THE VASCULAR CRYPTOGAMS 37 



homologies of the seed. One part of the seed, the testa, 

 is not represented in Selaginella, for the megasporangium 

 is without integuments. The megasporangium itself 

 corresponds to the nucellus of an ovule, as we have 

 already seen, but it never develops into anything of the 

 nature of a seed. This is because the megaspores are 

 normally set free from the megasporangium before fertili- 

 sation takes place, so that the seed-stage is never reached. 

 The megaspore, when it is filled with prothallus and 

 contains an embryo, bears a certain resemblance to a 

 seed, but there is no complete homology ; for, as we 

 have already seen, the development shows that the mega- 

 spore is homologous with the embryo-sac only. 



The typical seed, such as we find in the higher Flowering 

 Plants, represents a persistent, integurnented megasporan- 

 gium, containing a single megaspore, which produces a 

 prothallus, and, after fertilisation, an embryo, while still 

 in situ. The shedding of the seed thus corresponds to the 

 detachment of the entire megasporangium, together with 

 its integument and contents. Bodies closely analogous 

 with seeds are found in some fossil Lycopods allied to 

 Selaginella, but the true seed of the higher plants appears 

 to have been evolved among the Pteridosperms, Palaeozoic 

 Seed -plants allied to the Ferns. The primitive seeds, 

 however, probably only developed an embryo when ger- 

 mination set in, as sometimes happens in Cycads at the 

 present day. 



TYPE V 



THE MALE FERN (Aspidium Filix-Mas, L.) 



The Ferns are a vast group, enormously outnumbering 

 all the other Vascular Cryptogams put together. The order 

 in the widest sense includes at least sixty genera and three 

 thousand species. In our own native Flora seventeen 



