56 PLANT HABITS AND HABITATS IN THE 



TEMPERATURE. 



As would be expected from the geographical relations, including the 

 altitude of Oodnadatta and of the Lake Eyre Basin in general, extreme 

 temperatures are the rule. The summers are very hot, the daily 

 variation in temperature is relatively great, and the minimum tem- 

 peratures of winter are lower than might be expected for the latitude. 

 These features, of course, are to a large degree associated with a low 

 rainfall and low relative humidity. The stabilizing influences of 

 water and of water-vapor are largely wanting. Thus the air, and 

 objects on the surface of the soil, including the vegetation, are quickly 

 heated under the very favorable conditions of insolation obtaining, 

 and conversely the radiant energy rapidly escapes with the passing 

 of day. For these reasons there are, as table 7 indicates, many days 

 during the year when the air has a temperature of over 90 F., and, 

 on the other hand, many nights when an air temperature of less than 

 40 F. is recorded. 1 In fact, in every month, except June and July, 

 a temperature of 90 F., or above, is reached, and in 7 months of the 

 year the night temperatures drop to below 40 F. The extremes thus 

 far recorded at William Creek, about 100 miles from Oodnadatta, 

 are 119 F. and 25.3 F. for a period covering 28 years. 



A summation of the mean monthly temperature for Adelaide, Port 

 Augusta, Farina, and William Creek shows that the total amount of 

 heat received increases at these stations in the order given: Adelaide, 

 747.8; Port Augusta, 793.9; Farina, 808; William Creek, 822.6 F. 



It is not likely that the temperatures attained at Oodnadatta, or in 

 the Lake Eyre Basin, are so high as to materially injure the perennial 

 vegetation. On the other hand, a large amount of heat is required for 

 the growth and development of such plants. Aside from this feature, 

 it is probable that a leading effect of high temperatures on the vegeta- 

 tion, especially perennials, is an indirect one, namely, the effect of 

 such temperatures on the relative humidity of the air, and hence on 

 the rate of evaporation. The capacity of the air to absorb water-vapor 

 increases directly with the temperature. Hence, it is found that the 

 drying power of the air is markedly greater where the temperatures 

 are higher, other conditions being equal, than where they are less. 

 The following will illustrate the point: The average mean tempera- 

 ture for Adelaide is 63 F. (17.2 C.); for Port Augusta it is 66.2 F. 

 (19 C.) ; for Farina it is 67.3 F. (19.6 C.) ; and for William Creek it is 

 68.6 F. (20.3 C.). The weight of aqueous vapor in a cubic meter of 

 saturated air for these temperatures is as follows: 14.4, 16.1, 16.6, and 

 17.4 grams, from which it is seen that an amount capable of producing 

 saturation under the mean-temperature conditions at Adelaide, and 

 assuming the equality of other conditions, would not bring about satu- 

 ration at William Creek, but only about 83 per cent of saturation. 



1 See the records for William Creek, table 7. 



