MECHANISM OF STOMATA 307 



inner ones curving away from each other and thus opening 

 the pore (Fig. 51, A, B, heavy lines). This is the normal 

 condition in bright light and when there is plenty of 

 water in the leaf. If the air outside becomes very dry 

 water vapour diffuses out through the pores very rapidly, 

 and thus the vapour pressure in the intercellular spaces 

 of the mesophyll is reduced. This in turn increases 

 the rate of evaporation from the mesophyll cells into 

 the intercellular spaces and depletes the cells of water. 

 Water is drawn into the mesophyll cells from the 

 epidermal cells and eventually into these from the guard 

 cells of the stomata. The guard cells in consequence 

 lose their turgor and become flaccid, closing the pore by 

 the straightening of the extensible portion, of the walls 

 (Fig. 51, A, B, thin lines). 1 At night also, owing to the 

 cessation of sugar formation by the chloroplasts of the 

 guard cells, the osmotic pressure falls and the pore closes. 

 This automatic mechanism closing the stomata when 

 the supply of water is depleted does not always completely 

 stop the loss of water from the plant, as may be seen 

 towards the close of a hot dry day in summer, when the 

 leaves of some plants become limp and droop owing to 

 the continued loss of water to the air, a loss which they 

 are unable to make good from the root quickly enough 

 to maintain the turgor of the cells. This may be due 

 partly to the incomplete closure of the pores, and partly 

 to the fact that the cuticle is not completely impermeable 

 to diffusion of water vapour. A drooping plant will 

 gradually recover if a bell jar be placed over it. The jar 

 prevents the removal by currents of air of the vapour 



1 This mechanism can be imitated by cutting off a piece of the inner 

 tubing of a bicycle tyre 10 inches long (5 inches on each side of the 

 valve), tying up the two ends tightly and fixing them to a straight 

 piece of wood so that the concave side of the tube lies along the wood. 

 If air is pumped into the valve the tube curves away from the wood 

 as it becomes inflated, and straightens as it is deflated again. 



