248 LEAVES 



that the guard cells then tend to shrink through lack or loss of 

 water, since their power to draw in and retain water decreases 

 with the loss of dissolved substances from their cell sap. Re- 

 gardless of what the chloroplasts have to do with it, it is obvious 

 that when the guard cells are swollen with water they bow out, 

 that is, curve away from each other and make the slit larger. 

 On the other hand, when the guard cells are shrunken through 

 the loss of water, they straighten and make the slit smaller. 

 Hence the stomata tend to open when the water supply is abun- 

 dant and close when water is scarce. 



The importance to the plant of closing the stomata when water 

 is scarce is apparent, for much water can be lost 'through open 

 stomata. It would seem, therefore, that the guard cells regulate 

 the loss of water from the plant and this they do to some extent. 

 However, it has been found that stomata open in light and close 

 in dark, and this tendency of light to open, conflicts with the 

 tendency of water shortage to close them; for it is during bright 

 hot daytime when the light stimulus to open is probably strongest, 

 that there is the greatest shortage of water. That the guard 

 cells open and close just when they should in order to control 

 water loss is much doubted. The most important feature of 

 stomata is that they permit exchange of gases. 



Leaves having the horizontal position have their stomata much 

 more abundant on the under surface; often they are not found at 

 all on the upper surface. On leaves that stand more or less erect, 

 as those of the Grass family and Carnations, the stomata are 

 * about equally distributed on both sides, and on leaves which lie 

 on the surface of the water, like those of the Water Lily, they 

 occur only on the upper side. The location of the stomata on 

 the under surface of horizontal leaves is an advantage to the 

 plant, since here the stomata are less likely to become choked 

 with water during rains, and also less water is lost through them 

 by evaporation. 



The number of stomata varies much with different plants, but 

 about sixty thousand to the square inch is a fair average. On 

 the leaves of some plants there may be as many as four hundred 

 thousand to the square inch. In the table on the next page are 

 given the number of stomata found on a square millimeter of leaf 

 surface of some common plants. 



