THE GYMNOSPERMAE : CONIFERALES AND TAXALES 665 



rays, connecting the pith with the external cortex. No pericycle or endo- 

 dermis can be seen. Outside each bundle there is a large resin canal in the 

 cortex. The outer surface is covered by a very highly thickened epidermis, 

 beneath which is an equally thickened hypodermis. 



The fully formed cambium consists of a single layer of meristematic cells, 

 which repeatedly divide in a tangential direction. Of the two cells thus 

 formed at each division one remains cambial, the other, the tissue mother 

 cell, divides again tangentially and the two cells thus formed become trans- 

 formed into either xylem cells or phloem cells respectively, according to 

 whether they lie on the inside or the outside of the cambial cell. Exceptionally 

 there may be more than one division of the tissue mother cell, so that 

 three or even four cells may be added to the vascular tissue from one tissue 

 mother cell. Certain cambial cells produce only parenchymatous cells, 

 which form radial rows dividing the tissues of the bundle. By this 

 means portions of the primary bundle are separated off on each side and 

 these portions pass out through the cortex as the leaf traces of the scale 



leaves. 



The cambium also extends itself tangentially, so that the bundles gradually 

 increase in width (see Fig. 663), narrowing down the primary medullary rays 

 until neighbouring bundles make contact and close the ring. This brings the 

 edges of the bundle cambia together, and from this point we may date the 

 so-called secondary thickening, which develops a closed ring of vascular 

 tissues. It is well to emphasize, however, that there is no real discontinuity 

 between the primary and the secondary growth. 



The cambium having now formed a closed ring, its further development 

 produces a continuous zone of secondary xylem on the inner side and one 

 of secondary phloem on the outer side (Fig. 664). At many points these 

 two zones are traversed by secondary medullary rays, also produced from 

 the cambial cells, to replace the primary rays. They are mostly uniseriate, 

 that is, they are only one cell broad and they are usually less than a dozen 

 cells in height (Fig. 665). They are composed of rectangular cells with thick 

 walls and numerous simple pits, but each contains cytoplasm and a nucleus, 

 with many starch grains. The upper and lower margins of the rays are formed 

 of one or two rows of marginal ray-tracheids, running horizontally, but 

 otherwise resembling short tracheids of the xylem, from which they may be 

 derived (Fig. 666). In the cambium and phloem zones these marginal 

 tracheids are replaced by large, thin-walled cells which extend upwards and 

 downwards between the cells of the vascular tissue. They contain much 

 free protein and are apparently storage cells, like the starch-storing cells of 

 other parts of the ray. These are sometimes referred to as albuminous 



cells. 



Each successive year new medullary rays are formed, between those 

 previously laid down, thus, as the distance from the centre increases, the 

 average distance between adjacent rays remains approximately the same. 



The areas of contact between medullary ray cells and tracheids of the 

 wood are called the ray fields. Each is marked by a large thin area or simple 



22 A 



