SOIL AND SUBSOIL, 



phere through the ammonia in rain water. This was then 

 shown to be wholly inadequate ; and Boussingault proved con- 

 clusively that plants do not take up nitrogen gas from the air. 

 This was subsequently denied by Ville; but investigation at 

 the Rothamstead agricultural station by Lawes and Gilbert 

 definitely confirmed Boussingault's results. At the same time 

 they also proved very definitely that while grass and root crops 

 deplete the soil of nitrogen, clover and other leguminous crops 

 leave in the soil more nitrogen than was previously present, 

 even when the entire, itself highly nitrogenous, leguminous 

 crop is removed from the land. The improvement of lands 

 for wheat production by rotation with clover had long ago 

 become a practical maxim ; but the cause was not understood 

 until, in 1888, Hellriegel and Wilfarth announced that the 

 variously-shaped excrescences or tubercles which had long been 

 observed as frequently deforming the roots of legumes, are 

 caused by the attacks of bacilli capable of absorbing the free 

 nitrogen of the air and thus enabling the host-plant to acquire 

 its needed supply by absorbing the richly nitrogenous matter 

 thus accumulated in the excrescences. The minute rod-shaped 

 organism was named Bacillus radicicola by Beyerinck ; Rhizo- 

 biuui legwminosarum, by A. Frank, who has published an ex- 

 tensive treatise on the subject. 1 



Microscopic examination of the nodules shows their tissues 

 to contain partly motile, free 

 bacteria, partly others ( bacte- 

 roids), which hive assumed a 

 quiescent condition, and are 

 of much greater dimensions 

 than those of the motile form. 

 These relatively thick, and 

 sometimes forked, forms, dif- 

 fering somewhat in each of 

 the group adaptations men- 

 tioned below, constitute the 

 bulk of the cell-contents of the 

 nodules, and ultimately serve 

 for the nutrition of the host- 

 plant with nitrogen. When 



1 Uber die Pilzsymbcose der Leguminosen, Berlin, 1890. 



2 Original figure from drawing by O. Butler, Asst. in Agr. Dep't Univ. of 

 California. 



FIG. 21. Microscopic section of cell tissue, 

 from a nodule of Square-pod pea, showing cells 

 filled with Rhizobia. 2 



