62 THE SMALLEST LIVING THINGS 



vironmental conditions. In the Chrysophyceae the cyst wall, 

 made of silica, has a single opening or pore, and is first laid down 

 within the organism, a layer of protoplasm covering the cyst on 

 the outside. This outer protoplasm, after variously sculpturing 

 the outside of the cyst wall, re-enters through the pore, which 

 is then securely plugged from within with a glass-like stopper 

 cemented with silica. Within this glass bottle the monad may 

 reproduce by division or lie dormant for long periods. 



Class Bacillariales (Diatoms) 



In every collection of material for microscopical study se- 

 cured from a pond or a ditch, bright yellow-colored organisms of 

 definite shapes will always be found. These are the diatoms, 

 which constitute another group of algae with about the same size 

 variations as the desmids. They are far more numerous than 

 desmids, however.* They are also much more cosmopolitan 

 than desmids, occurring in seas, especially in the colder waters, 

 as well as in fresh water. In the sea they form a great propor- 

 tion of the plankton^ and serve as food for animals of all kinds 

 and sizes. In fresh water they occur in sediment, or attached to 

 filamentous algae, or float as suspended organisms in the clear 

 water. Their yellow color is due to the presence of chlorophyll 

 in which the green color is masked by the deposit of a special 

 yellow pigment known as diatomin. This may be removed by the 

 use of dilute alcohol and the true green color of chlorophyll is 

 then exposed. 



In structure they are not unlike the desmids, each individual 

 consisting of two valves called frustules — fitted together like a 

 pill-box and its cover. It is generally agreed, however, that this 

 resemblance is due to parallel evolution rather than to any ge- 

 netic relationship of the two groups. One valve, corresponding 

 to the cover of a box, is measurably larger than the other. Both 

 valves are richly impregnated with silica which makes them prac- 



* Karsten (Engler and Prandtl, 1928) enumerates no less than 189 genera, and 

 Fritsch (Encyclopedia Britannica, 1928, "Bacillariales") enumerates upward of 

 1500 species and this number does not include marine forms. 



t Plankton, the passively floating or weakly swimming animal and plant life of a 

 body of water ; it is chiefly minute organisms, diatoms and blue-green algae among 

 plants and protozoa, various eggs and larvae and the slow-moving jellyfishes. 

 Fishes and other large actively swimming animals are not included. 



