20 PLANT HABITS AND HABITATS IN THE 



The number of days during which the shade temperature may reach or 

 exceed 90 F. in the central portion of Australia may be very considerable 

 (table 7) . For example, in the northern part of Western Australia the 

 maximum shade temperature sometimes exceeds 90 for days or weeks at 

 a time. At William Creek, South Australia, there are, on the average, 

 114 days in each year when the thermometer registers 90 or over. 

 When it is recalled that the relative humidity of the air is a function of 

 the temperature, the significance of such long-continued high tempera- 

 tures for plant growth, more especially in the dry interior, is apparent. 



The daily range in the temperature of the air is especially striking in 

 regions where the rainfall is relatively small. Thus in 20 stations in 

 Western Australia and South Australia, whose average precipitation is 

 8.5 inches, the average daily range of temperature is 37 F. 



WINDS. 



/ 



The action of air currents, both directly and indirectly, upon plants 

 and their environment is of the greatest importance, especially in dry 

 regions. One of the pronounced characteristics of such regions is the 

 prevalence of winds. With little vegetation to impede their way, 

 they are nearly always blowing. During the seasons of rains this is of 

 comparatively little moment to plants, but with the return of dry 

 conditions, particularly during the summer, the winds operate to in- 

 crease the drought in a marked degree; and even in situations more 

 or less remote from the dry interior the ill effects of the "desert" winds 

 can frequently be seen in the withering of vegetation of all kinds. 

 Thus, in southern South Australia, distant from the dry interior over 

 250 miles, such winds are experienced occasionally and sometimes are 

 disastrous. 



Evidences of wind action are not wanting in other directions. 

 Crescentic-shaped dunes near Oodnadatta, the surfaces of which bear 

 ripple marks, and the moving of fine earth in other places, as at Copley, 

 where fences are buried beneath it, are further indications that the 

 winds are active as well as forceful. The flattening of the " gibbers," 

 which make up the desert pavement characteristic of large areas in the 

 central portion of the continent, may also be an indirect result of wind 

 action. The pavement itself is the result of the removal by the wind 

 of the finer soil particles, and, in fact, it is generally recognized as 

 probable that the wind is a very important agent of erosion in the dry 

 nterior, as evidenced in a great variety of ways (Jutson, 1914:142). 



SUBTERRANEAN ENVIRONMENT. 



The leading habitats of the desertic-semiarid regions are apparently 

 few in number. They are characterized and may be distinguished by 

 their physical nature and chemical content, as well as by their physio- 

 graphical relations. Thus, there are salt spots, salt plains, and salt 

 slopes in which the soil, often of rather fine structure, carries an excess 

 of salts. Such saline areas are often, but apparently not always, 



