EFFECT OF HEAT UPON TRANSPIRATION. 277 



alkalies. But as Sachs 1 showed in 1859, even a very little free 

 acid in water hastens, while an alkali retards, transpiration. 



Burgerstein 2 in a long series of experiments showed that 

 while a single salt added to water in less amount than .5 per 

 cent hastens transpiration, any per cent above this produces a 

 marked retardation. When a solution of nutrient salts is used, 

 even if its concentration is as low as .05 of solid matter, there 

 is a retardation, and this is greater when the solution is more 

 concentrated. 



In the experiments, the results of which are given below, 

 four plants of Indian corn were employed. The temperature 

 varied between 16.7, and 18 C., and the observations con- 

 tinued through one hundred and three hours. The amounts 

 transpired are given in percentages of the weight of the fresh 

 plants. 



Nutrient solution 247.4 



Distilled water 264.17 



Potassic nitrate 283.2 



Ammonic nitrate 334.2 



742. Temperature and transpiration. Rise of temperature in- 

 creases the rate of transpiration not only by affecting evaporation 

 in general, but indirectly also b}* augmenting the absorption of 

 water and heightening the turgescence of the cells. Burger- 

 stein shows that leafy twigs of yew can transpire even at a 

 temperature of 10.7 C., while the leafless shoots of horse- 

 chestnut are said by Wiesner to transpire at 13 C. 3 



Sudden changes of temperature greatly influence transpiration, 

 since the " atmosphere and the plant cannot follow the course 

 of temperature with equal rapidit}-, and a rarefication of the 

 air saturated with moisture within the plant must favor its 

 release." * 



743. Effect of light upon transpiration. Transpiration goes on 

 more rapidly in light than in darkness, even when the tempera- 

 ture in darkness is somewhat higher. But differences in the 

 intensity of diffused light do not produce very marked differences 

 in the amount of transpiration. "When, however, diffused light 



1 Versuchs-Stationen, i., 1859, p. 223. 



Sachs met with some anomalies in his experiments, in one case finding a 

 noticeable retardation of transpiration upon the addition of an acid. 



2 Sitzungsb. d. Wiener Akad., 1876 and 1878. 



8 Quoted by Pfeffer: Pflanzenphysiologie, i., 1881, p. 148. 

 * Pfeffer : Pflanzenphysiologie, i., 1881, p. 148. 



