38 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, HE) 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.! 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 textwre 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. . 
