Untitled - The Alfred Russel Wallace Website

ANTS AND PLANT-STRUCTURE 399
of tropical America, the knee of the petiole may
sometimes be seen hollowed and enlarged by ants ;
[but the action of these insects has not been
maintained with sufficient constancy to render the
swelling a permanent character in any species of
Cassia I have met with].
Ants congregate on the pods of some Cassias
and other plants which have seeds in sweet pulp ;
and on those parts of any plant where* they find
suitable food, in the shape of mucilaginous exuda-
tions, etc. ; but
they mostly sojourn there just so
long as that food lasts, and no longer ;
or otherwise
they merely visit the plants for the sake of collecting
their products and carrying them off at once to a
permanent
storehouse elsewhere.
3. Of Inflated Branches
Ants' nests in swellings of the branches are
found chiefly in soft-wooded trees of humble growth,
which have verticillate or quasi-verticillate branches
and leaves, and especially where the branches put
forth at the extremity a whorl or fascicle of three
or more ramuli ;
then,
either at each leaf- node or
at least at the apex of the penultimate (and some-
limes of the ultimate) branches, will probably be found
an ant-house, in the shape of a hollow swelling of
communication between the houses
the branch ;
being kept up, sometimes by the hollowed interior
of the branches, but nearly always by a covered
way along their outside.
The genus Cordia (Boraginaceoe) affords many
examples of this structure. One of the rather
artificial sections into which Cordia is divided in the