Chap, i Processes of Absorption of Water 263 



its own permeability by water in consequence of stimula- 

 tion, and can under the same influence affect the quantity 

 of osmotic substances in the cells. Darwin suggested indeed 

 that knowledge of the regulation of the production of the 

 latter, in ways not at present capable of explanation, may 

 justify us in expecting that the turgor of the guard cells 

 may be found to be regulated indirectly like other pheno- 

 mena of irritability. 



Darwin supported his view of the co-ordination of the 

 two factors for closing by showing that withering leaves 

 exhibit a gradually increasing closure, which in many plants 

 is preceded by a well-marked rise of transpiration. He 

 explained this as indicating a temporary opening of the 

 stomata due to diminution of epidermal pressure on the 

 guard cells as withering begins. The subsequent closing he 

 was inclined to attribute to the guard cells losing their turgor 

 in response to a possible stimulus caused by the slight 

 flaccidity of the rest of the leaf. 



The suggestion made by Von Mohl that the presence of 

 the chloroplasts in the guard cells is correlated with the 

 presence of osmotic substances in larger quantities than in 

 the other epidermal cells met with little or no confirma- 

 tion. It will be remembered that Von Mohl expressed 

 no opinion as to the nature of these substances, but pointed 

 out that they would probably accompany the increased 

 assimilation or photosynthesis induced by the light, and 

 so secure the opening of the stomata. It follows, there- 

 fore, that the withdrawal of the light would, if their 

 closing were due to the same cause, have to be im- 

 mediately followed by a certain and indeed considerable 

 destruction of these osmotic substances. Quantitative 

 considerations do not support this view of the mechanism, 

 for Darwin showed in the course of the work now under 

 review that half an hour's darkness is not likely to have 

 any great effect in destroying osmotic substances and so 



