54 THE HUMAN SIDE OF PLANTS 
played by plants in protecting themselves against 
their enemies is that of an acacia of South Amer- 
ica. This tree, sometimes known as “Bull’s-horn 
thorn,” is surrounded by such vast hordes of ene- 
mies that it is necessary for it to keep a standing 
army to guard against the depredations of its foes. 
To this end it has developed a most ingenious plan. 
Its branches are covered with curious hollow thorns, 
and at the base of each leaflet is a small gland 
which secretes honey. Colonies of ants occupy the 
cavities in the thorns, and feed from the honey. 
The ants are naturally interested in their home, and, 
when it is attacked by enemies—such as moths, or 
leaf-cutting ants—the occupants of the tree pour 
down upon them like an army of warriors. As a 
result, the wise acacia is well protected in return for 
the board and lodging supplied to its standing 
army. 
But plants which keep a standing army must 
tolerate the servants, guests, farmers, nurses, dogs, 
milk cows, horses, pets, scavengers and parasites— 
in fact, the whole retinue of camp-followers which 
is incident to every armed force. Possibly the most 
human traits among the habits of these intelligent 
ants are their well-known farming, animal-raising, 
and slave-holding habits. The very fact that ants 
keep these hundreds of servants and allies assures 
