22 PLANT HABITS AND HABITATS IN THE 



where they fall, as there are no appearances of washing and the main 

 instrument of detrital transportation is the wind, but owing to the 

 fairly abundant vegetation, as will be described below, the moving of 

 the sand, except where the vegetation has been disturbed, is not an 

 important matter. 



A notable class of plant habitats is that associated with an excessive 

 amount of salts of whatever kind in the soils. The immediate occasion 

 of the accumulation of the salts is also in part inadequate drainage, but 

 coupled with this are high evaporation and small rainfall. 



Beds of gypsum, hydrous calcium sulphate, and of travertine, or 

 desert limestone, calcium carbonate, are frequently to be found in the 

 dry regions. In certain regions outside of Australia, at least, travertine 

 is an important feature of the environment of plants in that it is not 

 easily penetrable by water and constitutes a fairly dry hardpan as a 

 subsoil. By travertine is meant "a deposit of carbonate of lime, laid 

 down on the surface of the ground by evaporating water containing the 

 substance in solution" (Jutson, 1914:228). In many places the traver- 

 tine is covered by soil and thus constitutes a subsoil. In appearance 

 the travertine strongly resembles the "caliche" of the more arid 

 portions of the United States, and is probably the same substance. 

 The exact soil horizon where the limestone is formed is in dispute 

 (Livingston, 1906 : 8) . In place of its being deposited on the surface of 

 the soil it may be deposited at the evaporating surface, which, in such 

 an arid country as southern Arizona, at least, probably lies somewhat 

 below that of the soil itself. 



The nature of the soil is another important factor of the subaerial 

 environment. It is dependent on the nature of the underlying rocks 

 from which the soil was derived by various geologic agencies. As an 

 important feature of the environment of plants it is not confined to 

 regions of small rainfall, but is to be found in the more moist regions as 

 well. Thus Osbom (1914: 113) observes in the vegetation of the Mount 

 Lofty Ranges near Adelaide that — 



"The second range of foothills, rising about 800 feet to a plateau, presents 

 several markedly distinct types of vegetation which appear to be correlated 

 with the geological formation. The slate hills are covered with grassland and 

 scattered 'gums,' having a parklike appearance. The absence of under- 

 growth and the maintenance of a sward may be partly due to grazing, but all 

 the difference observable can not be attributed to this cause. Grass is almost 

 entirely absent from the quartzite hills, which are covered by a scrub of many 

 species of shrubby plants." 



Jutson (1914:58), speaking of the vegetation of the central or salt- 

 lake division of Western Australia, which is arid or semi-arid, says: 



"[It is] divided into two main groups, viz: that growing on the basic and that 

 on the granitic rocks; the former being stronger and of a more varied character 

 and the latter often or mainly of a stunted and monotonous type, except that 

 in its annuals or small shrubs there is often both variety and beauty." 



