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 
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Fic. 178.—OvuLe or Pinus, Fic. 179.—OvuLE oF AN ANGIO- 
SHOWING THE PROTHAL- SPERM SHOWING THE MEGASPoRE, 
LIUM, end, IN THE MuGa- mac, WITH ITS PROTHALLIUM; 
SPORE, mac. 008, OOSPHERE. 
arch, archegonia ~ 
this divides into two, one of which travels to each end of the 
ovoid spore. Hach 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 
