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are covered with strong curved spines, set in pairs, from which it
receives the name of the bull's-horn, they having a very strong
resemblance to the horns of that quadruped. These horns are hollow, and
are tenanted by ants, that make a small hole for their entrance and exit
near one end of the thorn, and also burrow through the partition that
separates the two horns; so that the one entrance serves for both. Here
they rear their young, and in the wet season every one of the thorns is
tenanted, and hundreds of ants are to be seen running about, especially
over the young leaves. If one of these be touched, or a branch shaken,
the little ants swarm out from the hollow thorns, and attack the
aggressor with jaws and sting. They sting severely, raising a little
white lump that does not disappear in less than twenty-four hours.

These ants form a most efficient standing army for the plant, which
prevents not only the mammalia from browsing on the leaves, but delivers
it from the attacks of a much more dangerous enemy--the leaf-cutting
ants. For these services the ants are not only securely housed by the
plant, but are provided with a bountiful supply of food; and to secure
their attendance at the right time and place, this food is so arranged
and distributed as to effect that object with wonderful perfection. The
leaves are bi-pinnate. At the base of each pair of leaflets, on the
midrib, is a crater-formed gland, which, when the leaves are young,
secretes a honey-like liquid. Of this the ants are very fond; they are
constantly running about from one gland to another to sip up the honey
as it is secreted. But this is not all; there is a still more wonderful
provision of more solid food. At the end of each of the small divisions
of the compound leaflet there is, when the leaf first unfolds, a little
yellow fruit-like body united by a point at its base to the end of the
pinnule. Examined through a microscope, this little appendage looks like
a golden pear. When the leaf first unfolds, the little pears are not
quite ripe, and the ants are continually employed going from one to
another, examining them. When an ant finds one sufficiently advanced, it
bites the small point of attachment; then, bending down the fruit-like
body, it breaks it off and bears it away in triumph to the nest. All the
fruit-like bodies do not ripen at once, but successively, so that the
ants are kept about the young leaf for some time after it unfolds. Thu