The New Student's Reference Work/Water, Ascent of

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Water, Ascent of (in plants). Water to supply the evaporation from the larger land-plants enters by the roots, especially the root-hairs, and passes thence into the woody strands which lie in the center of the root. These connect with similar strands in the stem, which run into the leaves and branch profusely, following the so-called veins and constituting the veinlets. The water travels chiefly in the interior of the minute, elongated tubes (ducts or vessels) which make up the greater part of these strands. These ducts are not continuous tubes, however, so that the water has to traverse the partition walls which occur here and there (abundantly in pines and their kin). The water is not forced up from below by the roots, except in low plants and at some seasons; it certainly does not rise by capillarity. No satisfactory understanding of the forces concerned has yet been reached, though the problem has been attacked by many investigators. See Root-Pressure, Transpiration and Vascular Bundles.