ANT PLANTS; UNEATABLE PLANTS 415 



Certain tropical trees offer ants special inducements to estab- 

 lish 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 

 (Fig. 315), which furnishes little growths at the ends of the 

 leaflets which serve as ant food. These little growths are known 

 from their discoverer as Belt's bodies. The ants bore holes into 

 the large, hollow, stipular thorns shown in the figure, live in 

 these thorns, feed on the Belt's bodies, and protect the acacia 

 from insect and other enemies. A nectary on the leaf furnishes 

 additional food to the ant inhabitants of the tree. A great multi- 

 tude of plants, some of them herbs, offer more or less impor- 

 tant inducements to attract ant visitors ; the species which are 

 known to do this number over three thousand. 1 



387. Plants of uneatable texture. Whenever tender and 

 juicy herbage is to be had, plants of hard and stringy texture 

 are left untouched by grazing animals. The flinty-stemmed 

 horsetails (Equisetum, Sec. 316) and the dry, tough rushes are 

 familiar examples of uneatable plants of damp soil. In pastures 

 there grow such perennials as the bracken fern and the hard- 

 hack of New England and the ironweed and vervains of the 

 central states, which are so harsh and woody that the hungriest 

 browsing animal is rarely, if ever, seen to molest them. Still 

 other plants, like the knotgrass and cinquefoil of our dooryards, 

 are doubly safe, from their growing so close to the ground as to 

 be hard to graze, and from their woody and unpalatable nature. 

 The date palm, which can easily be raised from the seed hi the 

 botanical laboratory, is an excellent instance of the same un- 

 eatable quality, found in a tropical or sub-tropical plant. Other 

 good examples are the shrubs of heath lands and of such coria- 

 ceous, or leathery-leafed, thickets as the Australian scrub and 

 the California chaparral. 



i Possibly in many cases the attractiveness of plants for ants is only 

 incidental and has not been evolved with direct reference to the protection 

 to be rendered by these insects. 



