8 



OF WOOD IN GENERAL 



there is a central whitish mass, which on being magnified exhibits 

 a comparatively wide-meshed structure, there are round this a 

 ring of patches of a greyer, closer tissue. These grey patches 

 may be observed to be roundish or slightly wedge-shaped in out- 

 line, their longer diameter lying in one of the radii of the stem, 

 and they are wider across their outer parts. They appear grey 

 on account of the smaller diameter of their cells. Longitudinal 

 sections show these patches to be cross-sections of long strands or 

 bundles of cells, narrower and more elongated than those around 

 them. The central mass of tissue is tbd pith or medulla, and these 

 strands are known as procambium or desmogen. 



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Fig. 4.— Transverse section of the stem of Traveller's Joy Clematis ( VUdlba), showing 

 relatively large central pith and large vessels. 



The pith is relatively large in the stems of herbaceous plants or 

 in young shoots (Fig. 4), but does not increase in bulk as the tree 

 grows older. Its cells are at first full of fluid, and their walls 

 often remain thin. Those of its outer portion, near the pro- 

 cambium strands, are smaller, and all its cells are often two or 

 three times as long in the direction of the elongation of the stem 

 as they are broad. Thus in shape they are short, polygonal, 

 closely-packed prisms. In many cases, as in the Elder, the cells 

 of the pith die, losing their fluid contents, shrivelHng, and so 

 completely disorganizing the entire tissue that the stem becomes 

 hollow, or a mere line of dry powder in the centre of the inner- 

 most ring of wood marks this structural centre of the stem. In 



