LEGUMINOUS PLANTS AND SYMBIOTIC FIXATION IO5 
the formation of tubercles. Only such plants as develop tubercles 
are able to use atmospheric nitrogen, 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 
luxuriantly and show a larger increase in nitrogen. 
These facts led to experiments in regard to their formation of 
tubercles. The tubercles will not form upon the roots of legumes 
grown in sterilized soil, Under these 
circumstances the plants develop no tu- 
bercles, fix no nitrogen and, unless fed 
with nitrogenous food, make very little 
growth, being stunted and small. It 
was next shown that if the lezumes were 
sown in sterilized sand, without nitrog- 
enous food, and were then moistened 
by water which had been standing in 
contact with ordinary soil, results were 
quite different. Such water, sometimes 
called a soil infusion, is made by simply 
soaking soil in water and then filtering 
off the solid particles, using the filtrate "23-77%? Sop boon 
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 with a good yield. 
