[Chap. XLVIl THE ALGAE 627 



form of bricks is used in the insulation of boilers and smelting furnaces. 

 At high temperatures it is more effective than asbestos or magnesia. Metal 

 polishes, abrasive soaps, and toothpaste often contain diatom shells. 



Living diatoms are of major importance as the food of fishes, oysters, 

 and clams. Certain marine fishes feed almost entirely on them, or on the 

 small animals that have consumed diatoms. The commercially important 

 hake, for example, feeds upon herring; the herring subsist on copepods; 

 and the copepods are dependent upon the diatoms for their food 

 (Fig. 275). 



The accumulation and decay of diatoms in water resenoirs and ponds, 

 on the other hand, may result in compounds that are extremely obnoxious 

 to our senses of smell and taste. Some species are able to grow in waters 

 highly polluted with organic matter. Diatom abundance is often asso- 

 ciated with a plentiful supply of nitrates and silicates. Paucitv of diatoms 

 is sometimes directly due to the absence of soluble silicates in the pond. 



Cell division in the diatoms is an almost unique phenomenon anions; 

 plants. The new cell wall in the center of the dixiding cell consists of 

 two new valves. Each of the two resulting daughter cells then has one 

 new valve, and one older valve that belonged to the parent cell. As a 

 result, any given diatom cell may have a valve that has been a part of 

 many ancestral cells. Spores are fonned in the diatoms both asexually and 

 sexually; and from them, after a period of dormancy, new plants develop 

 directly. The spores of diatoms have been indiscriminately called auxo- 

 spores regardless of whether or not they are the result of sexual union. 

 The prefix "auxo" refers to the increase in size of this cell. The cell 

 formed from the auxospore is usually much larger than the cell, or cells, 

 from which the auxospore originated. Subsequent divisions of the cells, 

 in vegetative multiplication of some species, lead to decrease in size of 

 cells until auxospores form. 



The diatom cell is diploid, the monoploid condition being represented 

 only in the nuclei of the gametes, a condition not common among algae, 

 but usual in animals. 



The Yellow-green Algae: Xanthophyceae 



This comparatively small group of algae is almost entirely confined to 

 fresh water and consists of about 400 species. The yellow-greens are 

 similar to the greens"^ in many respects, but are characterized by chloro- 



^ Since many of the green algae may often appear yellowish green during certain stages 

 of development, it is sometimes difficult to separate species of the two groups with 



