262 The Physiology of Plants BOOK in 



cells. The latter must be regarded according to this view 

 as passive agents in opening and closure, and not as acting 

 automatically. The force at work is mainly indirect, being 

 the varying turgor of the whole of the epidermal cells, and 

 these working by a kind of traction and not by osmotic 

 changes in the guard cells. Leitgeb was led to this view 

 particularly by observations on the nocturnal closure of 

 stomata. He held that darkness exerts a tonic action 

 on the cells concerned. 



A somewhat animated controversy between these two 

 schools of thought was maintained during 1887 and 1888, 

 being conducted chiefly by G. Haberlandt and by Schaeffer. 

 It was revived in 1896 by Schellenberger. An important 

 contribution to it, which suggested a basis of agree- 

 ment, was made in 1898 by F. Darwin, who carried out 

 a number of experiments with great success. He introduced 

 a new instrument for work on the exhalation of vapour, 

 which was found capable of demonstrating extremely small 

 amounts and enabling comparisons to be made with much 

 greater accuracy than before. This was the hygroscope, 

 two varieties of which are described in his paper. 



In Darwin's work the idea of the vital character of the 

 action of the guard cells can be seen to have undergone 

 a very great degree of development. He held that the 

 mechanism depends upon correlation between the two fac- 

 tors claimed by the rival schools to be concerned, and that 

 the turgor of the guard cells plays the most prominent 

 part. The action is the autonomic action of the plant as 

 an individual organism, and these variations in turgor are 

 only the machinery by which it works. The changes of 

 illumination, temperature, &c., act as stimuli, not so much 

 perhaps immediately to the guard cells as to the whole 

 organism. 



According to this view, the changes in the turgor of the 

 guard cell are effected by their protoplasm, which can modify 



