212 STRUCTURAL BOTANY 



presence of such an envelope, the sexually produced fruit 

 owes its special name of cystocarp. ' 



In so far as the result of fertilisation is the production, 

 not of an obspore, but of an entire fruit, there is a certain 

 analogy between Floridese and Bryophyta ; but in the 

 former the fruit is always in organic connection with 

 the sexual plant, and can therefore scarcely be regarded 

 as a new generation ; whereas in the Mosses the sporo- 

 gonium always remains distinct from the oophyte, though 

 dependent upon it. It is not probable that the Florideae 

 have any direct relation with the Mosses, or with any of 

 the higher plants. Somewhat doubtful affinities with 

 certain green and brown Alga3 have been suggested, but, 

 as at present known to us, the Eed Seaweeds constitute 

 a very isolated and well-defined group. 



The carpospores germinate in precisely the same way 

 as the asexual tetraspores, but it is only very rarely that 

 the development of the plant has been traced far. Eed 

 seaweeds are difficult to cultivate successfully, and our 

 knowledge of their life-history is still extremely limited ; 

 so far as we know, however, there is nothing to show 

 that any regular alternation of sexual and asexual 

 individuals prevails among them. 



D. THE CYANOPHYCE.E 

 TYPE XVII. NOSTOC 



There remains to be considered a group of rather 

 obscure plants of simple structure, which resemble the 

 AlgaB in their habit and mode of life, and are therefore 

 best described in this place, though their real relationships 



