Water in Trees under Australian Conditions. 87 
or even 120 F. The curves appended 1 (Fig. 1) give some idea of the character 
of these changes, which usually end suddenly when the cool south wind sets 
in, the temperature often falling a degree a minute for a quarter to half an 
hour or at a less rate over a somewhat longer period. The rate of evaporation 
from a free surface of water is exceedingly rapid when the hot dry north 
wind is blowing from the interior, and since these hot spells often come 
when the ground is still quite moist, we should expect to find the rate 
of transpiration extremely high at such times. The hot spell rarely lasts 
long and is usually followed by a cool change often accompanied by rain. 
The fall of temperature is usually more rapid than the rise, but in the curve 
for November 19? t 897 > the reverse is the case. In neithei case does the 
suddenness of the change appear to operate injuriously upon the vegetation, 
while its rapidity and irregular occurrence would be sufficient to prevent 
slowly responding plants specially adapting themselves to it. 
Nevertheless a prolonged hot spell results in a regulatory diminution 
of the rate of transpiration even when the soil is kept moist. Cut branches 
supplied with water show the same peculiarity, although, as might be 
expected, they always transpire less actively under otherwise similar 
conditions than plants rooted in the soil. The amount of watei passing 
l Extracted from unpublished Observatory records with Mr. Baracchi’s kind consent. 
