i6o BACTERIA AND SOIL FERTILITY 



tile organism. It has been shown that the organism travels in 

 the soil at a comparatively rapid rate. Nevertheless, the num- 

 ber of root hairs infected is too small to lend support to a chemo- 

 tactic theory. However this may be, the organisms cluster at the 

 tip of the root hair and by some means dissolve the cell wall. This 

 enables the microbe to enter the root hair. As a result there is a 

 decided bending of the tip causing it to resemble a shepherd's 

 crook. This is taken as a sign of complete infection. Other 

 hairs which form after this infection appear to be resistant to the 

 attack of other legume bacteria. The bacteria once inside the 

 cell rapidly multiply, and the irritation thus produced causes en- 

 largement of the plant at this point. 



The size, distribution, and appearance of the resulting nodules 

 vary with a number of factors. They are large and more nu- 

 merous where aeration is best in the soil. In tight or water satu- 

 rated soils they occur at the surface and are often found colored 

 green similar to sunburned potatoes. At the period when seeds 

 are forming on the plant most of the nodules are soft and the 

 internal tissue sloughs off, leaving the more resistant outer tis- 

 sue a mere shell which later decays. 



The organisms do not seem to enter the root hairs of old plants 

 or even well-nourished plants. It would, therefore, seem that 

 the plant has to weaken thru the lack of food or for other causes 

 before the organism can become established on the roots. 



Conditions Favoring Growth. — The legume bacteria are all 

 aerobic and the nodules on the roots of the plants are usually 

 near the surface. Altho nodules will form on plants grown in 

 water cultures, yet they are not as large and active as when grown 

 m a well-aerated soil. The addition of oil to a soil or water 

 culture in which legumes are growing prevents the formation of 

 the nodules. Moreover, as shown by Whiting, the legumes get 

 their nitrogen thru the root and not the leaves. The result of 

 cultivation of legumes is, therefore, threefold: (i) the loosening 

 up of the soil makes available to the nodule bacteria atmos- 

 pheric nitrogen and oxygen; (2) the working of the soil increases 

 other bacterial activity which in turn renders soluble potassium, 



