50 Plant Life and Evolution 



tion of nitrogenous compounds suitable for plant 

 food. 



Blue-green Algae. Probably related to some of 

 the bacteria are certain common plants of low or- 

 ganization, called, from their color, the blue-green 

 algse (Cyanophyceae), and these, with the bacteria, 

 constitute a group of plants known as the Schi- 

 zophyta, or fission plants, as their only form of 

 propagation is by means of simple cell division. 

 Some of these blue-green algse occur as slimy, black- 

 ish green films on wet earth or on objects in the 

 water, while others give rise to jelly-like masses, 

 often of considerable size, in which the plants are 

 imbedded. Some of these organisms occur in hot 

 springs like those in the Yellowstone Park, and like 

 many of the bacteria they are able to endure tem- 

 peratures which are fatal to most plants. Like the 

 bacteria, also, the blue-green algse show a very 

 primitive cell structure, and are presumably very 

 ancient types, but it is by no means certain that 

 either these forms or the bacteria are related directly 

 to the higher plants. 



Flagellata. Many bacteria are actively motile, 

 being provided with cilia so that they resemble some- 

 what the low organisms known as the Flagellata. 

 The Flagellata are also presumably very primitive 

 organisms. In the structure of their cells they re- 

 semble the lower types of plants and animals, and 

 there is some reason to suppose that they more 

 closely resemble the progenitors of both the higher 



