CH. X] THE CONDITIONS OF LIFE IN THE SEA 219 



seas, and Murray and Irvine^ have made the interesting sugges- 

 tion that organisms, such as diatoms, are able to decompose these 

 and make use of the colloidal silica set free. Such clay particles 

 may adhere to the film of mucus covering the surface of a diatom, 

 or other organism, and may be decomposed by the sulphur of the 

 organic matter of the excretion of the organism, so affording 

 colloidal silica, which may be used by the latter. But we find 

 that inorganic particles in suspension in river water are rapidly 

 precipitated by the salts of sea water, when they enter the latter, 

 and therefore clay particles are very scarce in the open sea, where 

 nevertheless there may be abundance of diatoms. 



Calcium exists in solution in the sea in the form of both 

 sulphates and carbonates ; and, relatively to the proportions of 

 silica and phosphates present, may exist in considerable quantities. 



Other salts necessary for the nutrition of marine plants are 

 present in the sea in relatively large amount. 



Oxygen is present in the sea in proportions that depend on 

 the temperature, salinity and other factors. It is dissolved from 

 the atmosphere, or arises during the metabolism of marine plants. 

 At a given temperature and salinity sea water can only take up 

 a certain quantity of oxygen and this saturation quantity is seldom 

 found in the sea. Indeed in many parts of the latter, where it 

 is imperfectly " ventilated " because of the absence of currents, 

 the oxygen may be greatly deficient, and occasionally its place 

 may be taken by sulphuretted hydrogen. The proportion present 

 therefore varies to a considerable extent, and it would hardly be 

 appropriate here to discuss the results of estimations made in 

 different seas, and at different depths-. Putter found that the 

 water of the Bay of Naples contained, at temperatures of 13° C. 

 to 15° C, an average quantity of about 7*6 milligrammes per litre. 



Thus all the substances which are essential for the nutrition 

 of plant life on the land are also present in the sea. We may 

 therefore regard the latter as a dilute food solution which is 

 utilised by the producers, that is, the larger algae, the unicellular 

 algae, the diatoms, and the other protozoa and protophyta which 

 have a similar mode of nutrition. Now it must have occurred to 



^ Loc. cit. 



'■^ See however Kriimmel, loc. cit. p. 295. 



