216 



Bacteria in Relation to Country Life 



terial numbers in the soil and provides for a better 

 inoculation of the succeeding plants. 



The relations between legumes and the nodule-bacteria. — 

 The exact relations between the plant and bacteria in 

 the formation of tubercles are not yet fully explained. 

 The information thus far in. our possession indicates 

 that the plant offers more or less resistance to the en- 

 trance of the bacteria and that the latter must be suf- 

 ficiently vigorous to overcome this resistance. It is 



Fig. 34, Sections through root tubercles. — 1. Cell from tubercle of Pisum 

 sativum^ showing bacterial filament. 2 and 3. Cells with bacterial filaments 

 from tubercle of Trifolium Pannonicum. (Stefan.) 



well known that when an abundance of available nitro- 

 gen is present in the soil, or when nitrate is applied, 

 the formation of tubercles is partly or wholly suppressed. 

 The suppression of tubercle-formation is explained, 

 in this case, by the assumption that the bacteria can 

 enter the roots of leguminous plants only when the 

 latter are in a weakened state, as is true, for example, 

 of legumes growing in nitrogen-poor soils. 



The young plants in such soils finding but meager 

 quantities of combined nitrogen to supply their needs, 



