122 GROWTH AND WORK OF PLANTS 



free nitrogen which they have absorbed from the air in the soil, 

 with carbohydrates and other organic substances which they 

 have absorbed from the cells of the clover or other legume. The 

 bacteria thus obtain their carbohydrate food from their " host," 

 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 



Fig. 91. Fig. 92. 



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



condition. denticulata. 



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 pjant food. After a time stm* 

 of the root tubercles die and some of the nitrogen " fat" 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 205, 

 206). 



204. Inoculation of soil with the root tubercle bacteria. 

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



