SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 13I 
of the bundles, and are therefore subsequently found only in the medullary sheath (in 
Gymnosperms also) ; and since these were formed during the growth in length, 
they are considerably longer than the secondary elements. The secondary phloem 
consists of phloem-parenchyma, sieve-tubes, and sometimes of true bast-fibres ; 
but the latter are often wanting. The mode in which these elements combine to 
form the secondary tissues varies greatly in different plants, and is, at present, of 
subordinate importance. 
The elements of the secondary xylem of which we have now spoken, as well as 
those of the secondary phloem, are, like those of the primary xylem and phloem, 
elongated in the direction of the axis of growth. But elements also occur in the 
thickening tissue placed horizontally (z. e. at right angles to the axis of growth) and 
radially, out of which the radiate tissue is composed. In Fig. 105, C (p. 129) the 
secondary xylem and phloem are represented as crossed in a radial direction by dark 
lines, some of which pass through all the secondary layers, while others begin only 
in the secondary xylem and end in the secondary phloem ; the former are first formed, 
the latter subsequently, and constantly in increasing numbers. Each of these dark 
lines in the figure represents a ray of parenchymatous cells placed horizontally: each 
of the rays runs, as will be seen, uninterruptedly from the xylem through the cambium 
into the secondary phloem ; as long as it runs through the xylem it is called a 
Xylem-ray V its continuation into the secondary cortex is a Phloem-ray. These rays 
split up, as it were, the secondary tissue in the longitudinal and radial directions into 
sections which have a wedge-shaped form when cut through horizontally, and which 
increase in number as the cambium-ring increases in size. Each separate ray does 
not, however, by any means extend through the whole length (in the direction of 
growth) of the secondary tissue; but has generally only an inconsiderable height. 
If a thick stem is split longitudinally, the rays have the appearance, in many close 
woods, of glistening bands (the ' Silver-grain traversing the prosenchymatous 
woody tissue in a radial direction ; in a tangential section they have the appearance 
of wedges driven into the mass of the wood. Each ray is sharp-edged above and 
below (?'. e. in the direction of the axis of growth), thin, but usually thickened 
in the middle (in reference to its height), and sometimes composed of a number 
of layers of cells, as is shown in Fig. 97, p. 117. This and the position of the 
rays causes the elements of the secondary xylem and phloem which are elongated 
longitudinally to be more or less bent in different directions. If the rays were 
imagined to be altogether removed, the entire thickening-tissue would then consist of 
bundles penetrated by empty meshes, and anastomosing tangentially above and 
below them. A very good idea of this structure may be obtained by examining a 
piece of ordinary lime-bast, or stems, such as the cabbage, in which the soft 
medullary rays have decayed. 
Just as the elements of the secondary xylem and phloem which are elongated 
^ The term Medullary Ray should be avoided in reference to these ; since most of the rays are 
neither connected with the pith, nor have any of its properties. [It is difficult however to substitute 
any other expression, although tlie term is used in a somewhat conventional sense. It is convenient 
to distinguish as Primary Medullary Rays those whicli are connected with the pith, and as Secondary 
Medullary Rays those which only commence in the secondary xylem,] 
K 2 
