CH. XIl] THE CIRCULATION OF NITROGEN 297 



sea-area require a greater proportion of food-stuffs for the main- 

 tenance of their ordinary life-processes, it may be the case that the 

 structural metabolism suffers ; the assimilated food-substances 

 being restricted to the ordinary metabolism of the organism 

 (thus passing through a " current account "), rather than to the 

 structural metabolism (passing into a " sinking fund "). Therefore 

 it seems reasonable to expect that fewer individuals would be 

 generated in warm than in cold seas, that is to say, the production 

 would be less. 



The reader will see that the problems of the causes of the varia- 

 tions in density of animal and vegetable life in the sea are 

 exceedingly complex ones. But it seems clear enough that certain 

 principal factors are concerned in setting up these variations of 

 density of life : (1) the movements of sea water, either horizon- 

 tally as oceanic currents, or vertically, as it sinks to the bottom or 

 rises to the surface. This circulation of the sea water must distri- 

 bute the inorganic nitrogenous and carbonaceous food-salts, leading 

 to an abundance of these in some parts of the sea and a scarcity in 

 other parts. Marine life will, of course, vary according to the 

 variations in abundance of these ultimate food-stuffs. (2) The 

 marine bacteria, particularly those that act specially on nitrogen 

 and its compounds, are also factors. The denitrifying micro- 

 organisms are capable of reducing the nitrogen salts to such 

 a form that they can no longer be assimilated by the plants. On 

 the other hand there are also bacteria in the sea which can utilise 

 the free nitrogen which is dissolved in the water from the 

 atmosphere, converting this into compounds which can serve as 

 food-substances for the plants. (3) Finally the mass of life in any 

 part of the sea must depend to some extent upon variations in 

 physical conditions : on sunlight because the energy of this is 

 utilised by the plants in the process of photo-synthesis ; and upon 

 temperature, since with the rise of the latter, ordinary metabolic 

 processes become more wasteful. 



Resuming the main facts elicited in our study of the circula- 

 tion of nitrogen we find that the land areas are being depleted of 

 this substance (1) because in the metabolism of organisms, and in 

 the decomposition of the dead bodies of these, nitrogen salts are 

 formed which are washed down into the sea in the water of rivers ; 



