Chap. I] 



WATER 



*3 



Correlations among Xerophytes of Different Habitats. 



The contrivances for conserving water, which have been already described, 

 appear to be identical in xerophytes growing in habitats where the danger 

 of desiccation is due to most diverse causes — it may be to physical drought, 

 to coldness of the soil, to abundance of soluble salts or of humous acids in 

 the soil, or to reduced atmospheric pressure. That we have not here 

 merely a case of accidental external resemblance may be safely inferred 

 from the fact that many xerophytes are satisfied with physiologically dry 

 habitats of the most diverse kinds, but are never found in the much more 

 physically similar habitats of hydrophytes. 



This interchange of physiologically dry habitats may be observed in 

 West Java. The character of the vegetation in this area, as determined 



Fig. 15. Xerophilous structure. Jl'et saline 

 soil of favanese mangrove-swamp . Son- 

 neratia acida. Water-storing tracheids from 

 the end of a vascular bundle in the leaf. 



Fig. 16. Xerophilous structure. Vegetation 

 of dry bark (^epiphytes). Pleurothallis. 

 Water-storing tracheids in the leaf v Blu- 

 menau, Brazil. 



by the climate, is decidedly hygrophilous ; xerophytes are confined to very 

 limited stations, the physical character of which varies greatly. Such are, 

 for instance : — 



1. Dry lava-gravels and other stony substrata, as at Gunong Guntur. 



2. The bark of trees (epiphytes). 



3. The sea-shore, including mangrove-swamps which are still inundated 

 at ebb-tide. 



4. Solfataras. with wet clay soil, impregnated with alum and other 

 soluble salts. 



5. Alpine highlands with rarefied air and strong insolation. 



More dissimilar physical conditions cannot be well imagined than those 



