TRANSPIRATION UNDER NORMAL CONDITIONS 249 



barometric pressure does not affect the evaporation of water to so great an 

 extent as might be thought, at least within the limits at which terrestrial 

 plants grow, and even on lofty mountains the influence which the low 

 pressure exerts upon transpiration is not very marked l . 



Dissolved substances. Concentrated solutions diminish the rate of 

 transpiration, just as a dry soil does (Sect. 37), and even normal nutrient 

 solutions exercise a slight retarding action. According to Burgerstein a 

 slight effect is produced even when the percentage of salts present is less 

 than p'i per cent., but the effect in this case is probably not a direct 

 physical one, but an indirect one, causing an alteration in the plant's 

 transpiratory powers. Plants appear to react to many substances in this 

 manner, as is shown by various researches, especially those of Burgerstein 2 , 

 who found that the presence of slight amounts of certain substances might 

 cause a moderate rise or fall in the transpiratory activity. 



The effects which small traces of certain substances may exercise upon 

 transpiration will presumably be still produced when they are absorbed 

 from the soil. Nevertheless, for reasons that are easy to see, the results 

 produced by the addition of various substances to the soil are not always 

 the same, and are not sufficiently definite to enable any clear general laws 

 to be established. Hence it is only necessary to mention that, as Sachs 3 

 has shown, and as Burgerstein (I.e.) has since established more in detail, 

 transpiration is decreased by the addition of small quantities of tartaric, 

 oxalic, nitric, or carbonic acid to the soil, whereas it is increased by 

 alkalies such as potash, soda, or ammonia. This result may be produced 

 without any injury to the plant, although the 0-15 to 03 per cent, solutions 

 which Burgerstein used must often have been deleterious. The details are 

 given in Burgerstein's papers, and from these it appears that the effect 

 which a salt may exercise upon transpiration when added to a nutrient 

 solution in which a plant is growing, may be different to that produced 

 when a similar quantity of the same salt is offered to another plant 

 cultivated in distilled water. 



SECTION 40. Transpiration under Normal Conditions. 



In a condition of nature the transpiratory activity of a plant undergoes 

 marked alterations as the external conditions change, and, moreover, the 

 transpiratory power alters as development continues, for the relative amount 

 of transpiring surface may be increased, while the nature and power of 



1 The incorrect opinions of certain authors are referred to by Burgerstein, II, p. 49. 



1 Burgerstein, Unters. liber die Beziehung d. Nahrstoffe zur Transpiration ,, I. Reihe, 1876 

 (Sitzungsb. d. Wien. Akad., Bd. LXXIII, Abth- i) ; II. Reihe, 1878 (ibid., Bd. LXXVIII, Abth. i) ; 

 and Materialien zu einer Monog. d. Transp., 1888, n, p. 51. 



3 Sachs, Versuchsst, 1859, Bd. I, p. 203. For additional literature see Burgerstein, 1879, P- 5 1 - 



