SPECIAL ASPECTS OF NUTRITION OF PLANTS 129 



and to this extent they have lived as parasites at its expense. 

 But they do very little if any injury to the clover. In fact many 

 of these bacteria charged with this " fixed" nitrogen die within 

 the root tubercle, and the clover or pea, or other host as the case 

 may be, is able to absorb this nitrogenous substance and appro- 

 priate it to its own use. This is why leguminous plants thrive 

 so well in soils poor in nitrogenous plant food. After a time 

 some of the root tubercles die and some of the nitrogen "fat" 



Fig. 93. Fig. 94. 



Root tubercle organism from vetch, old Root tubercle organism from Medicago 

 condition. denticulata. 



bacteria (often called bacteroids) are set free in the soil and thus 

 enrich the soil. Some of the living bacteria are also set free in the 

 soil so that the soil contains numbers of them to attack succeeding 

 legume crops. Even when the crop of clover, or peas, etc., is 

 removed from the ground the soil becomes richer in nitrogenous 

 substance because of the bacteroids left in the root tubercles. 

 But more nitrogenous plant food is added to the soil in the pro- 

 cess of "green soiling" such crops, i.e., in plowing the clover or 

 peas under (see also Mycorhiza, and Symbiosis, paragraphs 

 206, 207). 



205. Inoculation of soil with the root tubercle bacteria. 

 If some of the soil, where clover or peas have grown with these 

 bacteria in their roots, be spread on soil poor in combined nitro- 

 gen where these or related crops have not been grown recently, the 

 clover or peas will develop a greater number of root tubercles and 

 consequently more nitrogen will be fixed, to the benefit of the 

 crop, and to the enrichment of the soil in combined nitrogen. 

 9 



