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VEGETABLE PHYSIOLOGY 



same structure as in the Gymnosperms and remains 

 enclosed in the sporangium or ovule. The development 

 of the prothalliuni is different. The megaspore has a 

 single nucleus as in other cases. When germination begins 

 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. 



FIG. 180. OVULE OF Pinus, 



SHOWING THE PROTHAL- 



LIUM, end, IN THE MEGA- 

 SPOBE, mac, 



arch, archegonia. 



FIG. 181. OVULE or AN ANGIO- 



SPERM SHOWING THE MEGASPORE, 

 mac, WITH ITS PROTHALLIUM ; 

 OOS, OOSPHERE. 



At this stage the prothalliuni ceases to undergo any change 

 (fig. 181) ; 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 

 which is the result of the fusion of the polar nuclei, and is 

 called the definitive nucleus of the embryo-sac. Each nucleus 

 is surrounded by protoplasm, the egg apparatus in parti- 

 cular showing three well-defined naked or primordial cells. 

 The antipodal cells become clothed with cell-walls. There 



