384 ANGIOSPERMS 



cisely the position of the vestigial prothallus of Gymnosperms 

 (Fig, 87, p. 374, D, prth, and E, end: and p. 376), differing 

 from it in the fact that it is only formed after fertilization. 

 We have here a case of retarded development : the degenera- 

 tion of the prothallus has gone so far that it arises, by free 

 cell-formation, long after the formation of the ovum which, 

 in both Gymnosperms and Vascular Cryptogams, is a 

 specially modified prothallial cell. 



The phyllula continues to grow and remains inclosed in 

 the megasporangium, which undergoes a corresponding in- 

 crease in size and becomes the seed. One or more seeds 

 also remain inclosed in the venter of the pistil, which grows 

 considerably and constitutes thefrm't. Finally the seeds are 

 liberated, the phyllula protrudes first its root, and then its 

 stem and cotyledons through the ruptured seed-coat, and 

 becomes the seedling plant. 



We learn from the present lesson that there is a far greater 

 uniformity of organization among the higher plants than 

 among the higher animals, not only in anatomical and 

 histological structure, but also in the fact that alternation of 

 generations is universal from Nitella and the mosses up to 

 the highest flowering plants. But as we ascend the series, 

 the gamobium sinks from the position of a conspicuous 

 leafy plant to that of a small and insignificant prothallus, 

 becoming finally so reduced as to be only recognizable as 

 such by comparison with the lower forms. 



