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 
