346 FOUNDATIONS OF BOTANY 



there are plenty of instances of structures, habits, or accu- 

 mulations of stored material in their tissue which plants 

 seem to have acquired mainly or entirely as means of 

 defense. Some of the most important are : 



(1) The habit of keeping a bodyguard of ants. 



(2) Mimicking the appearance of dangerous or uneatable plants, or 



imitating pebbles or earth, so that they may be overlooked. 



(3) Forming tough, corky, woody, limy or flinty and therefore 



nearly uneatable tissue. 



(4) Arming exposed parts with cutting edges, sharp or stinging 



hairs, prickles, or thorns. 



(5) Accumulating unpleasant or poisonous substances in exposed 



parts. 



414, Ant-Plants. Some ants live on vegetable food, 

 but most of them eat only animal food, and these latter 

 are extremely voracious. It has been estimated by a 

 careful scientist, an authority on this subject, that the 

 ants of a single nest sometimes destroy as many as one 

 hundred thousand insects in a day. The Chinese orange- 

 growers in the Province of Canton have found how useful 

 ants may be as destroyers of other insects, and so they 

 place ant nests in the orange trees and extend bamboos 

 across from one tree to another, to serve as bridges for the 

 ants to travel on. 



Certain tropical trees, in order to insure protection by 

 ants, offer them especial inducements to establish colonies 

 on their trunks and branches. The attractions which are 

 offered to ants by various kinds of trees differ greatly. 

 One of the most interesting adaptations is that of an 

 acacia 1 (Fig. 242), which furnishes little growths at the 

 ends of the leaflets which serve as ant food. These little 



1 A. sphaerocephala. 



