number of chromo- 
somes, have appeared as mutants, as in Oenothera, or in 
apospory, as in MARCHAL'S mosses; such sporophytes 
are distinctly larger and more vigorous than their 
normal ancestors. This is indirect confirmation of the 
chromosome hypothesis. Many hybrids are sterile, and 
in some cases cytology shows that this sterility is the 
result of, or at least associated with, a chaotic condition 
of the chromosomes in the germinal tissue of the hybrid, 
while everything else in the hybrid seems normal. 

BREEDING. (i) MENDEL'S theory of segregation, 
which must be regarded as well established, requires that 
the bearers of hereditary characters must behave in a 
certain particular way at maturation of the gametes. 
This necessary behavior is found to occur quite regularly 
among the chromosomes at the reduction division, and 
nothing but chromosomes can be shown to behave in 
this way. 

(2) Mention has been made of MORGAN'S unique 

work r linkage and crossing over. It is only necessary 

at his unusual results are associated with 

>gularities in the behavior of chromosomes, 

trong confirmation of the chromosome 



Bearers of Hereditary Characters 201 

(3) If chromosomes bear hereditary characters, and 
there are only two pairs of chromosomes in a plant, the 
result would obviously be nothing above the dihybrid 
ratio. Although the plant might show many more 
than two Mendelian characters these characters should 
be linked into two groups and dihybrid ratios result. 
If trihybrid ratios were obtained, one should have to 
conclude that something else than two pairs of chromo- 
somes was involved. It is a mechanical necessity that 
the ratios should not exceed the number of chromosomes. 
For example, do we ever have pentahybrid ratios in a 
plant with four pairs of chromosomes? This question 
cannot be answered fully for plants, because all the plants 
used in experiments have an ample number of chromo- 
somes to account for the highest ratios discovered. In 
MORGAN'S fruit fly, however, there are actually only 
four pairs of chromosomes, and in all the great mass of 
data collected on inheritance in the fruit fly tetra- 
hybrid ratios occur regularly, but pentahybrid ratios 
never. 

(4) Finally, the fact that the chromosome hypothesis 
is coming into more and more use in plant-breeding is a 
general evidence in its favor. The plant breeder does 
not usually question whether the theory of inheritance 
he uses is true or false, but merely whether it is useful 
or barren in leading to new results or connecting up 
known facts. On this basis the chromosome hypothesis 
is proving very useful. 

ARGUMENTS AGAINST THE CHROMOSOME HYPOTHESIS 

(i) Without going into "details the general fact may 
be stated that the chromosome hypothesis does not 



2O2 Plant Genetics 

appeal to many of the most prominent physiologists and 
physiological chemists. 

(2) Physiological chemists claim that chromosomes 
in all plants are chemically identical. If chromosomes 
differ so much in the factors they carry, one should 
expect them to show some chemical differences in the 
various races of plants, but they show no such differences. 
On the other hand, there are other plant constituents 
that show a remarkable chemical specificity. Plant 
proteins, for example, are chemically different in about 
every plant species. Plant starches, also, have been 
shown to be peculiar in their chemical constitution to 
the race of plants in which they occur. It is natural 
to ask why are not these proteins and starches, with 
their remarkable specificity, more like what the carriers 
of hereditary characters ought to be than the chromo- 
somes which are chemically the same throughout the 
plant kingdom. 

(3) Such work as that of RIDDLE is directly against 
the chromosome hypothesis. If it is found that char- 
acters which were supposed to be determined by chromo- 
somes can be controlled artificially by regulating the 
physiological conditions, it makes one conclude that 
chromosomes are not of controlling importance in 
inheritance. 

(4) Finally, there is the growing impression that 
chromosomes are consequences instead of causes. 
Instead of actually determining hereditary characters, 
chromosomes may well be merely a rather useful super- 
ficial index of the hereditary situation. It may be that 
this is all the claim made for the chromosome hypothe- 
sis by many cytologists. In this connection we may 



Bearers of Hereditary Characters 203 

quote from WILSON (3): "For the present, at least, all 
the requirements of investigation are met if we think of 
the chromosomes, or that which they carry, only as 
differential factors in heredity, not as primary or exclusive 
' determiners/ 77 It is a very dominant conception in 
biology today that all plant and animal phenomena are 
to be explained ultimately on a physico-chemical basis. 
Cytology, therefore, should not be satisfied with observ- 
ing and recording the peculiar contortions of chromo- 
somes in their different phases. Such a study is justified 
only in so far as it furnishes some useful working hy- 
potheses. The ultimate thing to be striven for is to be 
able to control plants and animals artificially," and this 
can be clone only by understanding the physics and 
chemistry that underlie the chromosomes as well as all 
other life-phenomena. 

With this presentation of the arguments for and 
against the chromosome hypothesis of inheritance, a 
conclusion seems to be difficult, but a suggestion may be 
made that seems reasonable and may be useful. That 
chromosomes are the visible bearers of hereditary char- 
acters is probably true, at least in most cases upon which 
there are data. Chromosomes, however, are not the 
ultimate things; behind lies the physico-chemical 
background, and it is this which is the ultimate aim 
of biology to understand. 

Such a statement may seem unsatisfactory. It has 
been said that chromosomes determine sex in some cases 
and that physiological conditions determine sex in other 
cases; then it was inferred that the same may be true of 
other plant characters as well. Can we not unify these 
two alternative conclusions in some single theoretical 



2O4 Plant Genetics 

scheme ? Two attempts to devise such a unifying con- 
ception will be presented briefly for what they are worth. 
Both are highly theoretical. 

The first of these theories is rather simple, sketching 
a picture which is easy to visualize as a whole but is 
inexact in its detail. Sex determination may be taken 
as an example of the more general situation. The deter- 
mination of sex may be likened to a pair of balances. If 
one side goes clown a male is the result; if the other side 
goes down a female is the result. What are the weights 
that operate the balance ? They are of two kinds; one 
is represented by the sex chromosomes, the other by 
the physiological conditions. The sex chromosomes are 
very heavy weights, so that if they are put on one side of 
the balance, it would require a great many physiological 
weights on the other side to counterbalance them. 
Nevertheless, this can be done. With the heavy chro- 
mosome weights all on one side they can still be over- 
balanced by piling all of the physiological weights on 
the other. In the organisms in which the sex chromo- 
some theory seems rigidly established, it means that the 
chromosome weights are relatively heavy and cannot 
be counterbalanced by all of the physiological weights 
on the other side; or it may merely mean that all of the 
physiological weights have not yet been discovered. 
In other forms the chromosome weights are not so heavy, 
and can be overbalanced by the known physiological 
weights. 1 

It may be asked why the chromosome weights are 
imagined to be so much heavier in some forms than in 
others. The explanation may be that where the chromo- 

ir This analogy of weights has been suggested by DONCASTER (i). 



Bearers of Hereditary Characters 205 

some weights are heaviest there is rigidity in inheritance, 
but where the physiological weights are more prominent 
there is plasticity in inheritance. Speaking in terms of 
ontogeny, all embryo tissues are plastic and mature 
tissues are rigid. Applying this thought to phylogeny 
it may be assumed that young unit characters are plastic 
and may be modified by physiological conditions; while 
old unit characters are fixed, rigid, and determined by 
chromosomes, against which the influence of physiological 
conditions is feeble and avails little or nothing. 

The second theory contains more physiological 
complexities, sketching a picture which is harder to 
visualize as a whole but is more exact in its details. 
As previously mentioned, the physiologist would like 
to believe that the protoplasts of each variety of plant 
show a certain specificity in their chemical make-up. 
The physiologist finds that the cytoplasm is the seat of 
the most important physiological activities, and con- 
sequently expects to find that the cytoplasm is also in 
some way the bearer of hereditary characters. He does 
not readily agree with the ' cytologist that hereditary 
characters are limited entirely to the chromosomes. He 
cannot, however, controvert the great bulk of data, 
collected by cytologists and Mendelians, indicating that 
chromosomes are the bearers of hereditary characters. 
He must reconcile his views with these undeniable facts. 
Perhaps he may be able to do this in the following 
manner. 

In the first place how must he visualize the so- 
called "determiners" ? Physiologically they seem to 
behave more like enzymes than anything else that we 
can think of. If, then, these enzymes are transmitted 



2o6 Plant Genetics 

from generation to generation in the orderly manner 
that the Mendelian data indicate, can we conceive of 
their being located in the cytoplasm ? Enzymes dif- 
fuse rather readily through the cytoplasm; but if each 
determiner were represented by an enzyme, what a 
chaotic mixture this cytoplasm would be, with the 
numerous determiner-enzymes intermingling, interact- 
ing, and probably losing their specificity. This is not 
the orderly picture required to account for the orderly 
Mendelian results actually obtained. 

If, however, we conceive of these determiner- 
enzymes inclosed in a linear arrangement along the 
chromosomes we can paint a much more orderly picture. 
The chromosome itself is not an active substance chemi- 
cally; it is merely a framework, an " apartment house" 
for determiner-enzymes, having, however, the power of 
reproduction. Each determiner-enzyme is contained 
(and "nourished") within its own compartment. When 
the chromosome reproduces, each determiner-enzyme 
reproduces within the chromosome and is passed on, at 
all times, however, being well bound in by the walls of its 
compartment and thus kept from reacting in the cyto- 
plasmic medium which surrounds the chromosomes. 
Sooner or later, as the automatic and inevitable result 
of a natural series of physiological changes that has 
taken place in the cell dynasty of the individual plant, 
certain physiological conditions appear in the cytoplasm 
which bring about the " release" of certain of these 
determiner-enzymes, which then react in the cytoplasm 
to produce the specific plant characters of which they 
are the determiners. If all these determiner-enzymes 
had been released at once, or if they had never been con- 



Bearers of Hereditary Characters 207 

fined, chaos rather than cosmos would have resulted. 
Actually we know that cosmos exists, and our highly 
theoretical scheme at least can account for that. After 
all, what is this scheme but the "intracellular pangene- 
sis" of DE VRIES (see chapter xiv) ? 

Applying the scheme still further, we may make two 
assumptions: (i) certain unusual physiological con- 
ditions are capable of " releasing" determiner-enzymes 
earlier than they would naturally be released. Thus 
they are free to react in an unusual medium, or to inter- 
act with other determiner-enzymes which they would 
not have met in the natural course of events. In 
either event the reaction is an unusual one, and the 
final result may be the appearance of an unusual plant 
character. (2) Unusual physiological conditions may 
per se bring about unusual reactions and result in 
unusual plant characters. The determiner-enzymes 
would not be involved in this. 

These two possibilities would account for some of the 
unusual (hereditary) responses that have appeared as 
the result of unusual physiological conditions, as, for 
example, the mutants obtained by MACDOUGAL (2) when 
he injected the ovaries of Onagra with various chemicals. 
In short, such a theory may help to reconcile some of the 
apparently contradictory data of the physiological school 
and the cytological school. 

To apply this theory to the matter of sex determina- 
tion would involve certain difficulties, since sex is a 
unique character and is inherited in a unique way. It 
is quite possible, however, to overcome these theoretical 
difficulties and to make our scheme reconcile the two con- 
flicting views even on the matter of sex determination; 



208 Plant Genetics 

but it would not be appropriate to enter into the 
theoretical complications any further. In this con- 
nection, however, we may recall the work of CIESIELSKY 
(see page 195), in which it is claimed that staleness 
of pollen affects sex. Staleness probably means an 
unusual amount of oxidation and would be a typical 
instance of " unusual physiological conditions." One 
might predict that staling of pollen might be found to 
affect other plant characters as well as sex. This sug- 
gests a new field of investigation. The male gamete 
of angiosperms, however, is pretty well insulated within 
the pollen coat. If work of this kind is to be done, 
the algae and fungi would furnish much more favorable 
material (see page 130). 

These are two theoretical pictures in which both 
chromosomes and physiological conditions may play 
their roles. The latter is more complex, but at the same 
time more exact and more suggestive of further investi- 
gation. Both are highly theoretical and should not be 
confused for a moment with any of the actual known 
facts of inheritance. At best either one of them may 
help to unify the situation that has been discussed, and 
unless some such scheme is constructed it will be found 
difficult to keep one's bearings among conflicting theories 
based upon apparently conflicting facts. 

1. DONCASTER, L., The determination of sex. Cambridge Univ. 

Press. 1914. 

2. MACDOUGAL, D. T.,see Heredity and eugenics (CASTLE, COUL- 
TER, DAVENPORT, EAST, TOWER). Chicago. 1912 (p. 205). 

3. WILSON, E. B., Heredity and microscopical research. Science 
N.S. 37:814-826. 1913. 



INDEX 



INDEX 



Acquired characters, inheritance 
of, 6, 1 6 

Adzuki beans, inheritance in, 45 

Albomaculata type of Mirabilis, 98 

Aleuronc layer, inheritance of color 
in, 149 >, 

Algae: inheritance in, 121, 130; 
sex determination in, 185; 
sexual structures of, 118 

Allelomorphs, 31 

Allen, C. E., 184, 187 

Antagonistic characters, 86, 91 

Antennaria, parthenogenesis in, 
116 

Anthocyanins in inheritance, 55 

Apogamy: and true-breeding hy- 
brids, 48; vegetative, 115 

Apospory, 116, 189 

Appetency, 16 

Artificial selection, effect of, 80 

Babcock, E. B., v 

Bacteria, isolation of, 131 

Barber, M. A., 131 

Bateson, W., 40, 42, 55 

Begonia: hybrids, 47; sex in, 196 

Belling, John, 138 

Biophores, 13 

Blakeslee, A. E., 45, 185 

Blends, 43/127 

Bolley, H. L., 23, 26 

Bryonia, sex determination in, 196 

Bryophytes, inheritance in, 130 

Burbank, Luther, 47, 158 

Burgeff, H., 186 

Camerarius, discovery of sex in 
plants, 4 



Cannabis, work on, 195 

Capsella and alpine characters, 

20 

Castle, W. E., 104, 105 
Cell architecture of Weismann, 

13 

Chlorophyll inheritance, 98, 101 

Chromogens in inheritance, 55 

Chromosomes: numbers in plants, 
126; as bearers of hereditary 
characters, 198; as differential 
factors in heredity, 203; isola- 
tion of, 131; theories of sex de- 
termination, 178 

Chrysanthemum, chromosomes in, 
126 

Cicsielsky, T., 195, 208 

Clausen, R. E., v 

Collins, G. N., 167 

Complementary factors, 52, 57, 
78, 127 

Compositae, parthenogenesis in, 
116 

Continuity of germ plasm, 7 

Continuous variations, 113 

Corneous endosperm, inheritance 
of, 154 

Correlation, 96; physical and 
gamctic, 73 

Correns, C., 40, 43, 98, 192, 196 

Coupled characters, 86, 91 

Crepis, chromosomes in, 126 

Crossing over, 90 

Cross-pollination, significance of, 
135 

Crypthybrids, 48 

Cumulative factors, 58, 61, 76, 78, 
109, 128 

Czapek, P, and M. E., 55 



211 



212 



Darling, C. A., 184 

Darwin, Charles, i, 4, 14, 68, 84, 

136, 158 

Determinants, 13 
Determiner, 32, 52 
De Vries, Hugo, 2, 14, 80, 84, 

207 

Dihybrid ratio, 37 
Disease transmission, 26 
Dominance, 30, 156; failure of, 45; 

and hybrid vigor, 170 
Doncaster, L., 204 
Dose, double and single, 34 
Double fertilization, 144 
Drosophila, work on, 93 
Duplex, 42 

East, E. M., 27, 40, 46, 48, 50, 5 2 > 
6g, 80, 112, 131, 142, 151, 153, 
154, 162, 173 
Elimination factor, 24 
Embryo, sources of, 117 
Emerson, R. A., 50, 86, 173 
Endosperm, inheritance in, 144 
Engler, Arnold, 22 
Environment, effects of, 20 
Enzymes in inheritance, 55, 205 
Equisetum, sex in, 190 

Factor hypothesis, 52 
Fagopyrum, heterochromosomes 

in, 184 

Failure of dominance, 45 
Ferns: apogamy in, *n6; sex 

determination in, 191 

Fixation of hybrids, 83 

Floury endosperm, inheritance of, 

154 

Fluctuating variations, 70 
Four-o'clock, inheritance in, 44 
Fruit fly, inheritance in, 93 
Funaria, sex determination in, 189 
Functionless organs and inherit- 
ance, 25 



Fungi, sex determination in, 185 
Fusion nucleus, 146 

Gartner, J., 158 
Galton, Francis, 16, 51 

Galtonia, heterochromosomes in, 
184 

Gametes, purity of, 31 
Gametic correlation, 73 
Gametophytcs, inheritance in, 123, 

138 - 
Gemmules, 6 
Genotype, 36 
Geranium, work on, 100 
Germ cells, origin of, 18 

Germinal continuity, 6, 8; selec- 
tion, 6, 9, 68 

Ginkgo, heterochromosomes in, 

184 

Hemp, sex in, 195 
Hertweg, Richard, 180 
Heterochromosomes, 178 
Heterosis, 164 

Heterozygosis and hybrid vigor, 
164, 169 

Hetcrozygote, 35 

Hieracium, parthenogenesis in, 

116 

Higgins, J. E., ^95 
Homozygote, 35 ". 

Houstonia, heterochromosomes in, 

184 . 

Hurst, C. C., 40 
Hybrid vigor, 158 

Idants, 12 

Ids, 12 

Indian hemp, sex in, n 

Inheritance: of acqi 

acters, 6, 16; in endosperm, 144, 

in gametophytes, 138 
Inhibitory factors, 57, 58, no, 128 
Intermediate hybrids, 43