464 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
live long as a seedling. The geneticist working in certain groups knows 
this latter type of seedling very well. Oenothera material is full of illustra¬ 
tions. One type is that of a seedling which expands green cotyledons but 
the hypocotyl is totally unable to develop a root. Other forms are delicate, 
frequently etiolated seedlings which live only a few days even when nursed 
along with most particular attention to their needs of suitable soil, careful 
watering, and cooler temperatures. Perhaps some of these seedlings are 
accidents of development, but it is clear from many studies that there are 
large groups in which the inability to develop rests on a genetical basis and 
is really the expression of a degree and form of sterility. 
Thus, from forms of zygotic sterility expressed by abortive embryos in 
seed-like structures, we pass insensibly to conditions illustrated by classes 
of weak seedlings which make a start only to die sooner or later. This is 
truly a form of sterility when the behavior is due to the germinal constitution 
of the seedling, and it is impossible to draw lines sharply in the wide range 
expressed by the various degrees of impotence. Examples of this form of 
sterility are well known to every plant geneticist who follows carefully the 
fate of seedlings from hybrid material when germination is complete. It 
not infrequently happens that large groups of etiolated seedlings appear, or 
weak dwarfs of various forms, or plants which make considerable growth 
but fail to mature. Most of these products die early even with the best of 
care; very few will live under the conditions of the open garden. 
In the foregoing sketch I have endeavored to make clear the fact that 
sterility in plants is expressed in a number of very different forms and may 
operate at several points in the life history. The fact that the life history 
of plants is made up of two generations, gametophyte and sporophyte, with 
the three ‘ critical periods of chromosome segregation, fertilization, and 
embryo development frequently associated with involved physiological and 
histological conditions adds greatly to the complications of observation and 
interpretation and makes the study of sterility in plants more difficult than 
that of animals. 
With so many forms of sterility known and expressed in such various 
ways, the specific causes must be very numerous. Some of them may be 
relatively simple, as when pollen fails to function because of delay in germi¬ 
nation or slowness of tube growth. Even a breakdown at the time of the 
reduction divisions may be something of an accident when due to chance 
irregularities of chromosome distribution. On the other hand, sterility 
based on inability of two sorts of germ plasm to work in harmony, whether 
in the beginning or at the end of a life history, presents problems that are 
difficult to vision* 
The geneticist postulates, as a form of expression, lethal factors in ex¬ 
pressing results of his experimental work when sterility appears, and his 
formulae are of course justified as steps towards an understanding of 
finalities. The lethals of the geneticist are placed as genes in the chromo- 
