TRANSPIRATION 187 



nothing of importance had been done on the subject from 

 his day until Sachs, in 1860, drew attention to the part 

 played by the intercellular space system in the process. 

 He was the first to show conclusively that transpiration was 

 a vital phenomenon, what might be described as evapora- 

 tion under protoplasmic control. He further showed 

 that the function was closely connected with absorption 

 by the root and more especially with the nature of the 

 salts absorbed. Sachs's conclusions were confirmed and 

 extended in 1876 by Burgerstein, and in 1880 by Vesque. 

 About the same time Haberlandt and Von Hohnel 

 carried out their often-quoted experiments on the amount 

 of water transpired from plants of different ecological 

 types. 



The study of transpiration and the variations observed 

 in the amount of water transpired naturally led to in- 

 vestigations of the agents chiefly concerned, viz. the 

 stomata. To Von Mohl is due the credit of having been 

 the first to observe that when the guard cells are turgid 

 the stoma is open, and when flaccid, closed, but beyond 

 suggesting that the variations in turgidity might be due 

 to the pressure of varying quantities of osmotic substances 

 in the guard cells, Von Mohl did not carry the matter 

 further. It was not indeed until physiology had clearly 

 grasped the idea of the plant as a sensitive organism 

 responding to stimuli from without that the opening 

 and closing of the stomata came to be looked upon as an 

 instance in point. The mechanism of the guard cells 

 was first of all examined with great care by Schwendener 

 in 1 88 1, and the fundamental importance of the mode of 

 distribution of the thickening on the walls clearly demon- 

 strated. In lecturing to you on transpiration I warned 

 you to note carefully how the whole efficiency of the 

 stoma as a regulator of the escape of water vapour depends 

 on the structure of the walls of the guard cells, so that 

 I need not recapitulate Schwendener's observations. 



In 1886 Leitgeb put forward the view that the periodic 



