246 THE MOVEMENTS OF WATER 



is but slight in amount, as is also the transpiration dependent upon it ; but 

 nevertheless only a very slightly higher temperature is necessary to enable 

 distinct transpiration to proceed in a saturated atmosphere, while on the 

 other hand a very slightly lower temperature than that of the surrounding 

 saturated air is sufficient to cause water to be condensed upon the plant 

 as upon any other cold body (Sect. 27). Any radiant heat absorbed by the 

 plant will naturally affect the rate of transpiration, and the loss of heat 

 by radiation will have the opposite effect. For these and other reasons the 

 amount of water transpired by the plant in a saturated atmosphere can 

 hardly be used to determine the amount of heat produced by it, as was 

 proposed by Sachs 1 . It has already been shown (Sect. 27) that the vapour 

 tension is but little affected by the condition of turgidity, so that even in 

 a flaccid plant a very slight difference of temperature above that of the 

 surrounding saturated air suffices to cause water to be evaporated. 



The temperature of the plant and of the air exercise in every case a direct 

 influence upon transpiration, just as the temperature does upon evaporation 

 in general. In addition, several indirect effects are produced, for the 

 turgidity of the tissues, the movements and absorption of water, the width 

 of the stomata, are all influenced by the temperature. Even at tempera- 

 tures below zero, plants are still able to transpire perceptibly ; Burgerstein 

 detected feeble transpiration in leafy branches of Taxus baccata at IC"7C., 

 while Wiesner and Pacher obtained similar results with leafless branches 

 of the horse-chestnut at a temperature which was always below zero, and 

 occasionally fell as low as i3C. 2 The rate of transpiration must be 

 different during, and just after, a change of temperature from what it 

 becomes subsequently, for the air acquires the new temperature more rapidly 

 than the plant, and a rise of temperature will cause the saturated air within 

 the plant to expand, and hence will temporarily accelerate the escape of 

 water-vapour. Moreover, a change in the temperature of the air influences 

 the rate of transpiration much more markedly than it does the absorption 

 of water 3 . 



The -visible rays of the spectrum, so far as they are absorbed and 

 converted into thermal vibrations, must influence transpiration according 

 to the heating effect they produce. Many other influences which light 

 exercises are not without effect upon transpiration, for light affects the 

 width of the stomata and the daily changes in the tissue tensions, while 

 the assimilation of carbon-dioxide accelerates gaseous exchange and thus 

 favours transpiration. As the resultant of these factors, it appears that 

 as a general rule exposure to light causes transpiration to increase, while 



1 Sachs, Sitzungsb. d. Wien. Akad., 1857, Bd. xxvi, p. 326. 



a Burgerstein, Oesterreich. Bot. Zeitung, 1875, No. 6; Wiesner u. Pacher, ibid., 1875, Nr. 5; 

 Kohl, Transpiration, 1886, p. 75 ; Eberdt, ibid., 1889, p. 42 ; Burgerstein, 1. c., II, p. 42. 

 3 Vesque, Ann. d. sci. nat., 1878, vi. se"r., T. vi, p. 189 ; also Eberdt, 1. c. 



