212 THE CONDITIONS OF LIFE IN THE SEA [PART III 



It will probably be fairly accurate, in the present state of our 

 knowledge, to regard the food contents represented by (1) as a 

 maximum value, and that represented by (2) as a minimum 

 value. 



The ultimate food-stuffs in the sea. Bearing in mind the 

 distinction implied in the terms producers and consumers, we see 

 that there is a certain mass of food-stuffs contained in the sea 

 in the form of plankton organisms, and that this represents the 

 nutrition available for the consumers. We have now to consider 

 what are the contents of the sea in the form of inorganic food-salts, 

 that is, the food-stuffs available for the producers. We find that 

 the following substances are the materials that are the essential 

 food-stuffs of plants, or of organisms exhibiting a similar mode of 

 nutrition : 



(1) Nitrogen compounds, such as salts of nitrous and nitric 

 acids and ammonia, and possibly amines. 



(2) Carbonic acid, which may exist in the air in the form 

 of carbon dioxide, or in simple solution in sea water, or in solution 

 in the form of bicarbonates ; and also other carbon compounds. 



These two classes of substances supply the plant organisms 

 with the carbon and nitrogen necessary for the building up of 

 their tissues, and for the development of the energy which is 

 manifested in their life-processes. 



(3) Phosphoric acid, which is present in the sea in the form 

 of soluble calcium phosphates. 



(4) Silica, which also exists in the sea in solution as colloidal 

 silicic acid ; and possibly also as particles of clay (aluminium 

 silicate) in suspension, capable of decomposition by the organic 

 matter excreted by plant organisms. 



(5) Calcium carbonate, also present in solution. 



The three latter classes of substances are utilised by marine 

 organisms. Phosphoric acid is necessary for the elaboration of 

 the nucleo-proteid, which is such a significant constituent of the 

 cell-nucleus. It is also required in the formation of the skeletons 

 of the organisms which possess limy substances. Silica is required 

 for the formation of the hard parts of those protophyta and 

 protozoa which have siliceous skeletons ; these are the shells, or 



