172 PLANT COMPANIONSHIP IN THE FOREST.
ammonium-compounds into nitrites, and, by a further process of oxidation,
to convert nitrites into nitrates. A most interesting recent extension of this
enquiry, however, points to the actual assimilation of the free nitrogen of the
atmosphere by these Nitro-bacteria, as they are collectively termed. I allude
to the functions of the root-tubercles which are present in all members of
the Pea-family (Leguminosae), as, for example, in the Clover (Figs. 3-7).
These gall-like tubercles, formed of hypertrophied cortical tissue on
the lateral roots, result from the attacks of a fungus through the root-
Fig. 5. Seedling clover plant showing nodules on the roots.
hairs. The poorer the soil is in combined nitrogen, the more numerous
and the larger are the tubercules. The cells of the tubercles are
rich in starch and sugar, and most of them are penetrated by branches of the
fungus-mycelium. These branches "abstrict" or bud-off innumerable
gemmules, which may probably be correctly termed gonidia, resembling,
as they do, those of a common mould, like Eurotium aspergillus. (Figs. 6-7).
So similar are these gemmules to Bacteria that they are called "bacteroids,"
and it is doubtful whether in this case the presence of any true Bacteria as