TEMPEEATUEE AND ITS CONDITIONS 321 



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. Eespiration 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 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- 



Y 



