358 BOTANY 



with the egg nucleus. The second generative nucleus is discharged 

 into the cavity of the embryo-sac, and sometimes, at least, fuses 

 with the endosperm-nucleus, formed by the union of the polar nuclei. 



Homologies of the Embryo-sac 



The embryo-sac represents the macrospore of the heterosporous 

 Pteridophytes, and the structures developed within it, the gameto- 

 phyte. From a comparison of the condition found in Peperomia with 

 that of the Pteridophytes and Gymnosperms, it is probable that the 

 egg-cell represents the archegonium reduced to a single cell. In the 

 typical Angiosperms the two synergids may probably be considered 

 as also, potentially, one- celled archegonia. All of the other nuclei, 

 endosperm-nuclei, and antipodals, represent the vegetative part of 

 the gametophyte. The fusion of the nuclei preliminary to the 

 formation of the endosperm is probably a stimulus to further active 

 division, but can hardly be considered a true fertilization, as is some- 

 times done. This is also true of the fusion of the second generative 

 nucleus of the pollen-tube with the endosperm-nucleus. Some inter- 

 esting experiments have been made in hybridizing Indian-corn, which 

 show that the endosperm of the grains resulting from cross-pollina- 

 tion combines the characters of the parent plants, indicating that in 

 all probability the endosperm-nucleus had united with one of the 

 pollen-nuclei. 



The Embryo 



The embryo of the Angiosperm shows a good deal of variation. 

 It may remain undifferentiated until after germination, or it may 

 become so large as to completely fill the cavity of the ripe seed. 

 Usually, but not always, a suspensor is developed, as in the Gymno- 

 sperms. 



Polyembryony. Polyembryony, or the development of several 

 embryos from a single ovule, which is the rule in many Coniferse, is 

 unusual in Angiosperm, but there are numerous exceptions. Thus in 

 Citrus (Orange and Lemon) several embryos are not infrequently 

 found in the ripe seed. It has been shown that these extra embryos 

 arise apogamously, by a budding of the tissue surrounding the embryo- 

 sac, and the same is true in Funkia. Jeffrey (15) has described 

 in Erythronium Americanum the development of several embryos 

 from a division of the egg itself, comparable to that in the Gymno- 

 sperms. In Iris Sibinca, and some Leguminosae, polyembryony has 

 been referred to a fertilization of the synergids, and in Allium 

 odorum embryos may be developed apogamously from the antipodal 

 cells. 



