106 RECLAIMING LOST NITROGEN 
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 
nitrogen-hunger stage. 
The Tubercle Bacteria~—These facts naturally suggest that 
bacteria, or other microérganisms, 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. 
Fic. 24—Showing 24). These filaments seem to be hollow tubes 
neuchecinthetuber, which contain smaller bodies, somewhat like 
cles. The figure bacteria. As the legume increases in size 
With the sass pene these bacteria-like bodies undergo a trans- 
(Siefew them formation in shape, growing larger and branch- 
ing somewhat, so as to form structures like 
those shown in Fig. 25. ‘hese 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 bacterio- 
logical 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 thusisolated 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 
