. Botany of the living plant. Botany; Plants. THE WATER-RELATION IOI well supplied with water, it is probable that transpiration has i ertain advantages. In the first place, evaporation has a cooling effect which may prevent harmful overheating of the leaf especially when exposed to strong sunshine. Secondly, the flow of water up the plant that is initiated by transpiration, probably accelerates the movement of mineral salts from the roots to the upper part of the plant (see also p. no). The fact that there is a subsidiary mechanism which tends to provide for a current of water through the pla
. Botany of the living plant. Botany; Plants. THE WATER-RELATION IOI well supplied with water, it is probable that transpiration has i ertain advantages. In the first place, evaporation has a cooling effect which may prevent harmful overheating of the leaf especially when exposed to strong sunshine. Secondly, the flow of water up the plant that is initiated by transpiration, probably accelerates the movement of mineral salts from the roots to the upper part of the plant (see also p. no). The fact that there is a subsidiary mechanism which tends to provide for a current of water through the plant in the absence of transpiration suggests that the maintenance of such a current is of importance to the plant (see Root Pressure, p. 108).. Fie. 69. Potometer of the Ganong pattern. When necessary the air-bubble (shown in black) can be driven back to the right-hand end of the capillary tube by admitting water from the funnel. The actual amount of water lost in transpiration may be consider- able. For example it has been estimated that a Birch tree may lose as much as 600 lbs. of water during a hot dry day. Measurements of transpiration can conveniently be made on plants growing in pots. The pot and soil must be carefully sealed up in metal shells roofed over with sheet rubber so that evaporation can occur only from the plant. The whole arrangement is weighed at intervals and the loss in weight during a period represents the transpiration. An instrument called the potometer is frequently used in experiments on transpiration, though what is thus measured is actually the rate of absorption of water by the cut shoot fixed into the apparatus (see Fig. 69). The whole apparatus being initially rilled with water, absorption by the. 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 Bower, F. O. (Frederick Orpen), 18
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