98 RECLAIMING LOST NITROGEN. 



with a good yield. Upon examining the roots of the plants they are 

 found to have developed tubercles, while the control plants, watered 

 with sterilized pure water, do not develop tubercles. These facts 

 of course indicate that in the soil infusion some agencies are 

 present which stimulate the development of tubercles and the 

 consequent fixation of nitrogen, and that the power of absorbing 

 atmospheric nitrogen enables the plant to recover from the nitro- 

 gen-hunger stage. 



The Tubercle Bacteria. These facts naturally suggest that 

 bacteria, or other microorganisms, are the cause of the tubercles. 

 Microscopic study of the tubercles shows a 

 somewhat perplexing structure. The tubercle is 

 the result of the excessive growth of the cells of 

 the root, but they are filled with peculiar bodies. 

 During the early growth of the tubercle, long, 

 thread-like sacs appear, which force their way 

 through the cells (Fig. 25). These filaments seem 

 to be hollow tubes which contain smaller bodies, 

 pouches in the tuber- somewhat like bacteria. As the legume in- 



cles. The figure ^ i TI i T 



represents two cells creases in size these bacteria-like bodies undergo 

 with the sacs pene- a transformation in shape, growing larger and 



tratmgthem(Ste/a). 



branching somewhat, so as to form structures 

 like those shown in Fig. 26. These are called bacterioids, and they 

 are characteristic of the tubercles of legumes. The next step was. 

 naturally, to isolate these bodies and study them by bacteriological 

 methods. It is easy to isolate from the tubercles bacteria that will 

 grow in culture media, and these organisms were named B. radicicola. 

 Experiments with the bacteria thus isolated have been extensive 

 and, on the whole, satisfactory, though occasionally they have been 

 conflicting. It has been proved many times that tubercles can be 

 produced upon legumes by the cultures thus obtained. Legumes 

 have been grown in sterilized soil and watered with bacterial in- 

 fusion from these cultures: the usual result has been the growth of 

 abundant tubercles and the fixation of nitrogen. Some striking 

 experiments have been made with germinating peas. Such peas, if 

 kept moist and warm, will grow for several days, sending out their 



