614 



ZONES AND REGIONS [Pt. Ill, Sect. II 



according to a communication for which I am indebted to Professor 

 Schweinfurth. At the time of the excavation of the Suez Canal, roots 

 were found in its bed belonging to trees growing high above it, on 

 eminences on its banks.' 



In the vast majority of plants dependent on subterranean water, the epi- 



geous parts, unlike those of the colocynth, exhibit a very marked xerophilous 

 structure which directly points to the influence of very dry air. Absence or 

 very weak development of leaf-blades, formation of thorns, felted tomentum, 

 succulence, thick cuticle, coatings of wax, reduced intercellular spaces, 

 protection of the stomata, and other xerophilous characters occur singly or 



