EFFECT OF HEAT UPON TRANSPIRATION. 277 



alkalies. Bat as Sachs ■" showed in 1859, even a very little free 

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



Burgerstein^ 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 tliis is greater when tlie 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 1G.7°, and 18° C, and the observations con- 

 tinued through one hundred and tliree hours. The amounts 

 transpired are given in percentages of the weiglit of the fresh 

 plants. 



Nutrient solution 247.4 



Distilled water 264.17 



Potassic nitrate 283.2 



Amnionic 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 by augmenting the absoi'ption of 

 water and heightening the turgescence of the cells. Burger- 

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

 tempei-ature of — 10.7° C, while the leafless shoots of horse- 

 ciiestnut are said by Wiesner to transpire at — 13° C.'' 



Sudden changes of temperature greatly influence tj'anspiration, 

 since the " atmosphere and the plant cannot follow the course 

 of temperature witti equal rapidit}', and a rareiioation of the 

 air saturated with moisture within the plant must favor its 

 release." * 



743. Effect of light upon transpiration. Transpiration goes on 

 more rapidh* 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 Versuelis-Stationen, i., 1859, p. 223. 



Sachs met witli 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. 



° Quoted by Pfeffer: Pflnnzen physiologic, i., 1881, p. 148. 

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



