326 VEGETABLE PHYSIOLOGY 
begin when the soil has a temperature but slightly above 
the freezing point of water; in the case of the Tobacco- 
plant it must be at 12°C. at least. The lowest temperature 
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 87° 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 vari- 
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 though much smaller amount 
is absorbed by conduction. It is further continually ex- 
pending 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 equalisation ig seldom reached, 
as both are varying simultaneously, and owing to the 
slowness of the conduction of heat along vegetable tissues, 
