444 General Botany 



in chains held together in large, irregular masses by the gelati- 

 nous sheaths; Anabcena, with short, curved chains of cells, very 

 frequent in ''water blooms"; and Tolypothrix, filamentous 

 forms that are often highly branched. 



THE DIATOMS OR BACILLARIACE^ 



The diatoms include about 12,000 species of one-celled algae, 

 remarkable for their abundance in moist places everywhere, 

 their small size, and their beautifully sculptured cell walls. These 

 algae differ from the other algae described in this book in having 

 siliceous cell walls. The cell wall consists of two overlapping 

 valves, like the two halves of a pill box. In cell division the 

 two valves are moved outward, and following the division of 

 the protoplasm, two new valves are formed between the two 

 halves of the original cell. Diatoms have distinct nuclei and 

 chloroplasts. Many of them are motile, but some have a stage 

 in which they are fixed by chitinous stalks and gelatinous sheaths 

 to under-water objects. 



Attention is directed to the diatoms because of their great 

 economic importance. They are one of the most important 

 sources of food, directly or indirectly, of marine and fresh-water 

 fishes. The gizzard shad, for example, consumes directly 

 enormous numbers of diatoms, while the hake feeds upon a 

 series of aquatic animals all of which directly or indirectly have 

 diatoms as their ultimate food supply. No matter how long the 

 chain of animals, up to the fish, the fundamental organism is the 

 diatom which changes inorganic compounds into food. 



Diatoms occur in vast numbers in the upper layers of the 

 ocean and in all kinds of streams and ponds. They sometimes 

 multiply rapidly in reservoirs, and when they subsequently die 

 become a source of annoyance by producing bad odors and tastes 

 in drinking water. Extensive deposits of diatom shells occur 

 in many parts of the world, and these shells are the basis of 

 powders and soaps for polishing metals. Diatomaceous earth 



