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, i 

 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 ant> 

 keep these hundreds of servants and allies assuresv 



