[Chap. XLVII THE ALGAE 625 



The Diatoms: Bacillariophyceae 



The vegetative cell of a diatom has highly silicified walls (i.e., glass 

 walls ) composed of two overlapping parts ( valves ) that fit together like 

 the halves of a petri dish. The wall is usually beautifully sculptured and 

 ornamented (Fig, 286). The plant may be unicellular, filamentous, or 

 joined in irregular colonies by gelatinous sheaths. Pectic compounds are 

 present, but there is no cellulose. The chloroplasts are yellowish to 

 brownish, and contain chlorophyll, xanthophyll, and carotene together 

 with accessory pigments of unknown chemical composition. Some 12,000 

 species have been described. 



Diatoms are distributed widely in both fresh and salt water. They are 

 the important constituents of the plankton of the cooler waters of the 

 sea, and at times are the principal algae of lakes and ponds, especially in 

 early spring and in the autumn. Owing to the mucilage from their cells, 

 diatoms may become attached to other plants and to all sorts of under- 

 water objects. Depending upon the amount and place of major forma- 

 tion of mucilage, the colonial species may form ribbon, chain, zigzag, or 

 branching aggregates. Although not as important as either greens or 

 blue-greens in the soil, some species of diatoms are distinctly terrestrial 

 and capable of withstanding desiccation for many years. In regions of 

 frequent rainfall diatoms may grow subaerially on tree trunks, on fences, 

 and on rocky cliffs. 



As noted above, diatoms may occur in prodigious numbers in both sea 

 and inland waters at certain times of the year. They are probably the 

 most important constituent of the ocean plankton. When the plants die, 

 the cell walls remain unaltered after the death and decay of the rest of 

 the cell. The empty "glass cases" may thus accumulate over a period of 

 years in great quantities on the bottom of seas, bays, and lakes. Such 

 deposits of fossil diatoms are known as diatomaceous earth and are found 

 in many parts of the world. One of the largest deposits is near Lompoc, 

 California; it covers about 12 square miles with a stratum of commer- 

 cially pure diatoms some 1400 feet thick, and in addition 3000 feet of 

 diatoms mixed with silt, sand, and gravel. 



Diatomaceous earth is quarried or mined, the annual output approach- 

 ing 100,000 tons. Its largest use is in the filtration of liquids, especially 

 those of sugar refineries. The powdered earth is added to the solution, 

 collects on the cloth screens of the filter press, and prevents the sus- 

 pended matter from passing through them. Diatomaceous earth in the 



