366 



THE NON-VASCULAR PLANTS 



ferentiation of cells begins among the many-celled algae and 

 this means differentiation of function. Ulothrix, a form not 

 uncommon in brooks, shows very well this differentiation 

 of cells. (See Figure 164.) You note that the lowest cell, 



which is quite dif- 

 ferent from the 

 others, serves as 

 an anchor. Other 

 cells serve espe- 

 cially in reproduc- 

 tion. Any single 

 cell which repro- 

 duces without 

 fusion with another 

 cell is called a spore. 

 Multicellular algae 

 reproduce chiefly 

 by spores. In 

 Ulothrix certain 

 cells of the fila- 

 ment, under condi- 

 tions which favor 

 this process, begin 

 to divide into 

 spores. What was 

 a vegetative pro- 

 toplast becomes a considerable number of spores. An 

 opening appears in the wall, and these spores swim out. 

 Their swimming is accomplished by the movement of their 

 cilia (singular, cilium) . A cilium is a thread-like extension 

 of protoplasm. The spores of Ulothrix have four cilia at 

 the top, The swimming spores of other algae have other 



A 

 Fig. 164. — Ulothrix. A, the base of a filament showing 

 the manner of attachment, and the differentiation of 

 the lowest cell. B, portion of a filament whose pro- 

 toplasts have formed spores ; note the two cells from 

 which the spores have already escaped. C, portion 

 of a filament showing zoospores of two sizes both be- 

 fore and after their escape into the water ; at the top 

 are shown two spores each of which has resulted from 

 the fusion of two of the smaller zoospores ; these are 

 oospores. See context. 



