RECLAIMING LOST NITROGEN. 143 



Up to the present time it has not been possible to isolate 

 them directly from the soil. That they are in the soil is of 

 course evident from the experiments already mentioned. They 

 may be isolated with ease from the tubercle on the root, but 

 the difficulties of separating bacteria from the soil have been so 

 serious that no bacteriologist has as yet been able with cer- 

 tainty to isolate the tubercle bacillus from the earth in which 

 it is known positively to be present. They are certainly soil 

 bacteria, although, thus far, found only in the root nodules. 



PRODUCTION OF TUBERCLES BY THE BACTERIA. 



Our next question is the relation of the microorganism to 

 the plant in whose roots it grows. There is no doubt that 

 these bacteria from the soil make their entrance into the roots 

 in the young condition of the legume, passing into its root ap- 

 parently through the delicate root hairs. Inside the root they 

 find favorable conditions and multiply rapidly. Their presence 

 serves as a stimulation to the root tissue of the plant which 

 results in an abnormal growth of the root. The cells of the root 

 tissues multiply more rapidly and more abundantly than usual, 

 and the extra growth produces the tubercles characteristic of the 

 phenomenon. The tubercle itself is thus really a growth of 

 the leguminous plant, stimulated by the presence of the micro- 

 organism. Inside of this growing tubercle the bacteria un- 

 dergo a series of transformations, which are regarded in part 

 as abnormal growths due to unfavorable conditions, the nature 

 of which is not well understood. They soon produce a muci- 

 laginous material which permeates the tissues of the plant in 

 the form of long, delicate threads containing the micro- 

 organisms. They subsequently develop peculiar forms, re- 

 ferred to as the Y and T forms, which are not normal charac- 

 ters of ordinary bacteria, nor of these bacteria under ordinary 

 conditions. (Fig. 21.) Microscopists have puzzled over these 



