EEPEODUCTION 



431 



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 Gymnosperms and the 

 Angiosperms respectively. Fig. 180 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 



FIG. 179. GERMINATION OF MEGASPORE OF Selaginella. 



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

 and the upper portion removed. 



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 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 



