788 



ECOLOGY 



beans, but if these bacteria are used to inoculate other bean roots, they grow as rig- 

 orously as did their immediate ancestors in the roots of peas. Similarly, bacteria 

 from legumes of calcareous soil do not infect legumes of siliceous soil, nor vice versa, 

 but if the acid content of a calcareous soil is increased slowly, a race of bacteria can 

 be developed (e.g. on alfalfa) that eventually will be able to infect lupines growing 

 in siliceous soil. It is the belief of bacteriologists that through similar progressive 



FlGS. uoi, 1102. noi, roots of the white sweet clover (Melilotus alba), showing the 

 characteristic root tubercles (t), which are induced by a special bacterial form, Bacillus 

 radicicola , note the grouping of tubercles in clusters; 1102, a longitudinal section through 

 a part of a root of the pea (Ptsum sativum) that has begun to tuberize by reason of the 

 stimulating influence of Bacillus radicicola; invasion occurs through the root hairs (r), 

 where infection threads (i) are formed; these penetrate the cortical tissues (c), where 

 branching takes place; 'note that the infected cortical cells have denser cytoplasm, larger 

 nuclei (n), and thicker walls than the uninfected cells; e, epidermis; highly magnified. 

 From FRANK. 



variability any race of Bacillus radicicola can be grown eventually on any legume 

 root, and this view is favored by the fact that when a leguminous crop is introduced 

 into an entirely new region, the roots soon become infected by bacteria which induce 

 tubercle formation. The bacteria of legume tubercles thrive only where free nitro- 

 gen and an abundant supply of carbohydrates are available, their development 

 being retarded by an abundance of nitrates. Similarly, the tubercles reach their best 

 development in soils that are poor in nitrates; they vary from the size of a pin head 

 in ordinary clovers to the size of a pea in Strophostyles helvola, a plant of sandy 



