208 SYSTEMS OF PERMANENT AGRICULTURE 



The clover plant then draws upon this combined nitrogen in the 

 root tubercles, and makes use of it in its own growth, both in the 

 tops and in the roots of the plant. 



These nitrogen-fixing bacteria live in tubercles upon the roots of 

 various legume plants, such as red clover, white clover, alfalfa, 

 sweet clover, cowpeas, soy beans, vetch, field peas, garden peas, 

 field and garden beans, etc. The tubercles vary in size from smaller 

 than a pinhead to larger than a pea, varying somewhat with the 

 different kinds of plants, being especially small upon some of the 

 clovers, and large upon cowpeas and soy beans. The tubercles are, 

 of course, easily seen with the eye, but the tubercle is only the home 

 of the bacteria, somewhat as the ball upon the willow twig is the 

 home of the insects within. The bacteria themselves are far too 

 small to be seen with the unaided eye, although they can be seen 

 by means of the powerful microscope. Several million bacteria 

 may inhabit a single tubercle. It is not necessary to see the 

 bacteria, because if we find the tubercles upon the roots of the plant, 

 we know that the bacteria are present within, otherwise the tubercle 

 would not be formed. 1 



It has also been demonstrated that, as a rule, there are different 

 modifications of nitrogen-fixing bacteria for markedly different 

 species of legume plants. Thus, we have one kind of bacteria for 

 red clover, another for cowpeas, another for soy beans, and still a 

 different kind for alfalfa. 



There are some noteworthy exceptions to this rule. Thus, the 

 bacteria of alfalfa (Medicago saliva) and of common sweet clover 

 (Mellilotus alba] are interchangeable, and apparently identical, 

 as are also the bacteria of cowpeas (Vigna unguiculata) and the 

 widely distributed native partridge pea (Cassia chamaecrista) , 

 relationships of much importance in connection with soil inocula- 

 tion for alfalfa and cowpeas. There is evidence that, by a compara- 

 tively long process of breeding, or evolution, the bacteria which 

 naturally live upon one kind of legume may gradually develop the 

 power to live upon a distinctly different legume to which they were 

 not at first adapted. This change which has been brought about 



1 A few plants form starchy nonbacterial tubers, which may be of large size, 

 like the potato and artichoke, or of smaller size, as on the rootstalks of nut grass 

 (Cyperus rotunda). 



