196 PLANT LIFE 



producing the nodular outgrowths in question. 

 It feeds and grows mainly at the expense of 

 the sugars and other substances supplied by 

 the host plant, these having, of course, been 

 produced as the result of the photosynthetic 

 activity of its leaves. 



But when thus provided with carbohydrate 

 food, the bacillus is able to manufacture the 

 essential nitrogenous compounds necessary for 

 the production of protoplasm by utilising the 

 free nitrogen of the air. Most plants have to 

 take in their nitrogen in a combined form, as 

 ammonia salts, nitrates, etc., for nitrogen is 

 a very inert element, and difficult to force into 

 combination with others. Bacillus radicola 

 is one of the very few organisms which can 

 perform this really stupendous task, provided 

 that it is supplied with the means of obtaining 

 the energy required for the process in the 

 form of appropriate carbohydrate nutrition. 

 There is no doubt as to the facts, for the 

 bacillus will do the same thing when culti- 

 vated outside the body of the plant, and 

 under the most rigidly controlled experimental 

 conditions. 



After the bacilli have thriven for a while, 

 mainly at the expense of the food supplied 

 by the root in which they are living and multi- 

 plying, a change comes over them. Many of 

 the individuals become weaker, and undergo 

 a sort of degeneration, whilst a few pass into 

 a resting stage in which they become highly 

 resistant to adverse conditions of life. The 



