v] NATURE OF PLANT-ANIMALS 143 



soil and also in the sea, possess the power of 

 causing free nitrogen to enter into combination with 

 other elements and so to serve as material for the 

 construction of the vitally necessary proteins. The 

 leguminous plants, clovers, peas, lupins, etc., do it 

 or rather get it done for them by entering into 

 association with a certain species of nitrogen- 

 fixing micro-organism. This organism, Pseudomonas 

 radicicola, enters the root and increases in its 

 tissues. Under the stimulus of this micro-organism, 

 the root swells locally to form nodules or tubercles. 

 Later, when the nodule-organism has accumulated 

 considerable quantities of organic, nitrogen com- 

 pounds, the tissues of the root destroy it, raid its 

 stores arid, living on the nitrogen-plunder, are able, 

 unlike other plants, to grow in soils which are 

 deficient or even lacking in inorganic, nitrogen com- 

 pounds. Thus, the gorse occupies vast tracts of 

 sterile, sandy wastes in Brittany and elsewhere, and 

 the traveller in spring may journey for miles between 

 tree-like groves of gorse ablaze with golden blossom, 

 every particle of which owes its presence in the air 

 to the nitrogen-fixing bacteria at work in the roots 

 underground. These bacteria it is which have provided 

 the essential, organic nitrogen compounds without 

 which the tissues of the flowers could not have been 

 formed. Large tracts of waste land in Germany, 

 America and other parts of the world have been 



