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 ^; 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 {i. 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 the term is used in a somewhat conventional sense. It is convenient 

 to distinguish as Primary Medullary Rays those which are connected with the pith, and as Secondary 

 Medullary Rays those which only commence in the secondary xylem.] 



K 2 



