Elementary botany . nge of gases takes placethrough them as we shall later large number of experiments showthat normally the stomata are openwhen the leaves are turgid. Butwhen plants lose excessive quantitiesof water on dry and hot days, sothat the leaves become flaccid, theguard cells automatically close thestomata to check the escape of watervapor. Some water escapes throughthe epidermis of many plants,though the cuticularized mem-brane of the epidermis largely prevents evaporation. In arid regionsplants are usually provided with an epidermis of several layers of cells tomore securely
Elementary botany . nge of gases takes placethrough them as we shall later large number of experiments showthat normally the stomata are openwhen the leaves are turgid. Butwhen plants lose excessive quantitiesof water on dry and hot days, sothat the leaves become flaccid, theguard cells automatically close thestomata to check the escape of watervapor. Some water escapes throughthe epidermis of many plants,though the cuticularized mem-brane of the epidermis largely prevents evaporation. In arid regionsplants are usually provided with an epidermis of several layers of cells tomore securely prevent evaporation there. In such cases the guard cellsare often protected by being sunk deeply in the epidermal layer. 92. Demonstration of stomates and intercellular air spaces.—A gooddemonstration of the presence of stomates in leaves, as well as the presenceand intercommunication of the intercellular spaces, can be made by blow-ing into the cut end of the petiole of the leaf of a calla lily, the lamina being. Fig. 56. The roots are lifting more water into the plant than can be given off in the form of water vapor, so it is pressed out in drops. From First Studies Plant Life. TRANSPIRA notf. 47 immersed in water. The air is forced out through the stomata and rises asbubbles to the surface of the water. the close of the experiment someof the air bubbles will still be in contact with the leaf surface at the openingof the stomata. The pressure of the water gradually forces this back intothe leaf. Other plants will answer for the experiment, but some are moresuitable than others. 92a. Number of stomata.—The larger number of stomata are on theunder side of the leaf. (In leaves which float on the surface of the waterall of the stomata are on the upper side of the leaf, as in the water lily.) Ithas been estimated by investigation that in general there are 40-300 stomatato the square millimeter of surface. In some plants this number is exceeded,as in the olive, where there
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