Mr. Francis Darwin, in a lecture on "Means of Self-Defense among
Plants," delivered lately at the London Institution, said that one of
the most curious forms of defense known is afforded by a recently
discovered class of plants, which, being stingless themselves, are
protected by stinging ants, which make their home in the plant and
defend it against its enemies. Of these the most remarkable is the
bull's-horn acacia (described by the late Mr. Belt in his book "The
Naturalist in Nicaragua"), a shrubby tree with gigantic curved thorns,
from which its name is derived. These horns are hollow and tenanted by
ants, which bore a hole in them, and the workers may be seen running
about over the green leaves. If a branch is shaken the ants swarm out
of the thorns and attack the aggressor with their stings. Their chief
service to the plant consists in defending it against leaf-cutting
ants, which are the great enemy of all vegetation in that part of
America. The latter form large underground nests, and their work of
destruction consists in gathering leaves, which they strip to form
heaps of material, which become covered over with a delicate white
fungus, on which the larvæ of the ants are fed, so that literally they
are a colony of mushroom growers. The special province of the little
stinging ants, which live in the thorns of the acacia, is, therefore,
to protect the leaves of the shrub from being used by the leaf-cutters
to make mushroom beds. Certain varieties of the orange tree have
leaves which are distasteful to the leaf-cutters, this property of the
leaves thus forming a means of defense. Other plants are unaccountably
spared by them--grass, for example, which, if brought to the nest, is
at once thrown out by some ant in authority. The bull's-horn acacia,
in return for the service rendered by the stinging ants, not only
affords them shelter in its thorns, but provides them with nectar
secreted by glands at the base of its leaves, and also grows for them
small yellow pear-shaped bodies, about one twelfth of an inch in
length, at the tip of some of its leaflets, which they use as food.
These little yellow bodies are made up of cells containing protoplasm
rich in oil, and afford the insects an excellent food. When the leaf
unfolds, the ants may be seen running from one leaflet to another, to
see if these little yellow bodies are ripe; and if they are ready to
be gathered they are broken up by the ants and carried away to the
nest in the thorn. Several small birds, also, build their nests in the
bull's horn acacia, thus escaping from a predatory ant which is
capable of killing young birds. The trumpet tree, another plant of
South and Central America, is also protected by a standing army of
ants; and, like the above mentioned acacia, grows for its protectors
small food bodies containing oil, but instead of secreting nectar in
its leaves it harbors a small insect (coccus), whose sweet secretion
is much relished by the ants. Dr. Beccari mentions an epiphytal plant
growing on trees in Borneo, the seeds of which germinate, like those
of the mistletoe, on the branches of the tree; and the seedling stem,
crowned by the cotyledons, grows to about an inch in length, remaining
in that condition until a certain species of ant bites a hole in the
stem, which then produces a gall-like growth that ultimately
constitutes the home of the ants. If the plant is not fortunate enough
to be bitten by an ant it dies. These ants, then, protect their plant
home by rushing out fiercely on intruders, and thus are preserved the
sessile white flowers which, in this plant, are developed on the tuber
like body.
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ADVANCE IN IRON.--At a meeting of the Philadelphia Iron Merchants'
Association, March 11, prices of all descriptions of merchant iron
were advanced fully 5 per cent.
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