116 PLANTS AND MAN 



ready to be used again by other green plants in their synthesis of 

 proteins (fig. 77). Another phase of the nitrogen cycle involves 

 certain species known as nitrogen-fixing bacteria, which can 

 extract the nitrogen from the air and "fix" it as a nitrate. Some of 

 these bacteria are free-living in the soil, subsisting upon car- 

 bohydrate debris which is used as the source of energy to fix the 

 nitrogen. Others live in nodules on the roots of various plants 

 related to the peas, beans, clover and alfalfa; when the host plant 

 dies and becomes part of the soil humus, these nodules decompose 

 and add nitrates to the soil. Since soils can be enriched in this 

 way by "green manure," it is possible to alternate crops so that 

 such nitrate-depositing plants make up for the other plants 

 which withdraw the nitrates from the soil. Here again the de- 

 positors in the account are the bacteria, the spenders are the 

 green plants; but also again animals are most interested bystand- 

 ers, since their protoplasm is derived from plant protoplasm, and 

 the latter depends upon nitrogen for its formation. 



Parasitism 



Another way in which plants influence the lives of animals 

 presents a less cheerful picture. The colorless plants are parasites 

 as well as saprophytes, and as parasites they gain their sustenance 

 from living plants and animals. In some cases this does not harm 

 the host organism, yet in many instances it results in the common 

 enemy of all living things — disease. As a result animals which do 

 not die of old age or by accident usually fall prey to one of hun- 

 dreds of species of bacteria. Plants themselves are also the prey of 

 these minute assassins, though in this case it is the fungi rather 

 than the bacteria, which are the chief off'enders. 



Man does not stand apart from this maze of inter-relations, 

 some of which are beneficial and some harmful. Dependent upon 

 green plants for his foods and food accessories, his beverages and 

 his drug plants, man has developed this relationship into a 

 complete dependence upon agriculture and allied activities. 

 Dependent also upon the bacteria, he has learned to use them in 

 keeping his soils rich in nitrates. But like his fellow animals, 

 his body at the same time is continually being attacked by hosts 

 of other bacteria which are attempting to use him in their food 



