THE ALG.E 165 



from the same. Hence we see that in this very simple 

 plant self -conjugation is avoided, just as self-fertilisation is 

 so often avoided among the higher plants. 



We will now see what becomes of the zygospore. It 

 soon acquires a cell-wall, and often puts out a colourless 

 root-hair (of variable length) by which it is attached to 

 the substratum. The germinating zygospore goes on 

 growing slowly for some weeks, but never attains any 

 great length, and always remains unicellular (see Fig. 

 70, F). Its contents become denser and of a dark-green 

 colour, while the cell-wall is much thickened, and now 

 the little plant enters on a stage of rest. 



It is a curious point about Ulotlirix that its dead 

 season is not the winter, but the height of summer. 

 Growth and asexual reproduction go on actively all 

 through the winter and spring, only stopping while the 

 plants are actually frozen hard. When the hot weather 

 conies, however, growth is checked, and it is especially 

 under these conditions that the zygospores are produced. 

 After a few weeks of slow growth they remain dormant, 

 until late in autumn or even into the winter. Then, 

 when cold weather returns, the plantlets produced from 

 the zygospores wake up, but they never grow into 

 Ulotlirix filaments. Their contents divide up into a 

 number of cells (from three up to about fourteen), and 

 in this case the division is simultaneous (Fig. 70, G). 

 These cells may germinate directly, or produce 

 zoospores, and thus reproduce the ordinary form of the 

 Ulothrix plant. 



We have already called attention to the close 

 similarity between the conjugating cells and the asexual 

 zoospores ; it will be remembered that in (Eclogonium 

 we already noticed how much the spermatozoids resemble 



