40 PLANT LIFE 



CHAPTER IV 



THE CELLS AND THE ORGANISM 



TURNING from the flattened forms to the 

 filamentous types of algae, we find a great 

 variety of forms, accompanied by a very 

 different degree of autonomy in the constituent 

 cells of the filamentous body. Moreover, we 

 see very clearly that closely analogous forms 

 have been reached by several evolutionary 

 routes. In other words, much the same 

 kind of organisation may have been arrived 

 at by plants which have descended from 

 several diverse simple stocks. This con- 

 vergence of type, or analogous similarity 

 between remotely related forms, is of fairly 

 wide occurrence both in animals and in plants, 

 nor is it by any means restricted to the 

 simpler members of either kingdom. 



Our first illustration of an alga organised 

 on the filamentous plan is afforded by the 

 species known as Hy drums jcetidus (Fig. 5). It 

 is an aquatic plant, rare in Britain, but fairly 

 abundant in many Alpine rills near the 

 melting s-now. The reason of this is that the 

 alga only thrives at a low temperature, soon 

 perishing in water above 12 C. (= about 

 54 F.). The plants are rather plumosely 



