DEVELOPMENT OF PLANTS 167 



from the other cells and frequently possess for a time a slight 

 motility that enables them to move away from the parent fila- 

 ment (Fig. 92, hr). In some species one or several rather large, 

 colorless cells, heterocysts, appear in the filament and may serve 

 to separate it into hormogonia (Fig. 92, h). In Oscillatoria 

 (Fig. 93) the filaments into which these hormogonia grow per- 

 manently retain their power of motion. No cilia have been 

 observed for a certainty in any of these forms and the cause of 

 the motion has been ascribed to the interchange of fluids attend- 

 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 162 that certain bacteria are independent of organic foods 

 and that they affect the decomposition of CO2 as in photosyn- 

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

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

 phyll-bearing plants. It seems altogether probable 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 a tendency towards the acquisition of chloro- 

 phyll 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 



