knotty, wart-like growths on the roots of the
plants just named. These tubercles are caused by tiny forms of life
called, as you perhaps already know, bacteria, or _germs_.

[Illustration: FIG. 23. TUBERCLES ON CLOVER ROOTS
The specimen at the right was grown in soil inoculated with soil from an
old clover field. The one at the left was grown in soil not inoculated]

Instead of living in nests in trees like birds or in the ground like
moles and worms, these tiny germs, less than one twenty-five thousandth
of an inch long, make their homes on the roots of legumes. Nestling
snugly together, they live, grow, and multiply in their sunless homes.
Through their activity the soil is enriched by the addition of much
nitrogen from the air. They are the good fairies of the farmer, and no
magician's wand ever blessed a land so much as these invisible folk
bless the land that they live in.

Just as bees gather honey from the flowers and carry it to the hives,
where they prepare it for their own future use and for the use of
others, so do these root-tubercles gather nitrogen from the air and fix
it in their root homes, where it can be used by other crops.

[Illustration: FIG. 24. SOY BEANS AND COWPEAS, TWO GREAT SOIL-IMPROVERS]

In the earlier pages of this book you were told something about the food
of plants. One of the main elements of plant food, perhaps you remember,
is nitrogen. Just as soon as the roots of the leguminous plants begin to
push down into the soil, the bacteria, or germs that make the tubercles,
begin to build their homes on the roots, and in so doing they add
nitrogen to the soil. You now see the importance of growing such crops
as peas and clover on your land, for by their tubercles you can
constantly add plant food to the soil. Now this much-needed nitrogen is
the most costly part of the fertilizers that farmers buy every year. If
every farmer, then, would grow these tubercle-bearing crops, he would
rapidly add to the richness of his land and at the same time escape the
necessity of buying so much expensive fertilizer.

=EXPERIMENT=

Take a spade or shovel and dig carefully around the roots of a
cowpea and a clover plant; loosen the earth thoroughly and then
pull the plants up, being careful not to break off any of the
roots. Now wash the roots, and after they become dry count the
nodules, or tubercles, on them. Observe the difference in size. How
are they arranged?