140 



THE FACTORS 



[Part I 



which appears to prove that certain ethereal oils attract the parasol-ants, 

 whilst others repel them. Such phenomena give indications regarding 

 the paths that natural selection must have followed in the struggle of 

 the vegetation against the parasol-ants. 



Whilst the protective contrivances belonging to the plant itself and 

 causing the species to be more or less completely avoided by the leaf- 

 cutters can at present form the subject of hypothesis alone, yet, in the 

 case of some species, it has been proved that in order to keep their foes 

 at bay, the plants enter into symbiotic relations with definite bellicose 

 ants by whom they are wellnigh perfectly protected. 



ii. MYRMECOPHILY. 



Plants with adaptations for attracting ants are termed myrmecophilous. 

 As is proved by the occurrence of such contrivances in the tropical zones 



Fig. 76. Acacia sphaerocephala. / Part of stem with stipular thorns, S, and a leaf with 

 Belt's corpuscles, F. On the petiole at JVz nectary. Reduced. //Single leaflet, F Belt's 

 corpuscles. Somewhat enlarged. From Strasburger's Text-book of Botany. 



of the Old World, myrmecophily has also been evolved for the purpose 

 of combating other enemies of vegetation, and chiefly, in fact, for the 

 protection of flowers from insect-bites. On the other hand, in temperate 

 zones, in accordance with the relative paucity in ants, definite allurements 

 for these insects are weakly indicated and only in a few plants. Typical 

 myrmecophily is one of the characteristic features of the tropical flora. 



Belt must be considered as the actual discoverer of myrmecophily, 

 although Delpino, on the basis of much less convincing material, nearly 

 simultaneously and quite independently published the same idea. In 

 Nicaragua, and on the Amazon, Belt became acquainted with several 

 species of myrmecophytes, but he devoted his attention chiefly to Acacia 

 cornigera, which is now one of the best-known examples of this class 

 of phenomena, as is also the very similar A. sphaerocephala (Fig. 76), 



