476 ANGIOSPERMS LESS, xxxiv 



formed is called the endosperm (F, end\ and occupies pre- 

 cisely the position of the vestigial prothallus of Gymnosperms 

 (Fig. 119, p. 453, v,prth, and E, end: and p. 458), differing 

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

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

 tion of the prothallus has gone so far that it arises long 

 after the formation of the ovum which, in both Gymnosperms 

 and Vascular Cryptogams, is a specially modified prothallial 

 cell. As a rule the tissue of the nucellus disappears as the 

 embryo grows, but in some cases, e.g., the water-lily, it is 

 retained, forming an additional store of nutrient material 

 and called the perisperm (Fig. 126, F, per}. 



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 the fruit. 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 this and the two preceding lessons that 

 there is a far greater uniformity of organisation 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 mosses up 

 to the highest flowering plants. But as we ascend the 

 series, the gamobium sinks from the position of a conspicu- 

 ous leafy plant to that of a small and insignificant prothallus, 

 becoming finally so reduced as to be recognisable as such 

 only by comparison with the lower forms. 



