BOTANICAL ECOLOGY OR TOPOGRAPHY 125 



fluted stems, and leathery or fleshy leaves, with few 

 sunken stomata, are, therefore, adaptations to check 

 transpiration or to store water. The drought to which 

 they are subjected may be mainly due to climate, or 

 mainly to soil. To the former master factor we may 

 attribute the sclerophyllous, or leathery erect-leaved, 

 vegetation of South Africa and much of Australia; the 

 maquis of the Mediterranean; the chaparral of South 

 California ; the savannahs of Eastern Tropical Africa, of 

 the Venezuelan llanos, and the Brazilian campos; the 

 dry " patanas " of the interior of Ceylon ; the " caatinga " 

 or thorn-forest of many warm dry regions; and much 

 of the steppe and desert areas of the globe. The soil of 

 a desert is by no means necessarily sand, though 

 generally non-retentive; but where precipitation is 

 reduced to a minimum edaphic conditions are only of 

 minor import. Xerophytic formations more directly 

 due to soil have been grouped according to whether the 

 soil is dry, or is only physiologically so. Lithophytes, 

 Algae, Lichens, and Mosses clinging to steep bare rock- 

 surfaces prepare the soil for such phanerogamic chasmo- 

 phytes, or crevice-haunting, species as Saxifrages or 

 Sempervivum ; and such chomophytes or rubble-dwelling 

 types as Sedum are found alike on mountain " screes " 

 and on maritime shingle-beds. Among psammophytes, 

 plants of porous sands, we may class the marram-grass 

 of the almost bare, drifting " white " dunes, the Sea- 

 buckthorn (Hippophae) and dwarf Willows of the more 

 fixed " grey " ones, the Ling of more inland heaths, and 

 by far the greater part of the coniferous forests of the 

 world. The thick fronds of the Wall-rue (Asplenium 

 Ruta-murarm L.), the small leathery leaves of the 

 Rock-rose (Helianthemum Chamtecistus Mill.), and the 

 distinctive hairiness of the Wayfaring- tree (Viburnum 

 Lantana L.) serve to indicate that calcicole, or lime- 

 dwelling, plants are decidedly xerophytic. 



There are three marked ecological sub-classes of 

 xerophytes which may grow in wet ground because the 

 soil is to them physiologically dry. These are the 

 Bog-xerophytes or oxylophytes, growing mainly in acid 

 humus, such as Sphagnum, Myrica Gale L., Eriophorum, 

 Calluna, and the insectivorous plants, Drosera, etc. ; the 

 majority of alpine plants or psychrophytes, growing in 

 soil that is often so cold as to hinder root-absorption; 



