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ROOT NODULE BACTERIA
that Leguminosae may benefit from the mere presence of rhizobia, regardless
of
whether nodules are formed. Other workers, however, have failed to confirm
this observation.
     Cross-inoculation groups. The specificity of the relationship existing
be-
tween the leguminous plant and its rhizobia is worthy of note. Members of
the genus Rhizobium are apparently able to infect only members of the family
Leguminosae, and conversely, practically all species of the Leguminosae are
infect-
ed by some species of Rhizobium.2 Not all species of Leguminosae are infected
by the same type of Rhiobiunim, however. Recognition of this specificity
has
lead to a classification of leguminous plants into groups, within which the
rhizobia are interchangeable.  In many cases a single plant group includes
several different species or even genera of the Leguminosae; in other cases
a
single plant species is the only known host to a particular Rhizobium.
     A detailed list of the plant-bacteria or so-called cross-inoculation
groups
has been given in Chapter 8. There is as yet no satisfactory explanation
for
the specificity which exists. A correlation between the acidity of the cell
sap and
the limiting acidity for growth of the rhizobia concerned has been suggested.
Baldwin, Fred, and Hastings, 1927, have established a certain correlation
between
the protein constitution of the seeds and the cross-inoculation grouping.
Neither
of these explanations establishes an exact correlation, and it seems probable
that
some other factor or factors in the physiological complex of the host determines
infection or non-infection.
     The work of Richmond, 1926c, is interesting in this connection. By grafting
the tops of navy bean plants, Phaseolus vulgaris, on the roots of the lima
bean,
Ph. limensis, and vice versa, he demonstrated that the conditions in the
root are
the factors determining infection. On grafted plants with lima bean roots,
only
rhizobia of the lima bean are able to induce nodule formation. And conversely,
on
plants with navy bean roots, the rhizobia of the navy bean alone may enter.
Seeds produced on such grafted plants were apparently altered in such a way
that the plants of the second generation could be infected by either the
rhizobia of
the navy bean or of the lima bean. The data supporting this latter statement
are meager. Recently Hansen and Tanner, 1931, have repeated Richmond's graft-
ing experiment and have arrived at negative conclusions as far as nodulation
is concerned. They have also done serological work on the seed proteins of
normal
and grafted plants and have found no indication of changed nature of the
seed
protein complex.
     Strain variation. Even before the specificity of the plant-bacteria
relationship
 was fully recognized, it was noted that nodules on the roots of leguminous
plants
 did not always result in fixation of atmospheric nitrogen and consequently
in-
 creased plant growth. Frank, 1892b, described two types of nodules on the
pea,
 distinguished as "amylodextrin and albuminoid nodules." The former
were
 characterized by their reddish-brown color with iodine, a reaction due chiefly
to
 absorption of iodine by numerous granulae of amylodextrin in the bacteroids
 and probably also by accumulated starchy reserve in the plant cells. Beijerinck,
 1888, divided nodules into two broad groups--one in which the plant gains
the
 2The few knowvn exceptions are listed in chapter 3.
169