. Foundations of botany. grooved form, and on the lower surfaceall of the stomata are placed. This plant, like someothers with the same characteristics, ranges far north intoregions where the temperature, even during summer,often falls so low that absorption of water by the rootsceases, since it has been shown that this nearly stops alittle above the freezing point of water (see Exp. XVII).Exposed to cold, dry winds, the plant would then oftenbe killed by complete drying if it were not for the pro-tection afforded by the woolly, channeled under surfacesof the leaves.^ 170. Operation of the Sto
. Foundations of botany. grooved form, and on the lower surfaceall of the stomata are placed. This plant, like someothers with the same characteristics, ranges far north intoregions where the temperature, even during summer,often falls so low that absorption of water by the rootsceases, since it has been shown that this nearly stops alittle above the freezing point of water (see Exp. XVII).Exposed to cold, dry winds, the plant would then oftenbe killed by complete drying if it were not for the pro-tection afforded by the woolly, channeled under surfacesof the leaves.^ 170. Operation of the Stomata. — The stomata serve toadmit air to the interior of the leaf, and to allow moisture,in the form of vapor, to pass out of it. They do this notin a passive way, as so many mere holes in the epidermismight, but to a considerable extent they regulate therapidity of transpiration, opening more widely in dampweather and closing in dry weather. The opening is 1 This adaptation is sufficiently interesting for class Plate IV. — A Cypress Swamp FUNCTIONS OF LEAVES 159 caused by each of the guard-cells bending into a morekidnej^-like form than usual, and the closing by a straight-ening out of the guard-cells. The under side of the leaf,free from palisade-cells, abounding in intercellular spaces,and pretty well protected from becoming covered withrain or dew, is especially adapted for the working of thestomata, and accordingly we usually find them in muchgreater numbers on the lower surface. On the otherhand, the little flowerless plants known as liverworts,which lie prostrate on the ground, have their stomata onthe upper surface, and so do the leaves of pond lilies,which lie flat on the water. In those leaves which standwith their edges nearly vertical, the stomata are dis-tributed somewhat equally on both surfaces. Stomataoccur in the epidermis of young stems, being replacedlater by the lenticels. Those plants which, like thecacti, have no ordinary leaves, transpire thro
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