. An introduction to vegetable physiology. Plant physiology. THE TEANSPOET OF WATEE IN THE PLANT 77 the stem so long as the cells are living. The stream in young plants passes along the whole suhstance of the wood, which in most cases forms a central mass of some size. In herbaceous plants the bundles do not usually form a continuous cylinder, but are more or less isolated in their course. In old trees the water-conducting area is limited to the outer regions of the central woody miass, which are known as the alburnum or sap-wood. The central portion of the wood is dead, and the ceU-walls are
. An introduction to vegetable physiology. Plant physiology. THE TEANSPOET OF WATEE IN THE PLANT 77 the stem so long as the cells are living. The stream in young plants passes along the whole suhstance of the wood, which in most cases forms a central mass of some size. In herbaceous plants the bundles do not usually form a continuous cylinder, but are more or less isolated in their course. In old trees the water-conducting area is limited to the outer regions of the central woody miass, which are known as the alburnum or sap-wood. The central portion of the wood is dead, and the ceU-walls are often very much altered in chemical composition. This region is known as the duramen or heart-wood ; it takes no part in the conduction, the tissue always remaining dry. The vascular bundles are seen to be continuous from the axis to the leaves, where they are no longer found arranged in a cylindrical manner, but are disposed in various ways as a much-branched net- work (fig. 58). The separate ramifica- tions are known technically as veins, and they are distributed in the various ways known, largely through the method of branching of the leaf axis. The latter, however, with very rare excep- tions, is flattened or winged throughout the whole or part of its length, and the wings or flattened portions are supplied with veins continuous with those of the branched or unbranched axis. The vascular tissue, therefore, if traced from below upwards, is seen to exhibit a separation of its constituent bundles, which continually appear to subdivide until they form a series of dehcate ramifications of considerable tenuity which per- meate the whole of the flattened portions of the leaves or other parts. The tenuity of the ultimate endings of the vascular bundles is attended with certain changes in the character of the constituent cells, but they remain woody and irregularly thickened as they are lower down in the axis. In the leaves these endings of the bundles, which are. FXG. 58.—VASCULA
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