The Lower Plants 63 



pendent developmental line, derived from some free- 

 swimming type, perhaps allied to the Flagellata. 

 The zoospores or asexual reproductive cells of these 

 plants differ from those of the green algae in hav- 

 ing cilia laterally inserted and, of course, possessing 

 a brown pigment. Among the lower alga-like 

 forms are certain types known as Peridineae, the 

 simpler forms of which are not unlike the zoospores 

 of the brown algae, having like them two laterally 

 placed cilia, and it is possible that the beginning of 

 the line which culminates in the great kelps and 

 rock-weeds is to be found in forms resembling the 

 simpler Peridineae, which in turn are presumably 

 allied to the flagellates. 



Another group which is sometimes associated 

 with the flagellates is that of the Diatoms, which 

 offer a large and widely distributed assemblage 

 of unicellular plants, occurring everywhere, both 

 in fresh and salt water. These diatoms, together 

 with the Peridineae, are the most important con- 

 stituents of the floating vegetation of the 

 sea, or the " plankton " upon which the animal life 

 of the ocean very largely depends. If any relation- 

 ship really exists between the diatoms and the 

 higher Phaeophyceae it must be extremely remote, the 

 diatoms themselves giving some evidence that they 

 are a highly specialized group of comparatively re- 

 cent origin. 



The Red Algae. The very characteristic red 

 algae, while they comprise a majority of the sea- 



