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, 
