I4O AGRICULTURAL BACTERIOLOGY. 



small. It was next shown that if the legumes were sown in 

 sterilized sand, without nitrogenous food, and were then watered 

 by water which had been standing in contact with ordinary 

 soil, results were quite different Such water, sometimes 

 called a " soil infusion," is made by simply soaking soil in 

 water and then filtering off the solid particles, using the filtrate 

 for watering the growing legumes. Plants watered with such 

 infusions showed two interesting stages of growth. The peas, 

 for example, sprouted readily and for a short time grew vigor- 

 ously ; then the vigorous growth ceased and the plant seemed 

 to be suffering for lack of food. This has been called the 

 nitrogen linnger stage, and represents a period in which the 

 plant has used up the nitrogen in the pea, and consequently 

 all that was within reach. Control plants, grown in simi- 

 lar soil and watered with pure water, never recovered from 

 this stage, but those that were watered with the soil infusions, 

 after a few days of such nitrogen hunger, recovered, began 

 once more a vigorous growth and eventually produced large- 

 sized plants with a good yield. Upon examining the roots of 

 the plants they were found to have developed tubercles, while 

 the control plants, watered with sterilized pure water, did not 

 develop tubercles. These facts of course indicated that in 

 the soil infusion some agencies were present which stimulated 

 the development of tubercles and the consequent fixation of 

 nitrogen, and that the power of absorbing atmospheric nitrogen 

 enabled the plant to recover from the nitrogen hunger stage. 

 Tubercle Bacteria. These facts suggested the agency of 

 microorganisms in the production of tubercles, and naturally 

 led to a careful microscopic study of these bodies. The mi- 

 croscopic structure of tubercles had been studied years before ; 

 as long ago as 1866 there had been found by Woronin small 

 bodies inside of the tubercles, the nature of which he did not 

 understand. A little later, in 1874, Krickson had found in 

 tubercles certain thread-like bodies, which were not intelligible 



