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 13 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. Kespiration 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 is seldom reached, 

 as both are varying simultaneously, and owing to the 

 slowness of the conduction of heat along vegetable tissues, 



