SYMBIOTIC BACTERIA AND LEGUMINOUS PLANTS. 
97 
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 
the formation of tubercles. Only such plants as develop tubercles 
are able to increase the amount of nitrogen in their tissues, and 
the amount of nitrogen fixation is roughly proportional to the 
development of tubercles. Plants without tubercles show no 
increase; those with a moderate number, a 
slight increase; and those with abundant 
tubercles grow luxuriently and show a larger 
increase in nitrogen. 
These facts led to experiments in regard 
to their formation of tubercles. The tuber- 
cles will not form upon the roots of legumes 
grown in sterilized soil. Under these cir- 
cumstances the plants develop no tubercles, 
fix no nitrogen and, unless fed with nitro- 
genous food, make very little growth, being 
stunted and small. It was next shown that 
if the legumes were sown in sterilized sand, 
without nitrogenous food, and were then 
moistened by water which had been stand- 
ing in contact with ordinary soil, results were quite different. 
Such water, sometimes called a soil infusion, is made by simply soak- 
ing soil in water and then filtering off the solid particles, using the 
filtrate for watering the growing legumes. Plants watered with such 
infusions show two interesting stages of growth. They sprout 
readily and for a short time grow vigorously; then the vigorous 
growth ceases and the plant seems to be suffering for lack of food. 
This has been called the nitrogen hunger stage, and represents a 
period in which the plant has used up the nitrogen in the seed, and 
consequently all that was within reach. Control plants, grown in 
similar soil and watered with pure water, never recover from 
this stage, but those that were watered with the soil infusions, 
after a few days of such nitrogen hunger, recover, begin once 
more a vigorous growth and eventually produce large-sized plants 
9 
FIG. 24. Tubercles on the 
roots of the Soy bean. 
