396 MORPHOLOGY OF GYMNOSPERMS 



cells. As it disorganizes, an irregular group of spindles first appears, 

 and then a number of minute cells (fig. 446). This ephemeral group 

 of cells is produced probably by division of the non-functioning male 

 nucleus, or it may be the joint product of the chromatin of this nucleus 

 and of some of the jacket nuclei. In any event, its appearance in 

 connection with the "second male nucleus" has suggested that it 

 may be the physiological equivalent of the endosperm of angiosperms. 

 The formation of the embryo of Ephedra trijurca occurs as follows 

 (23). The fusion nucleus gives rise to eight free nuclei, more or less 

 unequal in size, and three to five of the largest enter into the organiza- 

 tion of walled cells that can produce embryos (fig. 446). The free 

 nuclei are usually irregularly distributed through the egg, or they may 

 form a row through the middle. The proembryonal cells are isolated 

 from the surrounding cytoplasm by the development of cleavage 

 cracks, which finally carve out a more or less irregular sheath of cyto- 

 plasm for each nucleus (figs. 447, 448). In general the lower cells are 

 successful in producing embryos, but sometimes only the upper (micro- 

 pylar) ones are successful. Each functioning proembryonal cell 

 rounds out, and its nucleus divides. The two daughter nuclei are 

 at first equal, but one soon becomes noticeably larger than the other. 

 As a result of vacuolation the two nuclei become placed on one side 

 of the cell (if it is one of the upper cells), or on the bottom of the cell 

 (if it is the lowest one in the egg). Two papillae are put out by the 

 wall adjacent to the two nuclei, but the papilla related to the larger 

 nucleus elongates rapidly to form the suspensor and the other one 

 disappears (fig. 449). In one case four nuclei were seen in a pro- 

 embryonal cell. The suspensor develops directly toward the antipo- 

 dal region if the proembryonal cell is the lowest one, but it advances 

 outward and then downward if it is developed by one of the upper 

 cells. The larger nucleus passes nearly to the end of the tube, and a 

 cleavage plate, beginning at the wall, cuts it off in a terminal embryo- 

 forming cell (figs. 450, 451). The other nucleus now enters the 

 suspensor and functions as its nucleus, remaining at the lower end 

 until it disintegrates. The suspensor elongates rapidly, thrusting 

 the embryo-forming cell deep into the endosperm and remaining 

 turgid until the secondary suspensor appears, when it collapses. The 

 longest suspensor observed measured 3 . i mm. in length. 



