154 CLASSIFICATION OF THE ALGAE 



remain in a dormant condition until conditions are favorable for 

 growth. The gametospore is often called a resting spore for 

 this reason. 



So rapidly do the diatoms multiply that the larger part of the 

 soil and rocks in many localities is composed of their remains. 

 The constant casting off of the valves through reproduction or 

 death results in vast deposits, known as silicious earth, on the 

 bottom of ponds and in the sea. Beds of silicious earth formed 

 in this way are often seen in districts where ponds and lakes 

 have dried up. Earth of this nature is used as polishing powders 

 and as absorbents in the manufacture of some explosives. 



Subdivision 4. Euphyceae or Algae 



65. General Features. This subdivision includes a large num- 

 ber of plants that live chiefly in fresh or salt water.' These plants, 

 popularly known as Algae, vary greatly in form and structure 

 and range from microscopic unicellular forms to some of the 

 largest and most highly constructed plants found among the 

 Thallophyta. Their advance over preceding groups appears es- 

 pecially in their well-marked walls and distinct plastids and 

 nuclei (Figs. 100, 104). Chlorophyll is always present in the 

 cells, although in certain groups it is masked by brown or red 

 pigments. These latter pigments are supposed to adapt the 

 plant to varying intensities of light. This is supported by the 

 fact that the marine Algae exhibit a zonal distribution in the 

 water. Forms living near the surface of the water are predomi- 

 nantly green, at depths where they are alternately exposed and 

 covered by the tide, they are more commonly brown, while in the 

 shade of the brown Algae, or, below tidal limits, red forms occur. 

 The Algae appear to have developed along three lines which are 

 indicated by the green, brown and red colors, although the 

 basis for this classification rests upon structural and reproduc- 

 tive characters. These three classes are: A, Green Algae or 

 Chlorophyceae ; B, Brown Algae or Phaeophyceae ; C, Red Algae 

 or Rhodophyceae. 



