Stomatal Action.—As already stated the stomatal pore is bounded by two guard cells and it is by changes in their shape that the alteration of the pore is brought about. Their shape, the relative thickness of their inner and outer walls, and their attach ment to the epidermal cells are such that when they become more turgid their inner walls—those facing the pore—move apart and so the pore widens, while on a reduction of turgor these walls approach one another and the pore becomes less in size. When the cells become collapsed, as in drying or by plasmolytic agents, the size of the pore is very much reduced though probably not to zero.
The mechanism by which light brings about alterations of turgor in the guard cells has been much disputed. It has often been held that the stomata open in light owing to the increased turgor due to the accumulation of sugars produced by the as similatory activity of the chloroplasts of the guard cells. This view is quite untenable for the guard cells open in the light in the absence of external supplies of carbon dioxide. Lloyd (r908) also has shown that guard cells containing only leucoplasts behave similarly to normal ones. It was shown also by the same observer that the guard cells in the early morning, when the pore is closed, show much starch, which disappears during the day to reappear again in the evening when the stoma closes. This led to the view that the light affected the starch-sugar ratio in the guard cells. In the day insoluble starch would change into soluble sugar and the turgor of the cell rise, in the night the sugar would change back to starch and the turgor fall. The turgor changes would bring about the corresponding changes in the size of the stomatal pore. This view received support from the observations of the osmotic pressure of guard cells. In Rumex patientia, for example, it was found that the osmotic pressure of the guard cells is 23 atmospheres when they are fully open, about midday, while only 13-14 atmospheres when the pore is closed at night. It is
found that in addition to light other factors such as neutral salts and acids and alkalis affect the starch-sugar relation and so stomatal movement. This is rather what might be expected since the change from starch to sugar and the reverse is almost cer tainly an enzymic one. Further work by G. W. Scarth in 1926 indicate that acetic acid and ammonia, i.e., both acid and alkali, will cause the opening of the stomata; in an intermediate range where there is slight acidity the guard cells remain closed. This suggests that what the guard cells respond to is a change in the concentration of free hydrogen ions (i.e., the free hydrogen parts of the acid molecule when it is dissociated in water) of the solu tion. In this view light affects the "acidity" of the leaf as a whole either by altering the content of organic acids in the cells, or the concentration of carbonic acid by affecting the rate of as similation. There are indications that the effect is more complex and that the change in the starch-sugar ratio is too slow to explain the rapid turgor changes that occur. It is probable that there is present in the cell sap of the guard cells some colloidal material which absorbs water and that the amount of water the colloid takes up is altered by changes in the concentration of the hydrogen ions of the cell.
Control of Transpiration by Stomata.—The question of the control of the rate of transpiration by the stomata has also been much debated. The older view was that the opening and closing of these pores had an important controlling effect. This view was abruptly called in question by Lloyd in 1908 who denied the regulation of water loss of leaves through stomatal closure. It is clear from later work that the stomatal aperture may vary widely while the rate of transpiration shows no corresponding change; thus the stomata may go on opening in the middle of the day while the transpiration rate falls. The matter is again complicated by the fact that the rate of loss of water vapour by the stomata is also affected by the accumulation of moist layers of air upon the surface of the leaf. If the stomata open more widely when such layers are present there may be little possibility of any faster diffusion of water vapour through the pores. Calculations indi cate that in still air the layers of moist air on the leaf exert the most effect, and it is only when the stomatal pores have become very narrow that the pores can have a controlling action. In wind, however, these moist layers are removed and the stomatal pore can exert a controlling action at any degree of opening. It must be borne in mind however that the stomata are very sensi tive to light but their response to losses of water by the leaf are slow, so that they cannot keep the water content of the leaf constant by controlling the stomatal pore.