p was added to the sand, then
the leguminous plants flourished in the absence of nitrates and showed an
increase in nitrogenous material. They had clearly made use of the nitrogen
of the air. When these plants were examined they had small swellings or
nodules on their roots, while those grown in sterile sand without
soil-extract had no nodules. Now these peculiar nodules are a _normal_
characteristic of the roots of leguminous plants grown in ordinary soil.
The experiments above mentioned made clear for the first time the nature
and activity of these nodules. They are clearly the result of infection (if
the soil extract was boiled before addition to the sand no nodules were
produced), and their presence enabled the plant to absorb the free nitrogen
of the air.

[Illustration: FIG. 15.--Invasion of leguminous roots by bacteria.

a, cell from the epidermis of root of Pea with "infection thread"
(zoogloea) pushing its way through the cell-walls. (After Prazmowski.)

b, free end of a root-hair of Pea; at the right are particles of earth and
on the left a mass of bacteria. Inside the hair the bacteria are pushing
their way up in a thin stream.

(From Fischer's _Vorlesungen ueber Bakterien_.)]

[Illustration: FIG. 16.

a, root nodule of the lupin, nat. size. (From Woromv.)

b, longitudinal section through root and nodule.

g, fibro-vascular bundle.

w, bacterial tissue. (After Woromv.)

c, cell from bacterial tissues showing nucleus and protoplasm filled with
bacteria.

d, bacteria from nodule of lupin, normal undegenerate form.

e and f, bacteroids from _Vicia villosa_ and _Lupinus albus_. (After
Morck.)

(From Fischer's _Vorlesungen ueber Bakterien_.)]

The work of recent investigators has made clear the whole process. In
ordinary arable soil there exist motile rod-like bacteria, _Bacterium
radicicola_. These enter the root-hairs of leguminous plants, and passing
down the hair in the form of a long, slimy (zoogloea) thread, penetrate the
tissues of the root. As a result the tissues become hypertrophied,
producing the well-known nodule. In the cells of the nodule the bacteria
multiply and develop, drawing material from their host. Many of the
bacteria exhibit curious involution forms ("bacteroids"), which are finally
broken down and their products absorbed by the plant. The nitrogen of the
air is absorbed by the nodules, being built up into the bacterial cell and
later handed on to the host-plant. It appears