ARID PORTIONS OF SOUTH AUSTRALIA. 33 



or on the other to that of material capable of imbibing water in large 

 amounts. Thus a condition of succulence may be induced, and 

 possibly also the formation of mucilage cells frequently found in 

 xerophytes. The presence of heavy cell-walls, and possibly also the 

 condition of spininess characteristic of many plants of dry regions, may 

 thus, at least in part, find a rational explanation. 



The temperatures of the air and of the soil are of very great im- 

 portance in many physiological processes of xerophytes as well as 

 other types of plants. Certain of the temperature relations may be 

 here mentioned. For example, the critical temperatures for growth 

 are to a degree specific, and on this fact may depend in part the charac- 

 teristic distribution of the species, its time of vegetative and repro- 

 ductive activity, and, in certain instances, the type of root-system 

 developed (Cannon, 1914:81, 1914:83, 1915:62, 1915:87, 1915 3 :211, 

 1916:75, 1916 2 :435, 1917:82). In evaluating the temperature of the 

 soil as an environmental factor the critical temperatures for growth of 

 any given species must be known, as well as the soil-temperatures at 

 the depth normally attained by the roots. The total expected growth 

 during the growing season with the aid of these data can be easily de- 

 termined. In this manner also we may learn the relative efficiency 

 of two stations as regards any species, so far as the soil-temperature is 

 concerned; also, the biological significance of a summation of soil- 

 temperatures may be found by the same means (Cannon, 1917: 91). 



As to the immediate effects of temperature, only a few especially 

 applicable references need be given. The osmotic pressures increase 

 with an increase in temperature and the rate at which dissolved sub- 

 stances diffuse through protoplasm also depends on temperature. The 

 hydrolysis of starch is hastened by higher temperatures up to 45 to 

 50 C. The acid-content is lowered with higher temperatures. The 

 rate of gaseous exchange, in respiration, is nearly proportional to the 

 temperature. The maximum rate occurs at about 40 C., and the 

 minimum at 10 to 15 C. (Palladin, 1917). The carbohydrate equi- 

 librium of Opuntia sp. depends in part on the water-content and 

 in part on the temperature of the plant. An increase in the tem- 

 perature results in the more rapid using up of the available simple 

 carbohydrates, the monosaccharides (Spoehr, 1917:73). The rate of 

 water absorption in agar and in biocolloids increases in general with a 

 rise in temperature up to maximum swelling of the plates, which occurs 

 near 40 C. in agar and somewhat higher in the biocolloids (Mac- 

 Dougal, 1918:68). 



The position taken in the ground by the roots of certain species has a 

 very definite relation to the aeration conditions of the soil (Cannon, 

 1918:81) and the distribution of cultivated plants (Howard and How- 

 ard, 1915), as well as certain species native to a semi-arid region (Can- 

 non and Free, 1917: 178), may also be directly related to the root re- 



