166 MORPHOLOGY OF ANGIOSPERMS 



division. This nuclear fusion is one of the most striking fea- 

 tures of the Angiosperms as contrasted with Gymnosperms, and 

 especially since the discovery of so-called " double-fertilization " 

 the morphological character of the endosperm of Angiosperms 

 is in question. For this reason, we have preferred to discuss 

 it apart from the gametophytic structures concerning which 

 there is no question. 



As has been said, the endosperm of Angiosperms is usually 

 derived from a fusion nucleus, the constituent members being 

 the micropylar polar nucleus, sister to the egg, and the antipo- 

 dal polar nucleus. If the current homologies are true, this 

 fusion is that of a female and a vegetative nucleus. In many 

 cases a male nucleus also joins in the structure of the primary 

 endosperm nucleus, which is then the result of a triple fusion 

 (Figs. 36, H, and 71-73). How far this male nucleus is an es- 

 sential factor in the formation of the endosperm of Angiosperms 

 is at present unknown, but the rapidly increasing number of 

 plants in which triple fusion has been observed leads to the 

 belief that it may be of general occurrence. It should also be 

 remembered that in Peperomia pellucida (Johnson 31 ) (Fig. 

 38) the primary endosperm nucleus is the result of the fusion 

 of no less than eight of the sixteen free nuclei of the embryo- 

 sac; and that in Gunnera (Schnegg 47 ) (Fig. 39) the same sort 

 of multiple fusion occurs. The fusion-nucleus, therefore, may 

 be made up of a variable number of constituents of various 

 morphological character, and hence the significance of the 

 fusion and the nature of the resulting tissue are peculiarly dim- 

 cult to interpret. 



While the fusion of these nuclei seems to result in what has 

 been called a growth-stimulus, endosperm is sometimes formed 

 without any antecedent fusion. For example, in Balanophora 

 (Treub, 16 Lotsy 26 ) the polar nuclei do not fuse, but divide 

 independently, the embryo-sac becoming filled with endosperm 

 tissue; and in Helosis (Chodat and Bernard 33 ) after the first 

 division. of the nucleus of the megaspore the chalazal nucleus 

 disintegrates so that antipodal cells, and hence an antipodal 

 polar nucleus, are not formed, the endosperm being derived en- 

 tirely from the micropylar polar nucleus. In Antennaria alpi- 

 na Juel 22 found that the polar nuclei do not fuse, although they 

 behave normally in A. dioica, as the same investigator 35 has 



