296 STRUCTURAL BOTANY 



of what the simple algal predecessor of the lowest 

 Bryophyte may possibly have been like. 



(Edogonium makes in some respects a great step 

 in advance. Here the sexual cells are perfectly differ- 

 entiated; instead of two similar conjugating zoospores, 

 we find a small moving spermatozoid and a large 

 resting ovum. The casual difference in size sometimes 

 observed in Ulothrix has here become fixed, and other 

 differences are added. Evidently there is a more perfect 

 adjustment of function here ; for while both partners are 

 still on equal terms as regards the union of their 

 nuclei, it is the female cell alone which assumes the whole 

 duty of accumulating food-supplies for the next genera- 

 tion. In order to do this most effectually, it remains at 

 rest, in connection with the vegetative body of the plant. 

 So far as the sexual division of labour is concerned, 

 (Edogonium is as far advanced as any other plant, but its 

 spermatozoids still show external signs of their origin 

 from zoospores. 



The formation of dwarf male plants in some members 

 of this group is without exact parallel in any other 

 plants, and shows how far the specialisation of the 

 sexes may go, even in simple organisms. This, however, 

 is only a special case, for, as we have seen, there are some 

 species of (Edogonium without dwarf males. 



The division of the germinating oospore into four 

 swarm-spores is an interesting fact. These spores are 

 just like the zoospores produced by the plant in its ordin- 

 ary condition. The fact that they are always formed by 

 the sexually produced resting - spore immediately on 

 germination, is an important step towards the regular 

 alternation of sexual and asexual reproduction. In this 

 respect, however, (Edogonium shows no advance on Ulothrix. 



