THAU/)PHVTA: ALGAE 



21 



form thick-walled zygotes. These germinate immediately, giving rise 

 directly to a new vegetative body. Sometimes the gametes conjugate 

 before being liberated. 



6. BACILLARIOPHYCEAE 



The Bacillariophyceae,* or diatoms, constitute an isolated group whose 

 relationships to the other algae are very uncertain. They include over 

 5,000 species of unicellular plants occurring almost universally in fresh 

 and salt water, as well as on damp soil. Some of the more common 

 genera are Melosira, Coscinodiscus, Bid- 

 dulphia, Pinnularia, Surirella, Cocconeis, 

 Navicula, and Pleurosigma. Diatoms 

 may be either free-floating or attached. 

 Frequently they form slimy brown coat- 

 ings on mud at the bottom of shallow 

 bodies of water, as well as on sticks, 

 stones, shells, other aquatic plants, etc. 

 That they were more numerous in geologic 

 times is shown by the great accumula- 

 tions of diatomaceous earth found in var- 

 ious parts of the world. This consists of 

 the shells (cell walls) of dead diatoms. 

 Deposits of diatomaceous earth were 

 formed mainly during the Tertiary, but 

 the fossil record of diatoms extends as 

 far back as the Jurassic. 



Although most diatoms are solitary, 

 some form colonies of diverse types, the 

 individuals being held together by a 

 sheath of mucilage. Their color, usually 

 a golden brown, is due to the presence of 

 chlorophyll in association with an excess 

 of carotinoids, particularly carotin and 

 several brown xanthophyll pigments. 

 Diatoms are distinguished from other algae 

 by their silicified cell wall. This consists 

 of two valves, one overlapping the other like the lid and bottom of a 

 pillbox (Fig. 11). The place where the valves overlap is called the girdle. 

 The cell wall is composed mainly of pectin impregnated with a large 

 amount of silica. It is variously marked with numerous fine transverse 

 lines that form regular and elaborate patterns. These make diatoms 

 among the most striking and beautiful objects to be seen under the 



1 Also called Diatomeae. 



A B 



Fig. 11. Two views of the shell of 

 Pinnularia viridis. A, girdle view; 

 B, valve view; g, girdle; pn, polar 

 nodule, en, central nodule; r, raphe. 

 {After Pfitzer.) 



