SOME GENERAL PRINCIPLES 49 



soil, the presence of which are essential to enable clo- 

 ver plants to secure additional nitrogen to that found 

 in the soil and subsoil on which to feed. When 

 manure was applied, as stated above, the clover 

 plants secured much or all of their nitrogen from the 

 manure. Bacteria were introduced in very limited 

 numbers at first, it may be through the medium of 

 the seed or in some other way, and because of an in- 

 herent power which they possess to increase rapidly 

 in connection with continued sowing of clover at 

 short intervals, they came at length to be so numer- 

 ous in the soil as to make possible the growth of 

 good crops of clover where these could not be thus 

 grown a few years previously. 



Careful observers had noticed that certain warty- 

 like substances were found attached to the roots of 

 clover plants, and that the more vigorously the plants 

 grew, the larger and more numerous were these sub- 

 stances, as a rule. It was thought by many that these 

 warty substances, now spoken of as nodules, were 

 caused by worms biting the roots or because of some 

 unfavorable climatic influence or abnormal condition 

 of soil. It is now known that they are owing to the 

 presence of bacteria, whose special function is the 

 assimilation of free nitrogen obtained in the air 

 found in the interstices ; that is, the air spaces be- 

 tween the particles of soil. This they store up iri 

 the nodules for the use of the clover plants and 

 also the crops that shall follow them. 



The nodules in clover plants vary in size, from a 

 pin head to that of a pea, and they are frequently 

 present in large numbers, Bacteria are present 



