Legume Bacteria 215 



plants, they may appear as irregular rods, or pear-, X-, 

 or Y-shaped. These irregular forms, designated as bac- 

 teroids, are characteristic in some of the legumes. 



The organisms that find their way into the legume 

 roots multiply there rapidly and increase to enormous 

 numbers. The small rod-shaped forms are rather abun- 

 dant in the young tubercles but, as the latter grow older, 

 the larger irregular forms, the bacteroids, become more 

 and more numerous. Later still, the bacteroids are 

 dissolved and absorbed by the host-plant. Not all of 

 the organisms are thus absorbed, however, for very 

 considerable numbers remain in the partly emptied 

 nodules. On the decay of the latter, the surviving 

 bacteria find their way back into the soil where they 

 probably derive their nourishment from the humus 

 until a new opportunity is given them to enter the roots 

 of some legume. 



It is evident that soils which frequently bear crops of 

 legumes must necessarily be richer in nodule-bacteria. 

 This is so because the greater the number of such plants, 

 the greater the number of bacteria that escape back 

 into the soil from the decayed nodules. For example, 

 it is a matter of common observation that when two 

 crops of cowpeas or of soybeans are grown in succession 

 on the same soil, the second crop is frequently superior 

 to the first. Similarly, in the case of alfalfa, soils new 

 to this crop yield but a scanty stand of inoculated 

 plants. When, however, the latter are turned under 

 and the field reseeded, the new stand is usually much 

 better. It appears, therefore, that the decay of the com- 

 paratively few tubercles leads to an increase of the bac- 



