100 VEGETABLE PHYSIOLOGY 



The modification of the turgescence of the guard-cells 

 is caused by the transference of water between them and 

 the other cells of the epidermis, from which they are separated 

 by thin walls. The vapour which is in the intercellular 

 space below them does not penetrate them, the walls 

 abutting on the space being thick and cuticularised. The 

 changes in the turgor of the guard-cells are effected by 

 their own protoplasm, which modifies its permeability to 

 water in consequence of their receiving a stimulus caused 

 by the variations in the quantity of water in the plant and 

 especially in the epidermis. The commencement of wilting, 

 for example, causes the protoplasm of the guard-cells to 

 allow water to pass from them with increased facility, and 

 they consequently diminish the size of the opening, checking 

 the loss of water from the general intercellular space system, 

 and hence tending to restore the general turgidity. 



It was held till recently that the guard-cells regulated 

 their turgor by producing osmotic substances in their 

 interior in greater or smaller amounts, so setting up osmotic 

 currents between the guard-cells and the general epidermis. 

 This was supported by the presence of chloroplasts in the 

 guard-cells. Quantitative considerations do not support 

 this view of the mechanism. A short period of darkness 

 is sufficient to close the stomata, and it is unlikely that so 

 short a time can reduce to a sufficient extent to explain the 

 closure the amount of the osmotic substances which have 

 been drawing water into the guard-cells till the failure of 

 the light. Nor is there sufficient evidence that darkness 

 causes a diminution of the quantity of osmotic substance 

 at all. 



Transpiration is markedly increased by sunshine, rising 

 to many times its original amount when a plant is trans- 

 ported into it from a dim light. No doubt this is due in a 

 very large measure to the heat rays which then fall upon 

 the plant, and which would raise its temperature very 

 dangerously were they not applied to the evaporation of 

 the water. But it is not due entirely to them, nor to the 



