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xxa ANTS AND PLANT-STRUCTURE 397
its allies. The opposite leaves, 9 inches long, are
oblong-oval, obtuse with a short apiculus, near the
base abruptly panduriforni, and bearing a small ant-
sac on the midrib. All the other known species of
this large genus have non-sacciferous leaves.
In all the plants I have seen bearing sacs on the
leaves, to whatever order they belong, it is remark-
able that the pubescence consists of long hairs
having a tubercular base ; and
although I do not
see what connection that peculiarity can have with
the ants' choice of a habitation, it is probable they
find some advantage in it.
2. Of Inflated Petioles
A true swelling of the petiole, inhabited by ants,
and (as I believe) owing its existence to their
agency, I have seen only in two genera of Leguminose
Coesalpiniese,
viz. Tachigalia and Sclero-
lobium. The Tachigalia^ are low-growing riparial
trees, of black-water rivers, and have pinnate, often
silky foliage, and small, yellow, sweet-smelling, nearly
regular flowers disposed in
panicles. All have
trigonous petioles, which are mostly dilated at the
base into a fusiform sac tenanted by ants. T. caripcs,
sp. n., grows abundantly on the banks, and on
inundated islands, of the Uaupcs. It is a spreading
tree of 30 feet, and has the ramuli, petioles, and
leaves clad with a fine, close, silky pubescence.
The sacs of the petiole are inhabited by small black
ants, whose entrance is by a little hole on the
underside of the sac. T. ptychophysca, sp. n., grows
in moist sandy caatingas by the same river, and has
a similar sac on the petiole.