REPKODUCTION 



431 



opens a little at the apex when the archegonia are mature, 

 in order that fertilisation may be possible. 



\Vhen we pass ' to the Phanerogams 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 Phanerogams we have two types of prothallium 

 which are characteristic of the Gynmosperms and the 



FIG. 177. GERMINATION OF MEGASPORE OF Selaginella. 



arch, archegonia ; oos, oospheres; em', embryo. The spore has been ruptured 

 and the upper portion removed. 



Angiosperms respectively. Fig. 178 shows the structure in 

 the former ; the spore or embryo -sac is filled with the pro- 

 thallium, formerly called the endosperm, at the apex of 

 which are several archegonia each containing a female 

 gamete or oosphere. After fertilisation 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 



