ANTS AND PLANTS 197 



In ferns there appear to be two devices for har- 

 bouring ants. In the tropics a great many ferns grow 

 on trees, and their roots are more or less exposed to 

 the air, and in order to prevent their drying up some 

 of the sterile fronds are developed into roof-like 

 shelters. In Polypodium quercifolium these sterile 

 fronds are concave on their upper surface, like dried 

 paper in consistency, and they are pressed closely to 

 the trunk of the tree on which the fern grows; in the 

 cavity so formed are found rootlets, and ants. Beccari 

 asserts that in cultivated specimens these sterile fronds 

 are not so papery nor so concave, and he thinks 

 that these differences are due to the absence of ants. 

 But the conditions of life under cultivation are so 

 unlike natural conditions that these slight differences 

 may well be attributed to other causes than absence 

 or presence of ants. In Polypodium carnosum and 

 P. sinuosum it is the rhizomes which afford shelters 

 for ants, and Professor R. H. Yapp has made a very 

 careful study of these two species. 1 Both have large 

 fleshy rhizomes which are tunnelled by a system of 

 galleries, and both are invariably inhabited by colonies 

 of ants. The former species grows on the higher 

 branches of trees in thick encrusting masses, often 

 several feet in length, completely encircling the 

 branches of its host. P. sinuosum grows nearer the 

 ground on the trunk or lower branches of a tree, and 

 its creeping rhizomes do not form such massive 



species described by Beccari. The tubers are not hollowed out 

 or inhabited by ants, but when they have become cracked or 

 split, ants may at least temporarily make a nest there, as in 

 Ficus irregularis (Annals of Botany, XXIV. (1910), p. 482), H. N. R. 

 1 Annals of Botany, XVI. (1902), p. 185. 



