96 RECLAIMING LOST NITROGEN. 
eighteenth century and has been more or less fully realized by 
farmers since that time. To what this soil-enriching function is 
due has not been understood till within the last thirty years. It 
is now known to be due to the fact that the legumes increase the 
nitrogen present: As already noticed, experimental evidence 
indicates that ordinary plants are unable to assimilate atmospheric 
nitrogen. Long series of experiments were conducted to test the 
matter and the more rigidly the experiments were performed, 
the more evident did it become that such an assimilation does 
not occur in ordinary green plants. It was, however, shown 
in 1883-4 that this conclusion did not hold in regard to the 
great family of legumes. It was demonstrated very conclusively 
that peas and beans, growing in a soil free from nitrogen and fed 
upon food containing no nitrogen, did, in the course of a few weeks' 
growth, increase the amount of nitrogenous material present in 
the plant, and, inasmuch as the only possible source of this nitrogen 
was the atmosphere, the conclusion was unhesitatingly drawn 
that peas can assimilate atmospheric nitrogen. This conclusion 
was contradictory to the belief accepted at the time, and although 
vigorously disputed, was soon found to be strictly correct. Mnay 
of the plants of the great family of legumes certainly do have 
the power, under certain circumstances, of fixing atmospheric nitro- 
gen and absorbing it into their tissues. 
Root Tubercles. The next step was the observation that the 
fixation of nitrogen by legumes is associated with the develop- 
ment upon the roots of little nodules known as tubercles (Fig. 24), 
These tubercles are little swellings on the roots, sometimes very 
numerous, and varying from the size of a pinhead to the size of a pea. 
They can easily be found on nearly any legume growing luxuriantly 
in the soil, if the roots are carefully dug from the soil in such 
a way as to prevent the nodules from being destroyed, and if the 
soil is carefully washed away. They were at first regarded as 
galls upon the roots, similar to those that appear upon the leaves 
and branches of trees, and, therefore, were looked upon as a type 
of disease. It is, however, evident that if they are of the nature of 
a disease, they do the plants no injury, for the plants developing 
