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398 NOTES OF A BOTANIST CHAP.
The species of Sclerolobium are not usually
riparial, but one species (S. odoratissimum, sp. n.)
is eminently so, constituting a great ornament of
the. shores and islands of the Rio Negro towards
the mouth of the Casiquiari, and perfuming the
whole breath of the river with the abundance of its
pale yellow honey-scented flowers ; and it is notable
that this is the only species of the genus in which
I have found sacciferous petioles. The sac is large,
extending upwards from the knee of the petiole to
the base of the second pair of leaflets, and it has a
furrow along the upper face.
I presume the ants have been induced to take
up their residence on these particular trees on
account of the abundance and long persistence of
their honied flowers. On other species of Sclero-
lobium, inhabiting dry lands solely,
such as ,5. tincto-
riinn, Benth., and 5". paniculatum, Vog., I have seen
the flowering panicles infested with little fire-ants,
which, however, seemed to have their permanent
habitation in the Ground, about or near the treeo
roots, and never to perforate the leaf-stalks. Many
other Leguminosas, especially the woody climbing
Phaseoleae, are visited by ants when in flower, and
knobs or galls caused by the perforation of those
insects are frequent on the panicles of Dioclea
and allied genera ;
[but I have not remarked any
instance of such knobs having become hereditary,
except in Pterocarpus ancylocalyx, Benth., a small
tree on the banks of the Solimoes or Upper Amazon,
which has the rachis of <
the racemes thickened in
the middle, the swelling being sometimes (but not
always) tenanted by ants].
In the shrubby Cassias, which are common weeds