Chap. I] 



WATER 



i5 



Such manifold interchangeability of habitats among xerophytes, as is 

 found in Java, has not yet been established for other areas, possibly only 

 because the physiological — as opposed to the physical — aspect of xerophily 

 is quite new as a subject of study, and observers have bestowed very little 

 attention on such phenomena. But Battandier had already reported that 

 certain Algerian plants are confined to the alpine summits of the Atlas 

 mountains and to the sea-shore ; further, that certain ubiquitous plants, in 

 both these apparently so dissimilar habitats develop exactly similar ano- 

 malous forms ; and, finally, 

 that the cultivation of alpine 

 and littoral plants in or- 

 dinary lowland soil induces 

 similar modifications of 

 structure. 



The xerophilous character 

 of the vegetation of peat- 

 moors has hitherto been con- 

 sidered an incomprehensible 

 anomaly, and yet the rich 

 supply of humous acids in 

 the soil furnishes a condition 

 for its occurrence as com- 

 prehensible as it is necessary. 

 The presence of Scots pine 

 and heather on both dry 

 sand and on wet peat is 

 thus not more remarkable 

 than is that of Ledum pa- 

 lustre, Vaccinium uliginosum, 

 and other peat-plants on the 

 cold dry soil in the polar 

 zones. All these habitats 

 so very dissimilar in physical 

 character are dry to plants, and therefore suited for the well-being of 

 xerophytes. 



In spite of all the resemblances in the protective means employed, in 

 spite of the frequent interchange of the precise methods of protection, 

 in short, in spite of all similarity in the vegetation of various physiologically 

 dry districts and habitats, careful examination shows that certain forms of 

 xerophily are favoured by definite external conditions. The connexion 

 between structure and environment is, as a rule, easily intelligible in such 

 cases. Thus succulent plants occur chiefly in hot districts, and there alone 

 attain large dimensions, both in dry and in damp air (plants of deserts, 



Fig. 17. Xerophilous structure. Vegetation of wet saline 

 soil of the Javanese jnangrove-sivamps. Rhizophora 

 mucronata. Transverse section of leaf with aqueous tissue. 

 Magnified 70. 



