XXIV 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 Legu- 
minose Caesalpinieae, viz. Tachigalia and Sclero- 
lobium. The Tachigaliae 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. caripes, 
sp. n., grows abundantly on the banks, and on 
inundated islands, of the Uaupes. 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. 
