432 



VEGETABLE PHYSIOLOGY 



structure known as the seed. It becomes detached from 

 the parent sporophyte and disseminated in various ways. 



In the Angiosperms the formation of the seed is in the 

 main similar to the process described, but it has certain 

 peculiar features. The embryo-sac or megaspore has the 

 same structure as in the Gymnosperms and remains 

 enclosed in the sporangium or ovule. The development 

 of the prothallium is different. The megaspore has a 

 single nucleus as in other cases. When germination begins 



FIG. 178. OVULE OF Pinus, 



SHOWING THE PROTHAL- 



LIUM, end, IN THE MEGA- 

 SPORE, mac. 



arch, archegonia 



FIG. 179. OVULE OP AN ANGIO- 

 SPERM SHOWING THE MEGASPORE, 

 mac, WITH ITS PROTHALLIUM; 



OOS, OOSPHERE. 



this divides into two, one of which travels to each end of the 

 ovoid spore. Each of these gives rise by two successive 

 divisions to a group of four nuclei, and a single nucleus from 

 each group returns to the centre of the cell, where the two 

 fuse together. These are often termed the polar nuclei. 

 At this stage the prothallium ceases to undergo any change 

 (fig. 179) ; it consists of a group of three nuclei at the apex, 

 known as the egg apparatus ; another group at the base, 

 termed the antipodal cells ; and the nucleus in the centre 



