40 PLANT LIFE 
CHAPTER IV 
THE CELLS AND THE ORGANISM 
TURNING from the flattened forms to the 
filamentous types of alge, 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 Hydrurus fetidus (Fig. 5). It 
is an aquatic plant, rare in Britain, but fairly 
abundant in many Alpine rills near the 
melting snow. 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 
