. Botanik; Grundsätze und Probleme. Botanik. Das BLATT UND SEINE FUNKTIONEN 75 Strom von Wasser ist somit durch die Anlage Körper, Eingabe an der Wurzel - Haare und gehen durch die stomata. Die Gesamtmenge dieses Wasser oft beträgt mehrere hundert Mal so viel wie die letzten trockenen Gewicht der Anlage selbst (Abb. 39).. Abb. 38.-Tran. spiration aus dem Blatt - Blade. Kresse - Abschnitt einer Blade einschließlich einer Vene. Pfeile zeigen, dass kein Wasser aus der Vene, durch das Mesophyll Zellen, in die Luft - Räume und die durch die stomata. Dicken Pfeile, flüssiges Wasser; die gestrichelten Pfeile, Wasser-dampf. Die

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. Botany; principles and problems. Botany. THE LEAF AND ITS FUNCTIONS 75 stream of water is thus passing through the plant body, entering at the root-hairs and leaving through the stomata. The total quantity of this water often amounts to several hundred times as much as the final dry weight of the plant itself (Fig. 39).. Fig. 38.—Tran.spiration from the leaf-blade. Cress-section of a blade including a vein. Arrows indicate passage of water from the vein, through the mesophyll cells, into the air-spaces, and out through the stomata. Solid arrows, liquid water; dotted arrows, water-vapor. The Rate of Transpiration.—The rate of water-loss varies greatly according to the kind of plant, the soil conditions, the season of the year, the time of day, and various environmental factors. As a general rule, we find that the rate tends to increase under conditions which favor increased evaporation, such as high temperature, bright light, rapid air movement and low humidit}"; and to decrease under environments of the opposite character. Transpiration is by no means controlled entirely by factors which influence evaporation alone. The rate of water-loss from a given leaf-surface and from an equal area of free water do not rise and fall exactly together, the transpiration from the living leaf sometimes being relatively higher and sometimes relatively lower. There must, therefore, be factors in the leaf itself (as opposed to those in the external environment) which tend to accelerate or to retard transpiration. The most important of these is doubtless the opening and closing of the stomata, which we have already discussed. Changes in the concentration of the sap in the mesophyll cells also probably determine to some extent the rate at which water evaporates from their surfaces. The actual amount of water transpired during the growing season may be large or small, depending on the size of the plant, . Please note that these images are extracted from scanned page images that

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• bookcentury1900
• bookdecade1920
• booksubjectbotany
• bookyear1923