. Botany of the living plant. Botany; Plants. THE WATER-RELATION 109. (Root Pressure) may be developed in exudation from a root system, amounting in some cases to 1 atmosphere or more : it is possible that in the intact, uninjured plant Root Pressure may reach consider ably higher magnitudes. A phenomenon that is probably due to Root Pressure is to be seen in plants that are provided with water glands. Drops of water issue from the leaves of such plants when conditions are unfavourable to transpiration, though still favouring absorption, as during a warm night (Fig. 74 a). The water exuded in
. Botany of the living plant. Botany; Plants. THE WATER-RELATION 109. (Root Pressure) may be developed in exudation from a root system, amounting in some cases to 1 atmosphere or more : it is possible that in the intact, uninjured plant Root Pressure may reach consider ably higher magnitudes. A phenomenon that is probably due to Root Pressure is to be seen in plants that are provided with water glands. Drops of water issue from the leaves of such plants when conditions are unfavourable to transpiration, though still favouring absorption, as during a warm night (Fig. 74 a). The water exuded in this way from the leaves of Grasses at night is often mis- taken for dew. It frequently contains salts in solution, and these may form an obvious incrustation round the water gland, as for example in var- ious Saxifrages. The deposit here is largely calcium car- bonate. Exudation from leaves is very common in tropical rain-forests, where the humid atmosphere depresses trans- piration. It appears then that a mechanism is present in the roots of plants which forces water up through the plant when transpiration is not operating, as is the case at night or in a decapitated plant. One suggestion is that special osmotic arrangements cause the inner- most living cells to pump water into the xylem (see Figs. 68 and 72). The endodermis, with its radial walls made impermeable to water by the strip of corky material previously mentioned, may play an important part in preventing the water which is accumulated under pressure in the xylem vessels from leaking out of the stele along the cell-walls, and perhaps eventually finding its way back to the soil. The Root Pressure mechanism may co-operate with that contemplated by the Cohesion Theory in raising the transpiration stream when trans- piration is in progress. On the other hand, in many experiments only feeble Root Pressure has been detected, while it is known that during active transpiration the contents of the xylem are in a state of ten
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