SPERMATOPHYTA 387 



(1) Five successive nuclear divisions intervene between the megaspore 

 mother cell and the formation of the egg. (2) The embryo sac is derived 

 from a single megaspore, the innermost one. Numerous deviations from 

 this type of development are seen throughout the angiosperms. The prin- 

 cipal ones are as follows (Fig. 332) : 



Oenothera. In Oenothera and other members of the Onagraceae, a 

 linear tetrad of megaspores is formed in the usual way but, with few 

 exceptions, the outermost (micropylar) megaspore, rather than the inner- 

 most (chalazal) one develops into the embryo sac. Only four successive 

 nuclear divisions intervene between the megaspore mother cell and the 

 egg. The nucleus of the functional megaspore gives rise to two nuclei, 

 both of which remain at the micropylar end of the embryo sac, a vacuole 

 appearing below them. Each again divides and, of the four nuclei thus 

 formed, three are organized into the egg apparatus, while the fourth 

 becomes a polar nucleus. Because of the absence of a fifth nuclear divi- 

 sion, there is no second polar nucleus and there are no antipodals. 



Allium. This type of embryo sac occurs not only in certain other 

 genera of Liliaceae, such as Scilla and Trillium, but also in numerous 

 genera belonging to many other families. The megaspore mother cell 

 divides into two cells of which the upper one soon degenerates, while the 

 lower one undergoes three successive free-nuclear divisions to form the 

 embryo sac. When mature, this displays the usual kind of eight-nucleate 

 organization. It is apparent that only four nuclear divisions occur 

 between the megaspore mother cell and the egg and that two megaspore 

 nuclei participate in the formation of the embryo sac. 



Peperomia. In this genus, one of the Piperaceae, all four megaspore 

 nuclei are involved in the formation of the embryo sac, no walls being 

 formed between them. The four nuclei are arranged in a cross-like man- 

 ner, with a large vacuole between them; two successive nuclear divisions 

 follow, the resulting 16 free nuclei being arranged in various ways, 

 depending on the species. The egg apparatus consists of the egg and a 

 single synergid. In Peperomia pellucida eight nuclei form the fusion 

 nucleus and six degenerate, while in Peperomia hispidula 14 nuclei form 

 the fusion nucleus. 



A^arious modifications of the Peperomia type are seen in certain other 

 families. Thus, in Gunnera, three nuclei form the egg apparatus, seven 

 unite to form the fusion nucleus, and six degenerate. In the Penaeaceae 

 and certain species of Euphorbia the 16 free nuclei are arranged in four 

 groups of four each. One member of each group becomes a polar nucleus, 

 the four polar nuclei fuse, while the three remaining nuclei in each group 

 become organized as cells. The three cells at the upper end of the embryo 

 sac constitute the egg apparatus, the other cells finally degenerating. 



