XIIl] CONTINUITY OF TISSUES 103 



up the central and intervening parts ; just as skeletons of 

 leaves can be obtained by rotting away the softer tissues 

 of the leaf-blade, and leaving only the harder fibres and 

 vessels of the venation. 



We are not here concerned with the differences in 

 detail, met with in different plants, as to the exact course 

 pursued by the different vascular bundles in the leaf, or in 

 its petiole, or in the shoot-axis ; nor are we concerned with 

 the differences met with in the proportion of fibres to 

 vessels, or the kinds of vessels themselves, or of other 

 tubular or supporting structures in the vascular bundles. 

 Our object is simply to emphasize certain matters of 

 fundamental importance common to all cases, and the first 

 of these is the complete continuity of all the structures 

 referred to not only as between shoot-axis and leaf, but 

 also as between shoot-axis and other parts, buds, branches, 

 and even the root and its branches. 



Every part of the vascular system of the plant is in 

 continuity, so that fluids entering the root can be brought 

 up to the highest shoot or leaf, and fluids from the latter 

 can pass from leaf to shoot-axis and to any branch or bud 

 or root in communication with the leaves by these minute 

 pipes, the vessels. We may compare the whole fibrous 

 system to a system of engineer's girders, struts and columns; 

 and the whole vascular system to a complex network of 

 water-pipes and drains, which for purposes of convenience 

 and economy accompany the supporting columns, struts 

 and girders all over the plant. These matters will occupy 

 our attention further in another section of the work. 



At present we are concerned more particularly with 

 the question of continuity between leaf and shoot-axis. 



When the leaf has finished its life's work, it may be 

 and generally is in the trees and shrubs here dealt with 

 at the end of the autumn of the first year, or, in so-called 



