INFLUENCE OF EXTERNAL CONDITIONS ON TRANSPIRATION 245 



SECTION 39. The Influence of External Conditions upon 



Transpiration. 



The external conditions affect transpiration both directly and in- 

 directly, and hence the effect produced by them is by no means necessarily 

 the same as it would be in the case of evaporation from a free surface of 

 water. A change in the conditions may alter the characters of the transpiring 

 organ ; and when the external conditions are such as to favour very rapid 

 evaporation, the stomata may close, and hence the rate of transpiration 

 markedly decrease. Some such arrangement is essential in order that 

 a purposeful regulation of this function may be possible. Attention must be 

 paid not merely to the direct physical action of the external conditions, but 

 also to their indirect influences, for only by these means can a clear com- 

 prehension be obtained of the causes which produce any ascertained result. 

 In studying the simplest reaction, all the factors influencing it must be 

 taken into consideration, though this is often far from easy, and therefore 

 frequently neglected. The task is made all the more difficult by the fact 

 that alterations in the external conditions may not only cause transitory 

 variations changing as the conditions alter, but may also produce permanent 

 accommodatory modifications in the plant or its parts. 



A general account has been given of the ways and means by which 

 transpiration may be influenced, and of the power which the plant possesses 

 of reacting to changed conditions (Sect. 38). It now remains to describe 

 the general effects produced by changes in the external conditions,, special 

 attention being paid to the more permanent of these and to the effects 

 produced by the transitory variations they exhibit. 



Humidity of the air. It has been known from the time of Hales that 

 as the moistness of the air increases, transpiration decreases 1 . As the air 

 becomes drier the transpiration does not necessarily increase correspond- 

 ingly, for as soon as the water present in the plant begins to decrease 

 the transpiration-curve falls. A plant is able to transpire even in a 

 saturated atmosphere, provided that it is warmer than the surrounding air. 

 The respiratory activity always tends to warm the plant slightly, and 

 hence renders a certain amount of transpiration possible in saturated 

 air 2 , so that the spadix of an aroid, owing to its comparatively high 

 temperature, transpires actively, and causes water to condense on the 

 wall of a bell-jar placed over it 3 . In most cases the heat thus produced 



1 Experiments by Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 203. Further litera- 

 ture : Burgerstein, II, p. 45; Eberdt, Transpiration, 1889, p. 34. The influence of the supply of 

 water, and of the moistness of the soil, may be judged from Sects. 37 and 38. 



2 The literature is given by Burgerstein, II, p. 45, but is for the most part very inconclusive. 



3 Cf. Vol. n, The Production of Heat ; also G. Kraus, Bliithenwarrne bei Arum italicum, 

 1884, p. 54. 



