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- 
times of the ultimate) branches, will probably be found 
an ant-house, in the shape of a hollow swelling of 
the branch ; communication between the houses 
being kept up, sometimes by the hollowed interior 
of the branches, but nearly always by a covered 
way along their outside. 
The genus Cordia (Boraginaceae) affords many 
examples of this structure. One of the rather 
artificial sections into which Cordia is divided in the 
