18 PLANT HABITS AND HABITATS IN THE 



to the extreme under desertic conditions. But the plant is under the 

 necessity of manufacturing food through energy, in part derived from 

 light. This calls for a leaf expanse adequate to this end. Therefore 

 forces are in constant operation which, on the one hand, tend to decrease 

 the extent of leaf surfaces, and on the other tend in the opposite direc- 

 tion. But in the arid regions, as has just been remarked, light is 

 abundant and of proper quality, so that a relatively or actually small 

 expanse of leaves, made necessaiy by the high rate of evaporation, is 

 also sufficient for the manufacture of foods. 



According to Schimper (1903:714) light in high altitudes, which may 

 have many of the properties of that of the desert, is very intense and is 

 , rich in actinic rays. It operates to retard the growth of shoot axes and 

 of foliage. It induces the construction, on the part of the plant, of 

 certain pigments which may possibly act as a screen, and at the same 

 time it may bring about the more rapid destruction of the chlorophyll. 

 The development of palisade cells is apparently forwarded by such 

 conditions as obtain in the mountains and on the desert, thus tending to 

 increase the xerophytic character of the vegetation characteristic of 

 these regions. 



TEMPERATURE. 



Australia possesses a very equable climate; indeed, according to 

 Hunt (1914:124), it is the most pacific and equable of all the conti- 

 nents. This in part is owing to its insularity, in part to its geographical 

 position, and in part to the comparatively low relief of its surface. 

 Taylor (1918 2 :4) makes an interesting comparison as to the temperature 

 on parallel degrees of latitude between Australia and the average for 

 each hemisphere. It appears that the Australian tropics are hotter 

 than the average for either hemisphere. On the other hand, the tem- 

 perate regions of Australia are somewhat hotter than parallel latitudes 

 in other continents of the southern hemisphere, but the opposite is true 

 with regard to Australia and the northern hemisphere. However, one 

 of the hottest regions on the globe, according to the same author, is in 

 Australia. He states that only four localities are known with an aver- 

 age annual temperature over 84 F. Of these, Timbuctu has an average 

 temperature of 84; from Massowah to Khartum the average is 86; 

 Tinnevelly, India, has an average temperature of 84.3, and, finally, 

 the average at Wyndham, northwest Australia, is 84.6 F. 



In the annual north-south progress of the seasons the 75 isotherm 

 sweeps nearly the entire continent (figs. 9a and 96) . The cooling effects 

 of the sea and of the highlands are to be seen in the curving of the iso- 

 therms. As a whole, however, the interior has relatively high tem- 

 peratures. Table 6 gives some of the highest shade temperatures 

 reported in Australia previous to 1912, and probably among the 

 highest reported anywhere. In table 7 are presented data relative to 

 the temperature at several stations, mainly in regions of small rainfall. 



