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 



FIG. 177. GERMINATION OF MEGASPORE OF Sdagindla. 

 arch, archegonia ; oos, 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 



