164 BOTANICAL GAZETTE [SEPTEMBER 
is attained by the male gametophyte, but the spore is shed. It 
should be noted that even in the case of the microspore the male 
gametophyte is usually completely organized before pollination; 
but the fact remains that the reduction does not compel reten- 
tion. It has seemed to me that this phenomenon is to be 
explained by Bower’s law of sterilization, developed in reference 
to the strobilus. This law certainly finds expression in the 
megasporangia of heterosporous pteridophytes, in which the 
sterilization of mother cells is conspicuous. This method of 
increasing the nutrition of the fertile cells is too commona 
phenomenon to need illustration; but it is a tendency that would 
seem very consistent with the development of megaspores, 
whose peculiar work holds so definite a relation to abundant 
nutrition. For this very reason high numbers of microspores 
may be continued, anda diminishing number of megaspores 
produced. This would reach its culmination in the production 
of but a single megaspore by a sporangium, and a proportionate 
increase in the size of the megaspore. With the development 
of a single spore imbedded in a sterile tissue, shedding becomes 
not only mechanically difficult, but meaningless, since the neces- 
sity of scattering a brood of gametophytes, to avoid competi- 
tion, has disappeared. It is further true that the development 
of such a spore involves nutritive supplies from numerous 
neighboring cells, and a certain amount of retention becomes 
necessary for this reason. Still further, the advantage to 4 
single megaspore in being retained, thus securing more abundant 
outside nutrition during germination, would fix the habit if any 
selective process wereat work. For these various reasons it would 
seem evident that when the sterilization of a megasporangium 
had reached its extreme limit, by organizing a single spore, 
retention is likely to follow sooner or later. If this line of 
reasoning be true, the seed habit might have been developed in 
any heterosporous line. 
With the retention of the megaspore pollination became 
necessary, but its gymnosperm expression differs in no way from 
the scattering of aerial spores in all the lower groups. The new 
