MODIFICATION OF THE ORGANS OF PLANTS 



87 



tropics, especially in South America, plants are 

 much exposed to the ravages of leaf -eating ants 

 (Atta). In order to protect themselves, they provide 

 other kinds of ants with shelter and nourishment, 

 and this necessarily entails several modifications. 



FIG. 48. Part of a leaf of Acacia sphaerocephala (after Schimper, Die Wechsel- 

 beziehungen zwischen Pflanztnund Ameisen im tropischen Amerika. Fasc. 1 

 of Schimper 's botanische Mittheilungtn aus den Tropen. Jena, 1888). 



In Acacia sphaerocephala (fig. 48), for instance, the 

 blades of the leaves are bipinnated and the proximal 

 leaflets terminate in small cavities filled with a nutri- 

 tive secretion. The stipules, which no longer contain 

 chlorophyll, are modified into hollow thorns in which 



