188 HISTORY OF BOTANY 



opening and closing of stomata was passive, and that the 

 real agents concerned were the surrounding epidermal 

 cells, whose turgidity and flaccidity exerted an alternate 

 squeezing and pulling effect on the guard cells. These 

 two, to some extent, antagonistic views were at last 

 brought into harmony by Francis Darwin, who, in 1898, 

 showed that both factors were operative, although in his 

 opinion the varying turgidity of the guard cells was the 

 chief agent. Perhaps the most important feature in 

 Darwin's paper was the emphasis he laid on the stoma 

 as a piece of vital machinery, extremely sensitive to all 

 changes in the environment. Once again the explanation 

 was seen to be deducible from the responses given by 

 the protoplasm to variations in light intensity, amount 

 of water vapour, changes in temperature, and other 

 stimuli. 



Recognition of the enormous amount of water evapor- 

 ated from the leaf surface, water which must have been 

 pumped out of the soil, led botanists to enquire as to the 

 pathway of the transpiration current and the nature of 

 the forces concerned in the ascent of the water to heights 

 that could not be accounted for either by " root pressure " 

 or the suctional effect of transpiration the vis a tergo 

 and the vis a fronte of the earlier physiologists ; but 

 this, perhaps one of the most hotly debated questions 

 in the whole range of plant physiology, I must leave over 

 to the next lecture. 



