440 VEGETABLE PHYSIOLOGY 
opens a little at the apex when the archegonia are mature, 
in order that fertilisation may be possible. 
When we pass to the Spermophytes two further advances 
may be seen. The spore never escapes from the sporangium, 
and the prothallium does not emerge even in part from the 
spore, which does not open. In these plants the megaspore 
is represented by the cell known formerly as the embryo-sac, 
the sporangium being the ovule. Among the Spermophytes 
we have two types of prothallium which are characteristic 
Fic. 177.—Grrmtnation or Mraaspore or Selaginella. 
arch, archegonia; 008, oospheres; em’, embryo. The spore has 
been ruptured and the upper portion removed. 
of the Gymnosperms and the Angiosperms respectively. 
Fig. 178 shows the structure in the former; the spore or 
embryo-sac is filled with the prothallium, formerly called 
the endosperm, at the apex of which are several archegonia, 
each containing a female gamete or oosphere. After fertili- 
sation the resulting zygote gives rise to a young sporophyte 
or embryo, which becomes embedded in the endosperm. 
The structure thus formed consisting of the sporangium or 
ovule, with the solitary spore it contains, the latter having 
in its interior the embryo surrounded by the prothallus, 
constitutes the structure known as the seed. It becomes 
