In the forests of South America there grow 'Imbauba,' or candelabra-trees, species of the genus _Cecropia_, which well deserve their name, for their bare branches stretch out like candelabra, and bear little bunches of leaves only at their tips. These leaves are menaced by the leaf-cutting ants of the genus _Œcodoma_, which attack numerous species of plants in these regions, often in tens of thousands, biting off the leaves, cutting them in pieces on the ground, and carrying them on their backs piece by piece to their nests. There they use them to make a kind of compost heap, on which fungi, to which the ants are very partial, readily grow. The candelabra-tree protects itself from these dangerous robbers, inasmuch as it has established an association with another ant (_Azteca instabilis_), which finds a safe dwelling-place in its hollow, chambered stem (Fig. 37, _A_), and feeds on a brown sap which oozes from the inside. On the stem there are even little pits regularly arranged in definite places (_E_), through which the female of _Azteca_ can easily bore her way into the interior. There she lays her eggs, and soon the whole interior of the trunk teems with ants, which come trooping out whenever the tree is shaken. [Illustration: FIG. 37. _A_, a piece of a twig of an Imbauba-tree (_Cecropia adenopus_), with the leaves cut off. At the leaf-bases are the hair-cushions (_P_). _E_, the opening for the associated ant (_Azteca instabilis_). _B_, a piece of the hair-cushion with the egg-shaped nutritive corpuscles (_nk_). After Schimper.] This alone would not suffice to protect the tree against the leaf-cutting ants, for how should the Aztec ants living inside notice the presence of the lightly climbing leaf-cutters? But that is provided for, for the Aztecs also frequent the outside of the trunk, and just where attack would be most disastrous, namely, at the stalks of the young leaves. At these places there is a peculiar velvet-like cushion of hair (_P_), from which grow little stalked white papillæ (Fig. 37, _B_), which are rich in nourishment, and are not only eaten by the ants, but are harvested by them, being carried off into the ants' dwellings, presumably to feed their larvæ. In this case, then, a particular organ, offering special attraction to ants, has been developed by the plant at the places more especially threatened; while, as regards the ants, it is probable that only the instincts of feeding and habitat require to be modified, since courage and thirst for battle are present in all ants, almost any species being ready at any time to throw itself on any other which intrudes into its domain.