ANTS AND PLANT-STRUCTURE 403 
calyces, is common all along the Amazon, both on 
•the river banks and in marshy inland sites ; and 
solitary trees of it are often seen standing out above 
the Cacao plantations. T. Schombttrgkii, Benth., a 
smaller tree, grows in the same way on the Upper 
Orinoco and Casiquiari. These trees, as well as 
the other arborescent Polygoneae, have slender 
elongated tubular branches, often geniculate at the 
leaf-nodes, and nearly always with perforations, like 
pinholes, just within the stipule of each leaf, which 
are the sallyports of the garrison, whose sentinels 
are besides always pacing up and down the main 
trunk, as the incautious traveller finds to his cost 
when, invited by the smoothness of the bark, he 
ventures to lean his back against a Tachi tree. 
I suspect that the remote progenitors of these 
ants have at first sheltered in the ocrea (sheathing 
stipule) w^hich is so characteristic a feature of the 
Polygoneae ; but, having found the wood soft and 
thin and the pith easy to scoop out, have made 
their more secure abode within the stem and 
branches. 
Some Tachi trees seem as if they were actually 
trying to run away from the ever-encroaching ants. 
Coccoloba pa7Hinensis, Benth., found by Schomburgk 
in British Guayana and by myself on the river 
Uaupes, is an arbuscle with a stem 15 feet long, 
that tapers upwards and arches over so as finally 
to touch the ground, the ants all the while hollow- 
ing it out, as it stretches away apparently in the 
hopeless attempt to escape their invasion. Some 
slender Coccolobas climb high into the adjacent 
trees, not by twining but by crooking their branches 
and thereby hoisting themselves up ; others are 
