30 THE FLOWERING PLANT. 



the way from pith to cortex. It must not be supposed that 

 in the thickened stem a medullary ray has a great vertical 

 extent, for it is often only a few cells high. If we imagine a 

 lath-shaped mass of cells running through the vascular tissue in 

 a radial direction, possessing convex sides which face sideways, 

 and edges facing upwards and downwards, we shall have some 

 idea of such a ray. In cross-section (fig. 5, E) it will look like a 

 narrow streak or " ray " running in a radial direction. A radial 

 section will display it as a broader band running at right angles 

 to the direction of the elongated vascular elements ; and, lastly, 

 a tangential section will cut across it, giving an outline resem- 

 bling in shape the cross-section of a biconvex lens. 1 The "silver 

 grain " of wood is due to the presence of medullary rays, which 

 are always excessively numerous. In the oak they are of un- 

 usually large size, and give the characteristic grained appear- 

 ance. It is a familiar fact that in a piece of stick, a log, or a 

 tree-trunk the wood presents, as seen in cross-section, a series 

 of concentric layers in "annual rings," surrounding a more or 

 less evident pith, which is generally dead and dry. Generally 

 speaking, one ring is formed per annum, hence the name (fig. 

 7, K). It is not usual for the pith to occupy the geometrical 

 centre, so that the rings are not exactly circular, nor is any 

 one ring necessarily of uniform thickness all the way round. 

 Since the new layers are formed by the cambium, it is evi- 

 dent that the inner rings are older than the outer. In many 

 timber trees the internal " heart- wood " or duramen is extremely 

 hard and dry, while the outer " sap-wood " or alburnum is much 

 softer, and, as the name indicates, full of sap. The appearance 

 of annual rings is caused by the difference in texture between wood 

 formed at different seasons of the year. Spring wood is com- 

 posed of comparatively large, thin-walled elements, but, as growth 

 proceeds, smaller and smaller elements are produced, with thicker 

 and thicker walls, till, in the autumn, growth ceases altogether. 

 Hence the dense autumn wood of one year is abutted upon by 

 the much looser spring wood of the next year, and, as the colour 

 also is generally rather different, the boundary between the two 

 is distinctly seen. Annual rings of different years may vary 

 very considerably in thickness. Owing to this method of increase 

 in the wood, viz., by the addition of layer after layer to the out- 

 side, dicotyledons (and gymnosperms too, which were formerly 

 grouped with them) were called exogens, or outside growers. 

 Monocotyledons received the converse name of endogens. This, 

 however, was founded on mistake. The plants in question rarely 



1 The very large medullary rays of oak are much flattened from side to 



side. 



