82 THE CIRCULATION OF NITROGEN IN NATURE. 
innumerable bacterium-like bodies, or rhizobes, which form 
the means by which new plants are infected. 
These organisms can be cultivated by the usual bacterio- 
logical methods, and the cultures so obtained act just as well 
as the tubercles themselves in producing tubercles on new 
plants. 
Some confusion has, however, been created by various other 
investigators, who have produced evidence to show that the 
nitrogen may be fixed by the plants independently of the 
tubercles, and even by soil containing organic matter 
independently of plants altogether. 
The question may, however, be said to have been rescued 
from the chaotic condition into which it threatened to 
relapse by the admirable investigations of Schloesing and 
Laurent at the Pasteur Institute. 
It will be noted that the class of evidence already referred 
to does not establish the whole truth ; it proves that the plants 
plus the tubercles have acquired an excess of nitrogen over 
that furnished in the soil, and it proves also that this close 
relationship is necessary for the assimilation of this nitrogen, 
but it does not prove that the nitrogen is derived from 
the air. This direct proof can only be obtained obviously 
from a careful measurement of the nitrogen supplied in the 
gaseous state to the plant during its growth, a matter of 
great difficulty, considering the length of time — some three 
months — over which an experiment must extend. 
If, however, these two sets of measurements can be carried 
out simultaneously — that is, if on the one hand the plant is 
proved to have assimilated a quantity of nitrogen over and 
above that contained in the soil, the seed, the water, and 
manurial matters, and on the other hand the air in which 
the plant is immersed has lost a certain quantity of nitrogen 
whichj within the limits of experiment, is the same as that 
