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 
Te pet cos wings or flattened portions are supplied 
or Lemar. 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- 
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