72 VEGETABLE PHYSIOLOGY 



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 

 FIG. 58. VASCULAR wings or flattened portions are supplied 



BUNDLES (VEINS) ... . .. ... .. 



OF LEAF. 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 some- 

 times free, and sometimes disposed in the form of an open 

 network, are surrounded by delicate parenchymatous tissue, 

 whose cells are in immediate contact with the woody ele- 



