IO4. RECLAIMING LOST NITROGEN 
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 green plants are unable to assimilate 
atmospheric nitrogen. 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. Many 
of the plants of the great family of legumes certainly do have 
the power, under certain circumstances, of fixing atmospheric 
nitrogen 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 or tubercles (Fig. 23). 
These nodules 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. Itis, however, evident that if they are of the nature of 
a disease, they do the plants no injury, for the plants developing 
these tubercles are as luxurious as those without them. Indeed, 
as soon as the nitrogen-fixing power of legumes was demonstrated, 
it became evident that the fixation of nitrogen was associated with 
