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 puts out a colourless root-hair 

 (of variable length) by which it is attached to the sub- 

 stratum. 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 UlotTirix 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 

 comes, however, growth is checked, and then 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 co]d weather returns, 

 the plantlets produced from the zygospores wake up, but 

 they never grow into Ulothrix 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, Gr). These cells are zoospores, and when they 

 become free they no doubt reproduce the ordinary form 

 of the Ulothrix plant, though all the stages have not yet 

 been observed. 



We have already called attention to the close 

 similarity between the conjugating cells and the asexual 

 zoospores ; it will be remembered that in (Edogonium 

 we already noticed how much the spermatozoids resemble 



