THE ASCENDING SAP 



97 



The supply of nitrogen to plants in an accessible form is not nearly so 

 plentiful as the plant requires, and nitrogen-hunger is frequently experi- 

 enced by them. " Nor is the origin of this nitrogen deficit far to seek. 

 The nitrogen contained in the soil comes in the plant to form a con- 

 stituent of the organic nitrogen compounds, such as the proteins. The 

 plant dies and decays, or is eaten and the eater decays. . . . The organic 

 nitrogen compounds of the dead animal or plant are broken down by 

 the bacterial and fungous agents of decay into a series of simpler 

 forms which, acted on by yet other of the ordered army of saprophytic 

 micro-organisms, yield finally ammonia and nitrogen. The nitrogen 

 leaks away into the atmosphere and contributes to the 79 per cent, of 

 nitrogen gas which is contained in the air. The ammonia may leak 

 away also as every dunghill testifies or it may be fixed in the soil by the 

 agency of certain nitrifying micro-organisms. These bacteria convert the 

 ammonia into nitrates, and the nitrates so formed become available to 

 the roots of the green plant. On the other hand, the nitrates of the soil 

 may be seized upon by yet other, denitrifying micro-organisms and, becoming 

 converted into ammonia compounds, may be lost to the vital circulation. 

 The constant leakage of nitrogen from combined forms to the free and 

 inert form of nitrogen gas results in a shortage of nitrogen available for 

 the formation of the nitrogenous food of plants. We may thus speak of 

 the problem which besets all living organisms that of obtaining adequate 

 supplies of organic nitrogen compounds as the nitrogen problem, and we 

 may well believe that the 

 sum-total of life supported 

 on our planet is deter- 

 mined ultimately by the 

 amount of available nitro- 

 gen present in the earth 

 and sea. Occasionally, 

 organisms are met with 

 which have solved the 

 nitrogen problem in a fun- 

 damentally satisfactory 

 manner. Among such 

 organisms are nitrogen- 

 fixing bacteria, leguminous 

 plants, and man. Each of 

 these organisms has evolved 

 methods of bringing back 

 into vital circulation the 

 nitrogen which has escaped 

 as nitrogen gas into the air."* 

 * Keeble, Plant Animals, 141. 

 10 



Photo by} 



IE. Step. 



FIG. 131. DBOSEKA INTERMEDIA. 



ill Dragon-fly (Agrion puella) has been caught by the 

 efforts of sereral leaves of the Sundew. 



