of Fertilization in Angiosperms . 701 
postpone the formation of an endosperm until fertilization is 
accomplished in order to avoid the waste of material which 
must occur if this massive tissue were thrown off by the plant 
with every unfertilized ovule. This is a very probable ex¬ 
planation, but it does not affect the morphological question 
at all. For the plant in need of a food-tissue, to be formed 
after fertilization, is as capable of adapting a modified second 
embryo to that purpose as of postponing the formation of 
a prothallus. In the latter case indeed the expedient of a 
second conjugation to stimulate development appears cum¬ 
brous. In some species of Gnetum ( Gn . Rumphianum and 
Gn. ovalifolium , Karsten 28), the formation of a prothallus 
begins before fertilization, is checked for a time, and proceeds 
after fertilization, without any fusion between a generative and 
a prothallial nucleus. Both generative nuclei in fact conjugate 
with egg-nuclei, and form two rudimentary embryos. Here 
the stimulus which excites the prothallus to renewed activity 
must be carried in some other way. 
In short, the problem from the morphological point of view 
is this. We have to account historically for the endosperm 
of Angiosperms. If it arose from a belated formation of 
prothallus, we must trace the origin of the triple nuclear 
fusion which precedes its development. Such a fusion is 
an interpolation in the history of the prothallus which needs 
explanation. If, on the other hand, the endosperm is a 
modified embryo, derived from a fertilized egg-cell, we have 
to account for the interference of the lower polar nucleus with 
the act of fertilization, and for the subsequent development of 
a body unlike a normal embryo. 
As regards the last point it is perhaps worth notice that the 
common mode of endosperm-formation among Angiosperms 
recalls that of the pro-embryo in Cycas circinalis (Treub, 39, 
p. 5 , figs. 9 , 12 , 13 , PI. I) and Stangeria paradoxa (Lang, 28 a , 
p. 293 ). That the pro-embryo is formed within an archego- 
nium, and the endosperm within the embryo-sac, is of less 
importance than appears at first, for the archegonium is 
completely suppressed among Angiosperms. 
