52 Plant Life and Evolution 



colorless flagellates show a close structural resem- 

 blance to the cells of such low animal types as the 

 simple sponges, and the derivation of these animals 

 from flagellate ancestors is readily conceivable. The 

 flagellates may thus be said to represent an ex- 

 tremely ancient structural type from which have 

 been developed, on the one hand colorless animal 

 forms, and on the other green plants. 



The pigment-bearing flagellates show a tendency 

 to approach the non-motile condition of the typical 

 plants. This tendency toward the immobile condi- 

 tion may perhaps be correlated with the presence of 

 chlorophyll and the resulting ability to assimilate 

 inorganic food. The colorless flagellates, not hav- 

 ing this property, would require greater power of 

 movement in their search for food; i.e., their nutri- 

 tion is typically animal. With the establishment of 

 these two great lines of development plants and 

 animals the characteristics of each become more 

 pronounced, and above the flagellates any organ- 

 ism may be easily referred either to the animal or 

 vegetable kingdom. 



Volvocales. In studying the various forms of 

 life that abound in fresh- water ponds, we often 

 find, actively swimming about, certain green cells 

 which may be solitary, or which may be united 

 in globular bodies large enough to be seen with the 

 naked eye. The best known of these organisms is 

 Volvox, whose globular body may contain thou- 

 sands of cells and is easily seen. Most of these 



