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 



