Ch. XII.] ADAPTATION OF A TREE TO ITS AXTS. 219 



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 he seen running about, especially over the young 

 leaves. If one of these be touched, or a branch shaken, 

 the little ants (Pseudomyrma bicolor, Guer.) 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 brows- 

 ing on the leaves, but delivers it from the attacks of a much 

 more dangerous enemy the leaf-cutting ants. For these 



V 



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 mid-rib, is a 

 crater-formed gland, which, when the leaves are young, 

 secretes a honey-like liquid. Of this the ants are very 

 fond ; and 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 pro- 

 vision 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 con- 



