148 ORIGIN OF THE SCHIZOPHYTA 



ant upon the absorption and digestion of foods. A simple type 

 of spore production is also to be found in the majority of forms. 

 This consists merely of a slight enlargement and thickening of 

 the walls of the ordinary cells (Fig. 92, s). These simple spores 

 are able to tide the plant over unfavorable conditions and germi- 

 nate, forming new plants when conditions are again suitable. 



The Bacteria and Cyanophyceae are evidently very primitive 

 and ancient forms of life. The fact that they frequently occur 

 in hot springs and that they can endure greater extremes of 

 heat than higher plants may indicate that they are related to 

 forms that appeared upon the earth when just such conditions 

 existed and at a time when the environment was not suitable 

 for the development of higher types. It has been stated on 

 page 142 that certain bacteria are independent of organic foods 

 and that others effect a decomposition of CO a as in photosyn- 

 thesis while in the purple and red sulphur bacteria we have pig- 

 mented forms that are perhaps in a transition state to chloro- 

 phyll bearing plants. It seems not at all improbable that the 

 earliest forms of life could build up organic compounds without 

 chlorophyll and in such forms as these bacteria we have perhaps 

 an illustration of the gradual acquisition of chlorophyll as seen 

 in the higher plants. 



Subdivision 3. Diatomaceae or Diatoms. 



64. The Nature and Structure of Diatoms. — The diatoms are 

 among the most common and widely distributed plants (Fig. 

 94). While microscopic they exist in such large numbers as 

 to form the familiar brown coatings on the bottom of ponds and 

 sluggish streams and often render sticks and stones in the water 

 slippery with slimy deposits. They are widely distributed on 

 • lamp soil and in fresh and salt waters. Note should also be 

 made of the peculiar species that float upon the surface of the 

 ocean and form the larger part of that organic life known as 

 the plankton (Fig. 95). This is especially abundant in northern 

 waters and has been compared to great pasture lands since it 

 furnishes the principal food of surface feeding fishes and other 



