320 VEGETABLE PHYSJOLOGY 
for the germination of the seed varies between 5° C. for the 
Wheat and 18° C. for the Vegetable Marrow. The upper 
limit for this function in the cases of these two plants has 
been ascertained to be 37° C. for the former and 42° C. 
for the latter. The optimum point for the growth of the 
roots of a seedling of Maize is 27° C., while the correspond- 
ing temperature for that of the Barley and Wheat is about 
23° C. Respiration seems to show similar limits, but very 
few observations have been made upon it from this point 
of view. The optimum appears to be a little over 30° C., 
and the maximum 25 degrees higher. 
The temperature of a terrestrial plant is subject to 
great and frequent fluctuations, and there is considerable 
difficulty in securing for it for any length of time the 
optimum temperature for any of its vital functions, and 
indeed sometimes of maintaining it within the limits which 
are essential. As a rule such a plant only secures a 
general approximation to the optimum point. The difficulty 
is due to the fact that there is a continual and yet vayi- 
able interchange of heat between itself and its environment. 
During the daytime it is constantly receiving supplies of 
radiant energy from the sun, and as the air surrounding it 
becomes warmer, a certain amount is absorbed by conduc- 
tion. It is further continually expending heat on the 
maintenance of transpiration, losing it also from time to 
time by radiation and conduction. In its own metabolic 
processes it is sometimes rendering heat latent, and always 
liberating it by the processes of respiration, fermentation, 
&c. Naturally, its temperature relationships are continually 
varying. On the whole, such a plant tends to approximate 
its temperature to that of its environment, but an equalisa- 
tion is seldom reached, as both are varying simultaneously, 
and owing to the slowness of the conduction of heat along 
vegetable tissues, the processes of adjustment only take 
place with difficulty. The trunk of a tree is during the 
day often cooler than the air and warmer than the latter 
during the evening and night. The mean annual tempera- 
