. Botany of the living plant. Botany; Plants. io8 r\N\ «i| THE LIVING PLANT bubbl< «'t bubbles would d< stroj i ohesion in the • emarked that, in point ct, the water-columns lin the • 'i<ii include air-bubbles: but the upholders of the Cohesion Theory maintain that the contents are free from bubbles m .1 sufficient proportion of the vessels to provide the necessary degree of con- tinuity : moreover, that the effect of the air-bubbles is only local. Any tension in the xylem vessels will spread downwards into the roots, and will there promote the flow of water into the xylem from the ad
. Botany of the living plant. Botany; Plants. io8 r\N\ «i| THE LIVING PLANT bubbl< «'t bubbles would d< stroj i ohesion in the • emarked that, in point ct, the water-columns lin the • 'i<ii include air-bubbles: but the upholders of the Cohesion Theory maintain that the contents are free from bubbles m .1 sufficient proportion of the vessels to provide the necessary degree of con- tinuity : moreover, that the effect of the air-bubbles is only local. Any tension in the xylem vessels will spread downwards into the roots, and will there promote the flow of water into the xylem from the adjacent living cells. Tins will activate the osmotic chain which across the cortex, and continued absorption of water from the soil will ensue (see Fig. 72, also p. 99). It appears, however, that there is an- other mechanism capable of inducing movement of water into the vessels of the root and up through the plant. This is suggested by the phenomena of Exudation and of Root Pressure. If a plant is severed a few inches above the soil on a warm day, and the cut sur- Fic. 74. face immediately examined with a lens,. ggSj it will appear dry. A drop of water placed upon it will at first be absorbed, but in a short time it will be found that :it>> tin- tube, the prcsvirr can !*• measured fan I the h' .1 mei water emerges from the cut surface, 1-suing from the region of the vessels. The -tump i- .nd to 'bleed," and may do so very extensively. Thus a vine ha- been observed to exude an average of 500 of liquid per day over .1 period of several week-. As might \ii<late is not pure water, but contains small quan- titi' mineral and of organic matter. Occasionally the latter is more abundant, a- in the Sugar Maple, where the exudate from incisions made into the trunk in spring, before the leaves are fully formed, may contain .; per cent, of sugars. The use of apparatus similar to that ill -1 in Fig. 74 -hows that a considerable pressure. Please note that these imag
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Keywords: ., bookcentury1900, bookpublis, booksubjectbotany, booksubjectplants
