070 SECONDARY GROWTH IN THICKNESS 



unless it ensures that the two tissue- systems interfere with one another 

 as little as possible. Now, in the case both of the mechanical and the 

 conducting systems, it is above all necessary that the component tissues 

 should be continuous; it follows that the mutual interpenetration of 

 stereome and mestome must not involve any interruption of either 

 system. The longitudinal strands of the stereome network must not 

 be broken at any point ; similarly, conducting elements must never 

 become completely isolated. In other words, every tracheide or vessel, 

 and every cell of the xylem-parenchyma, or of a medullary ray, must have 

 direct access to the stream of nutrient material. Xylem-parenchyma or 

 tracheidal tissue could not possibly remain active, if completely 

 enveloped by mechanical tissue. 



The continuity of the conducting system in the woody cylinder of 

 Dicotyledons has been made the subject of detailed study by Troschel, 

 and subsequently by F. W. Krah. 357 Both these investigators come to 

 the conclusion that the structure of the woody cylinder of deciduous 

 Dicotyledonous trees provides in a very satisfactory manner for this 

 continuity which, as we have already explained, is demanded by 

 fundamental anatomico-physiological principles. As a matter of fact, 

 the principle of continuity finds a three-fold application in the 

 construction of secondary wood. In the first place, the vessels 

 and tracheides are not only longitudinally continuous, but also com- 

 municate with one another laterally ; secondly, the medullary rays 

 always abut directly against xylem-parenchyma on their flanks, as well 

 as along their upper and lower edges ; lastly, the vascular elements 

 invariably have conducting parenchyma (xylem-parenchyma and 

 medullary ray tissue) closely associated with them. 



As regards the first-mentioned point, stress may be laid upon the 

 existence of tangential communication between the water-conducting 

 elements their longitudinal continuity requiring no comment. Anato- 

 mically, this tangential continuity is most marked, where the water- 

 conducting portion of the wood consists largely or exclusively of fibrous 

 tracheides ; in this case the bordered pits are chiefly massed on the radial 

 walls, so that diffusion in the tangential direction is facilitated as much 

 as possible. It is clearly to the advantage of a tree that this should be 

 the case ; for if the transpiration current is interrupted on one side of 

 the trunk by some local injury, or if there is any serious irregularity 

 in the development of the root-system, the ill effects of an interrupted 

 water-supply upon the foliage are almost entirely obviated, owing to 

 the ease with which water can move tangentially in the trunk. In this 

 respect the woody cylinder resembles the primary vascular system; for, 

 in the latter case, the tangential vascular anastomoses act in the same 

 way as the bordered pits on the radial walls of the secondary tracheides. 



