CHAP, i Processes of Absorption of Water 265 



The phenomenon of nocturnal closing does not, therefore, 

 seem to be one of stimulation entirely. Darwin found 

 evidence that it shows an amount of inherent periodicity 

 in nyctitropic plants, a certain fundamental rhythm existing, 

 comparable with the nyctitropic rhythm, but not so well 

 marked. He found also that though transpiration is checked 

 at night by stomatal action, the closure is not generally 

 so complete as to stop it entirely. 



Other evidence of the ultimate dependence of the regula- 

 tion of transpiration on the behaviour of the protoplasm 

 has been forthcoming. Baranetzky showed, in 1872, that 

 shaking a branch temporarily increases its loss of water. 

 Wiesner failed to observe this in 1876, but the phenomenon 

 was again recorded by Kohl in 1886 and by Eberdt in 1889. 

 Both Baranetzky and Kohl said that if the shaking is 

 prolonged the rise of transpiration is followed by a fall. 

 Baranetzky attributed the rise to the rapid expulsion of 

 the saturated air through the stomata at the time of shaking. 

 On the other hand it was ascribed by Vines, in 1886, 

 to the effect of the shaking on the protoplasm ; acting as 

 a stimulus, it makes the latter modify its permeability and 

 so allows a more rapid evaporation from the cells to take 

 place. F. Darwin (1898) suggested that it may be due to 

 a temporary opening of the stomata, taking place pre- 

 sumably in response to the stimulus. 



The vital theory of transpiration received support also 

 from the observation of Von Hohnel in 1876, that the 

 process is independent of the quantity of water at the 

 moment present in the leaf. 



The general effect of light upon the transpiratory pro- 

 cesses, apart from its local action on the stomata, was 

 investigated by Wiesner in 1876. He found that when 

 a plant is kept at a constant temperature, and when the 

 air around it is kept at a definite degree of saturation with 

 moisture, light affects transpiration directly and not through 



