TEMPERATURE 121 



the respiration activity. At higher temperatures variation 

 obtains and it was found that the respiration intensity rapidly 

 changed with the time of exposure and that leaves in pre- 

 cisely the same external conditions gave varying results due 

 to some internal factor. This is illustrated by the following 

 results of certain experiments on isolated cherry laurel leaves 

 exposed to light and carbon assimilating. The respiration 

 values represent the carbon dioxide given off by 2 grams of leaf 

 per hour :— 



Also it was found * that the leaves of this plant in their 

 respiration exhibit the same relation to temperature as they 

 do in their carbon assimilation in that the initial value at 

 higher temperatures is not maintained but falls away, the 

 higher the temperature the steeper the fall. 



An increased temperature must be operative for some time 

 before the stimulation of the respiratory process is noticeable, 

 a sharp variation in temperature is without effect and a change 

 in the respiration intensity after transference from one tem- 

 perature to another is gradual. f 



Muller-Thurgau and Schneider-Orelli % in their investi- 

 gation on the respiration of potatoes found much the same 

 phenomena : mere variation in temperature effected by 

 heating and cooling is without, result unless the exposure to 

 the higher temperature be sufficiently prolonged. Potato 

 tubers, after an exposure to a temperature of 40 C. for 

 several hours and then transferred to room temperature, 

 show a gradual increase in respiration intensity which reaches 

 its maximum after a lapse of twenty-four hours. Exposure 

 to higher temperatures, e.g. 44 C, results in a permanent 

 increase in the intensity of respiration which indicates a per- 

 manent change in the organism. To obtain this stimulation 

 the temperature must be sufficiently high, no effect being 



* See Blackman : " Ann. Bot.," 1905, 19, 281. 



t Blanc : " Rev. gen. Bot.," 1916, 38, 65. 



X Muller-Thurgau and Schneider-Orelli : " Flora," 1910, IOI, 309. 



