. Principles of modern biology. Biology. Fig. 13-18. The action of guard cells. Above, the guard cells are turgid, leaving the stoma open; below, the guard cells are wilted (, less turgid), closing the stoma. remain open in the daytime—provided the water supply is adequate—but at night they tend to be partially closed. The quantity of water transpired by an average plant in sunlight is about 50 grams per square meter of leaf surface per hour. Thus a single corn plant puts forth more than 50 gallons of water in the course of one summer; or an acre of corn transpires about 300,000 gallons of
. Principles of modern biology. Biology. Fig. 13-18. The action of guard cells. Above, the guard cells are turgid, leaving the stoma open; below, the guard cells are wilted (, less turgid), closing the stoma. remain open in the daytime—provided the water supply is adequate—but at night they tend to be partially closed. The quantity of water transpired by an average plant in sunlight is about 50 grams per square meter of leaf surface per hour. Thus a single corn plant puts forth more than 50 gallons of water in the course of one summer; or an acre of corn transpires about 300,000 gallons of water in the same time. An average tree transpires more than 1500 gallons annually, and the total quantity of water vaporized from the vegetation of a forested region has a significant influence upon the rainfall, humidity, and tempera- ture of that region. As the sun beats down, the leaf absorbs about 75 percent of the impinging light. However, roughly only 3 percent of this energy is utilized in photosynthesis. The rest is transformed into heat—the heat that va- porizes water and leads to transpiration. This vaporization is most important, not only because it dissipates the heat that otherwise would kill the tissues of the leaf, but also because it generates an osmotic force that evacuates the ducts of the leaf and brings about a further flow of sap upward from the roots. Transpiration and the Flow of Sap. Transpira- tion motivates the upward flow of sap by altering osmotic conditions in the leaves. When the chlorenchyma tissues lose water, the cells become hypertonic to the sap in the veins, which lie in close contact with the chlorenchyma (Fig. 13-16). During transpira- tion, consequently, water tends to be drawn from the ducts into the chlorenchyma tissues. Such a forceful evacuation of the ducts of the leaf creates a lifting force that helps to ele- vate the whole column of sap in its ascent from the roots. At certain times, in fact, when the solute content of the
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Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
