THE IMPENETRABLE SEA 



numerous that they are now recognized as the most im- 

 portant plants of the sea. Each diatom is a tiny pill-box — 

 or perhaps "jewel case" would be a better description. 

 They occur in an infinite variety of forms, numbers of 

 them singularly beautiful. The living contents of a 

 diatom are enclosed within two similar valves of silica, 

 which fit tightly together like the top and bottom of a 

 box. Myriads of the tiny siliceous "boxes" form the main 

 part of the oozes of the world's sea-floors, particularly in 

 the North Pacific and the Southern Ocean. You can see 

 and handle quantities of these microscopic cases if you 

 obtain some ordinary fuller's earth, which consists of the 

 cases of myriads of what were once the shells of marine 

 diatoms, deep in the sea. 



Those diatoms — the majority — which are enclosed in 

 silicon, look like crystals under the microscope: some 

 are shaped like rods, some Hke discs, some are cubes, 

 some cylinders. Others have spines or other curious 

 adornments : all are complicated structures of ingenious 

 design, and many are exquisitely lovely. The smaller 

 varieties, called nannoplankton or ultraplankton, which 

 pass easily through the finest silk, are (according to the 

 most recent scientific pronouncements) among the most 

 important of all food providers. 



Diatoms, like land plants, are quickened by the advent 

 of spring, come to their most prolific activity and abun- 

 dance at this time, and then decline in summer, having 

 exhausted the mineral salts held in solution in the water 

 around them. Eventually the surface waters of the sea 

 cool and sink, and are replaced by waters from below 

 which have not been exhausted of their nitrogenous and 

 phosphoric salts. These waters from below form the new 

 surface layer, in which new plants will flourish abun- 

 dantly (though invisible to our eyes) in the following 

 spring and summer. 



Planktonic plants need to capture light energy from 

 the sun, like all green plants on land. For this reason 



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