DEVELOPMENT OF PLANTS 163 



walls and dense cell contents which enable them to remain in a 

 dormant state 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 w^here ponds and lake? 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 (Fig. 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 vary- 

 ing intensities of light. This is supported by the fact that the 

 marine Algae exhibit a zonal distribution in the water. Forms liv- 

 ing near the surface of the water are predominantly 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 indi- 

 cated by the green, brown and red colors, although the basis for 

 this classification rests upon structural and reproductive charac- 

 ters. These three classes are: A, Green Algae or Chlorophy- 

 ceae; B, Brown Algae or Phaeophyceae ; C. Red Algae or 

 Rhodophyceae. 



