Chap. VI] EDAPHIC INFLUENCES IN THE TROPICS 387 



alpine species in the solfataras steadily increase in numbers (Rhododendron 

 retusum, Gaultheria leucocarpa, Myrsine avenis). Besides these, isolated 

 ferns with leathery leaves, and lycopods, are always present in such spots. 

 It is remarkable, as Junghuhn has already recorded, that the stems of the 

 solfatara-bushes are entirely free from all epiphytic vegetation, even from 

 mosses and lichens ; as the above-mentioned author expresses it, a clean 

 sweep is made of the last-named plants — a phenomenon that is also 

 exhibited on the sea-shore. 



As has been previously explained 1 , the physiological cause of the 

 occurrence of a completely xerophilous vegetation on a wet soil, in 

 a climate with a heavy rainfall, in the midst of a most luxuriant rain- 

 forest, is supplied by the abundance of very soluble salts, especially alum 

 and other sulphates, in the solfataras. 



2. FORMATIONS OF THE TROPICAL SEA-SHORE. 



i. CLASSIFICATION OF TROPICAL LITTORAL FORMATIONS. 



Among the edaphic formations of the tropics, only those of the sea-shore 

 have as yet been accurately investigated. They owe their marked 

 characteristics partly to physical and partly to chemical causes, and, as 

 these are very variable, the formations exhibit a physiognomy that 

 frequently varies at short distances. 



Tropical littoral formations may be divided into four groups : — 



1 . Open formations of the stony and rocky shore. 



2. Open formations of the sandy shore. 



3. Littoral woodland above high-tide mark. 



4. Littoral woodland belotv high -tide mark. 



The formations on rocky and stony ground of the shore must be left 

 out of consideration, as observations regarding them are wanting. 

 According to my own occasional observations, they appear to afford 

 little that is characteristic. 



ii. OPEN FORMATIONS OF THE SANDY SHORE. 



The flat sandy shore on open coasts, exposed to the wind, is only poorly 

 clad with vegetation. This is also true of the outermost series of dunes, 

 whereas dunes further from the sea, and, still more, the valley-like 

 depressions between them, show a vegetation that increases in density 

 landwards. The most characteristic plants occur where the environment is 

 unfavourable, namely on the loose shifting sand of places exposed to the 

 full violence of the sea-wind. The oecological character of the vegetation 

 on shifting sand by the sea has been already described 2 in a general 

 manner. It was there stated that the first settlers are chiefly creeping 



1 See p. 91. ■ See pp. 179-84. 



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