^ or T 

 THE STELK UNIVER* 9 



\ -> F H\bX 

 the surface of the stem. These are known as . $i&. ptfiblem. On 



tracing them backwards down the shoot we find them continuous 

 with tissues which immediately beneath the epidermis are 

 commonly green, and which often have their cells much 

 thickened in the corners in herbaceous plants or shoots, whilst 

 still further back, on older parts of woody shoots, the green layer 

 is often buried under one or more layers of brown cork. These 

 tissues which thus originate in the periblem are known collec- 

 tively as the cortex. 



It is with tissues originating from the central plerome or stele 

 that we are mainly concerned. If we cut a young shoot across, a 

 little below its entirely embryonic apex, we shall see that, whilst 

 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 

 outline, 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. Longi- 

 tudinal 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 the pith or 

 medulla, and these strands are known as procambium or desmogen. 



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, shrivelling, and so 

 completing 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 

 other cases, as in the Oak, the cells of the pith have their walls 



