THE TRANSPORT OF WATEB IN THE PLANT 77 



the stem so long as the cells are living. The stream in 

 young plants passes along the whole substance 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 mass, which are 

 known as the alburnum or sap-wood. The central portion 

 of the wood is dead, and the cell-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 

 delicate 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 



FIG. 58. VASCULAR 

 BUNDLES (VEINS) 

 OF LEAF. 



