THE WATER-RELATION [03 



more important to the action of the stoma, is as follows. Observation 

 shows that in darkness abundant starch grains are present in the 

 chloroplasts of the guard-cells. When light falls on the leaf, 

 these starch grains disappear owing to their conversion to sugar, 

 probably through enzymic action. The increase in sugar content 

 of the guard-cells thus produced leads to an intake of water from 

 the neighbouring epidermal cells : for these have as a rule no chloro- 

 plasts and no starch reserves, and experience no increase in sugar 

 content when exposed to light. This intake of water and consequent 

 increased turgor in the guard-cells produces the opening, as described. 

 Observation indicates that when the light fails in the evening the 

 sugars in the cell-sap of the guard-cells are largely converted back 

 into starch, with a resultant fall in turgor and closure of the pore. 

 So that the effect of light intensity on stomatal aperture is due to 

 its exertion of a close control on the starch-sugar balance of the 

 guard-cells, though at present it is somewhat uncertain how this 

 control arises. That the opening of the stoma depends on turgor 

 is readily proved by treating a living preparation from a leaf, showing 

 open stomata, with a 5 per cent, solution of common salt. The 

 osmotic withdrawal of water and loss of turgor quickly results in 

 closure of the stomata, the guard-cells ultimately becoming plas- 

 molysed. 



The mechanics of stomatal movement are complex, and are bound up 

 with the structure of the guard-cells. While the latter varies a good deal, 

 the chief features are these. The two guard-cells, attached at their ends, 

 are usually curved, the wall facing the pore being shorter than that in contact 

 with the adjoining cells. The inner and outer slopes of the face next the 

 pore bear each a projecting ridge (Fig. 71). The open stoma, with its tense 

 cells, requires more room than the closed stoma. That room has to be gained 

 by forcing the adjoining cells aside. Where the cell-walls are thick, special 

 thin areas of cell-wall are found which are effective as joints or hinges, allowing 

 the cells to adjust themselves mutually when the pore opens. It is the in- 

 crease in turgor, produced as explained above, acting on cells of the form and 

 structure of the guard-cells, that makes the pore open in the presence of 

 light. The internal pressure being equal over the whole internal surface, 

 since the convex wall presents a larger area, it will stretch more than the 

 concave wall when turgor increases ; this would in itself produce a greater 

 curvature and opening of the pore, even if the walls were all of the same 

 thickness. But they are not. The ridges of thickened cell-wall on the outer 

 and inner faces of each guard-cell make the average thickness of the wall 

 greater on the side next the pore. This will accentuate the curvature of the 

 cells when the turgor rises. A further effect of the increased turgor will 

 be to make the section of the cell-cavity approach as nearly as possible to 

 the circular. This too will result in withdrawing the projecting surfaces 



