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 Legu- 

 minose 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. 



