v] NATURE OF PLANT-ANIMALS 145 



These compounds, put into the soil, are each a source 

 whence the green plant may obtain the raw materials 

 for the synthesis of organic nitrogen and thus increase 

 the supplies of material essential for the development 

 of brain and muscle in animals and man. 



The fact of nitrogen-hunger is, then, no small matter 

 of mere academic importance. It touches the future 

 of man himself and presents a problem which every 

 living organism must solve. The supply of available 

 nitrogen is a limiting factor of life. Let us see what 

 bearing the fact of nitrogen-hunger has on the 

 economy of C. roscoffensis and C. paradoxa. 



That nitrogen-hunger presses as hardly on marine 

 organisms as on those which live on the land is 

 undoubted. Recent investigations have shown that 

 the amount of combined nitrogen present in sea- 

 water, in a form available to plants for synthetic 

 purposes, is extremely low. Thus, according to 

 Johnstone (1907), the amount of nitrogen compounds 

 in Baltic and North Sea water may be taken as about 

 *2 millegrams (= *003 grains) in a litre, or about two 

 parts in a million. No wonder that marine animals 

 are always hungry ! No wonder either that the 

 free, flagellated infecting organism of C. roscoffensis 

 settles down on the egg-capsules to avail itself of any 

 crumbs of nitrogen compounds that it may find there. 

 Nor is it remarkable that, finding a certain amount 



K. 10 



