PLANT HABITS AND HABITATS IN THE ARID 

 POETIONS OF SOUTH AUSTRALIA, 



INTRODUCTION. 



Australia, especially South Australia, holds much of interest to the 

 student of the vegetation of arid regions. Where rain is abundant 

 plants compete with one another in a very real way for room in which 

 to live and for sunlight by which they gain energy for various life 

 processes, but in regions of scanty rainfall, as in portions of South 

 Australia, there is abundance of room and of light. Here the "struggle" 

 is associated with the water relation ; it is that of the individual plant 

 with an arid environment, and not individual with individual. When 

 viewed from this standpoint the island continent is seen to be the field 

 of a vast botanical experiment in which may be observed the reaction 

 of numerous species and innumerable individuals to a physical en- 

 vironment, a leading characteristic of which is a relatively small water- 

 supply. Moreover, owing to the great age of Australia, it is possible 

 that nowhere else have plants been exposed to and influenced by an 

 arid environment for a longer period of time. 



The physical background of the Australian plants is in a measure 

 unique. The dry region is very extensive. Some idea of its size can 

 be had by the statement that it has nearly as great an area as all Arabia, 

 and as a matter of fact is larger than all other regions of the kind south 

 of the equator. Living and developing under an environment of which 

 the keynote is aridity, the flora of the continent as a whole bears a 

 xerophytic stamp and appears to possess a degree of uniformity which 

 constitutes one of its most marked characteristics. Wherever one 

 goes in Australia, he encounters trees and shrubs with leathery, ever- 

 green leaves. In the better- watered portions the trees are large and 

 numerous and there is an extensive transpiration surface, but in 

 portions less favored in this particular the trees are not large, a shrubby 

 type of vegetation prevails, and the transpiration surface is also much 

 restricted in area. Between the two extremes there are innumerable 

 intermediate conditions in which the gradations are quantitative 

 rather than qualitative. When studied in some detail, however, there 

 may be found a bewildering variety of adjustments to the environment, 

 often monotonous perhaps in outward appearance, and which have in 

 a measure the force of belying the generalization just made. 



The physical and biological complexes which enter into our concep- 

 tion of what constitutes an arid, more especially a desert, region are 

 made up of many features. It is true that they center around the 

 important factor of a small water-supply, but they all should be 

 logically included in any definition of such regions with scanty rainfall. 



