DEVELOPMENT OF PLANTS 173 



in life, such as conditions unfavorable for growth. The spores 

 are especially to be looked upon as devices to bring about a rapid 

 increase in the number of individuals while the gametospores 

 more usually serve to carry the plant over periods of drought or 

 extremes of temperature which would prove fatal to the plant. 

 For this reason the gametospores are often provided with thick 

 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 where ponds and lakes have 

 dried up. Sometimes these beds reach enormous proportions, 

 the city of Richmond, Va., being built upon such a deposit. 

 Some of the western beds are quite 300 feet in thickness and yet 

 it takes about 40 million of these plants to make a cubic inch. 

 Silicious earth is used as polishing powders and as absorbents in 

 the manufacture of some explosives. 



Subdivision 4. Euphyceae or Algae 



65. General Features. TJiis 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 larg- 

 est and most highly constructed plants found among the Thallo- 

 phyta. Their advance over preceding groups appears especially 

 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 

 living near the surface of the water are predominantly green, at 

 depths where they are alternately exposed and covered by the 



