GENETICS: E. M. EAST 
99 
some standpoint of heredity the cells of the mother plant are duplex in 
their organization: they contain N pairs. The cells of the gametes 
contain N chromosomes, one coming from each pair of the mother cell; 
but they are all parts of the mother cell and contain nothing that that 
cell did not contain. These gametic cells cannot reach the ovaries of 
flowers on the same plant because they cannot provoke the secretion of 
the direct stimulant from the somatic cells of that plant. 
All gametes having in their hereditary constitution something different 
from that of the cells of a mother plant, however, can provoke the 
proper secretion to stimulate pollen-tube growth, reach the ovary before 
the flower wilts, and produce seeds. Such differences would be very nu- 
merous in a self-sterile species where cross-fertilization must take place; 
nevertheless like hereditary complexes in different plants should be found, 
and this should account for the small percentage of cross-sterility actually 
obtained. It must be granted that this hypothesis satisfies the facts, 
but that is not all. It is admittedly a perfectly formal interpretation, 
but from a mathematical standpoint — granting the generality of Mendel- 
ian inheritance — it is the only hypothesis possible that can satisfy the 
facts. 
In conclusion it should be mentioned that the cross-pollinated pistils 
show a considerable variation in the rate of growth of individual poUen- 
tubes, though our curves of growth have been made by taking the aver- 
age rate of elongation. Is this variation a result of chance altogether or 
must one assume a differential rate of growth increasing directly with the 
constitutional differences existing between the somatic cells and the vari- 
ous gametes? If it is assumed that any constitutional difference between 
the two calls forth the secretion of the direct stimulus to growth, chance 
fertilization by gametes of every type different from that of the mother 
plant will ensue; if there is a differential rate, selective fertiHzation will 
occur. This question cannot be decided definitely at present, but two 
different lines of evidence point toward chance fertilization: 
1. Flowers from a single plant pollinated by different males show no 
decided difference in rate of fertilization. 
2. Color differences are transmitted to expected ratios. 
Further, it will be recalled that beginning with the F2 generation sister 
plants crossed together have given us our F3 and F4 populations, and that 
these F3 and F4 populations apparently have given a constantly increas- 
ing percentage of cross-sterility. This is what should be expected imder 
the theory that a small difference in germ plasm constitution is as active 
as a great difference in causing the active stimulation to pollen tube 
