148 PLANT PHYSIOLOGY 



factors." Yet it is easily conceivable owing to the different 

 rates at which water may be absorbed or given off by the 

 different cells of the epidermis, that in some plants, or under 

 certain conditions of culture, etc., there may be such differ- 

 ent conditions in the epidermal cells that while the guard- 

 cells may be expanding and so tending to open the stoma, 

 the other epidermal cells may also be expanding and so 

 tending to push the guard-cells closer together and to close 

 the stoma.* According to Benecke,t the auxiliary cells 

 assist in the opening and closing of the stomata only indi- 

 rectly and by neutralizing the mechanical strains brought 

 to bear on the guard-cells by the changes in form of the 

 epidermal and even of other leaf cells. 



Granting that changes in the turgor of epidermal cells, 

 and especially of those peculiar epidermal structures, the 

 guard-cells, bring about the opening and closing of the 

 stomata, we must enquire how these changes are effected 

 and what are the results as regards transpiration. 



Whenever a cell loses more rapidly than it absorbs water, 

 the turgor of the cell will decline proportionally. This dif- 

 ference will occur whenever a cell is unable to supply itself, 

 directly or through its neighbors, with water enough to 

 make good the loss by evaporation. The decrease in turgor 

 of the guard-cells of the stomata, and their consequent 

 flattening, are the result of such external conditions; the 

 stomata are closed. The difference in the amount of water- 

 vapor given off from leaves with open and with closed stom- 

 ata has been estimated by various means. The most striking 

 way of demonstrating this is perhaps Stahl's cobalt paper 

 method.^ Leaves with closed and with open stomata are 

 compared, under as like conditions as possible, as to their 

 rates of changing the color of dry filter paper impregnated 

 with cobalt chloride. Whereas the sheet of cobalt paper 

 placed against the leaf with open stomata will change 



• Stahl, E. 7. r.. p. 121. 



t Benecke, W. Die Nebenzelleu der Spaltoffnungen. Bot. Zeitung, p. 

 602-3, 1892. 



t Stahl, E. Einige Versuche fiber Transpiration und Assimilation. Bot. 

 Zeitung, 1894. 



