DIV. I 



MORPHOLOGY 



149 



PARENCHYMA (hp), which forms longitudinally-running strands, and 

 in the medullary rays is also directed radially ; this constitutes the 



WOOD PARENCHYMA and PARENCHYMA OF THE MEDULLARY RAYS. 



Corresponding to this the wood serves (1) for water- conduction, (2) 

 to render the stems and roots rigid against pressure and bending, and 

 (3) for the storage of organic materials. The properties which make 



g 



t 



I 



ef hp 



FIG. 173. Tracheae, tracheides, wood-fibres, and wood parenchyma of a Dicotyledon with 

 transition-forms between the various elements. Diagrammatic, Explanation in text. (Modified 

 after STRASBURGER.) 



wood such a valuable building material depend upon its natural 

 function as a mechanical tissue. 



The various kinds of cells of which the wood is composed can be most readily 

 studied by treating wood with SCHULTZE'S macerating mixture (cf. p. 42). 



The vessels are pitted or less commonly reticulately thickened. 

 The tracheae may be wide and composed of short segments, or narrow 

 and formed of more or less elongated cells (Fig. 173 g, tg}\ the 

 tracheides are narrow and elongated and serve both for conduction 

 and as mechanical tissue. The wood-fibres (h) are usually very long 



