248 THE MOVEMENTS OF WATER 



that the chloroplastids are organs developed especially for the purpose of increasing 

 the transpiratory activity during illumination. 



Wiesner (1. c.) found that when green plants were exposed to light of various 

 colours, the most marked increase in the transpiration was caused by the rays 

 which were most freely absorbed, and Comes l has confirmed these results by using* 

 coloured flowers. Since a large part of the radiant energy which the chloroplastids 

 absorb is converted into potential chemical energy in the assimilation of carbonic 

 acid, it follows that assimilation must diminish the rate of transpiration to a certain 

 extent. The decrease can, however, be but slight, and hence it is doubtful 

 whether it caused the increased transpiration which Deherain and Jumelle 2 observed, 

 when, owing to the absence of carbon dioxide, no assimilation was possible. Indeed 

 Kohl, as well as E. and J. Verschaffeldt, obtained the very opposite result under 

 such conditions 3 . It must be borne in mind that the direct or indirect effects 

 due to the absence of carbon dioxide may easily outweigh the slightly increased 

 heating effect produced by the cessation of photosynthetic assimilation. 



Air-currents and mechanical vibrations. It is well known how remark- 

 ably the wind accelerates evaporation, and hence transpiration also 4 . This 

 is primarily due to the continual removal of the water-vapour as fast as it is 

 formed, a marked difference of potential being thus maintained. Mechanical 

 movements of the branches, &c. caused by a strong wind accelerate the 

 diffusion currents of the intercellular air, and hence more especially favour 

 diastomatic transpiration. All mechanical vibrations must act in a similar 

 manner, but in some cases shaking causes the tissue tensions to diminish, 

 and the plants to become flaccid, while the changes of volume thus produced 

 influence the gaseous interchanges in the intercellular spaces. When turgid 

 plants are vigorously shaken, the rate of transpiration is increased to a 

 certain extent, but the action is rather a complex one, and hence it is not 

 surprising that the effects produced differ in detail, and that in some cases 

 the rise in the rate of transpiration is transitory, in others permanent. 

 Shaking does not appear to cause a closure of the stomata (Sect. 31), but 

 nevertheless it may possibly induce a narrowing of the apertures, while 

 the increased transpiratory activity may call the regulatory mechanism 

 into play. 



The variations in the atmospheric pressure are in general too slight 

 to influence the intercellular diffusion of gases to any great extent. The 



1 Comes, Bot. Centralbl., 1880, p. 121. The other literature is given by Burgerstein, II, p. 39, 

 but is frequently inconclusive. 



* Deherain, Ann. d. sci. nat., 1876, vi. sen, T. IV, p. 177; Jumelle, Rev. ge'n. de bot., 1889, 

 T. i, p. 37 ; 1890, T. ir, p. 417; 1891, T. Ill, p. 241 ; Kohl, Transp., 1886, p. 44; E. n. J. Ver- 

 schaffeldt, Dodonea, 1890, Jahrg. n, p. 334. See also Burgerstein, II, p. 434. 



3 For literature see Burgerstein, n, p. 46; Eberdt, Transpiration, 1889, p. 78; Stahl, Bot. 

 Zeitung, 1897, p. 100. 



4 Baranetzky, Bot. Zeitung, 1872, p. 89; Kohl, Transpiration, 1886, p. 86; Eberdt, ibid., 

 1889, p. 68. 



