. An introduction to vegetable physiology. Plant physiology. TKANSPIEATION 91 two dry sheets of glass of somewhat larger area. In a very short time, often in less than a minute, the paper in con- tact with the lower side of the leaf becomes pink, while the other piece remains blue for a considerable time. The amount of water given off by transpiration varies in different plants. In the sunflower (Helianthus) the amount has been stated to be T^ cubic inch of water per square inch of surface in twelve hours. V. Hohnel has computed that a birch-tree with about 200,000 leaves may transpire 60 to 8
. An introduction to vegetable physiology. Plant physiology. TKANSPIEATION 91 two dry sheets of glass of somewhat larger area. In a very short time, often in less than a minute, the paper in con- tact with the lower side of the leaf becomes pink, while the other piece remains blue for a considerable time. The amount of water given off by transpiration varies in different plants. In the sunflower (Helianthus) the amount has been stated to be T^ cubic inch of water per square inch of surface in twelve hours. V. Hohnel has computed that a birch-tree with about 200,000 leaves may transpire 60 to 80 gallons of water during a very hot day. Doubtless, however, individual plants show a considerable variety in the amount. This copious evaporation readily explains why the bleeding of plants from wounds can Beldom be observed when the leaves are expanded and active. When transpiration is exces- sive the leaves and branches lose their turgescence, become flaccid, and droop. A branch which has reached this condition may be revived by forcing water into it, which can be done by fastening fig. 65.—apparatus to show .... n , , , i Dependence of Withering it into one arm oi a U-tube con- UP0N LoBs of watee. taining water (fig. 65), and pour- ing mercury into the other. The restoration of the water restores the turgescence of the tissues, and the branch regains an erect position. The exhalation of the water accumulated by root- pressure in the closed system of the vessels leads to a diminution of the pressure of the air which they contain in addition to the water. Indeed it is by such a suction that the air is originally enabled to enter the vessels, being drawn into them from the intercellular spaces. Conse- quently, while transpiration is active, there is a negative. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original wo
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