464 



FOURTH GROUP. SEED-PLANTS. 



thickness of the stem by increase in circumference combined with radial longi- 

 tudinal divisions of their cells, as in Viscum, Helianthus annnus, etc. The 

 masses of xylem and phloem produced by the activity of the cambium-ring are seen 

 to be traversed longitudinally in radial direction by secondary medullary rays 

 formed of horizontal cells which are not always lignified in the wood, and in 

 the secondary phloem are usually soft and parenchymatous ; they are called xylem- 

 rays in the wood, and phloem-rays in the phloem, and are adapted to receive and 

 store up assimilated food-material. In proportion as the cambium-ring increases, the 



FIG. 407. Part of a transverse section from the fully developed hypocotyl of Ricinus communis. The vascular 

 bundle consists of phloem by and xylem t g\ between them is the cambium layer c c, which is continued into the fun- 

 damental tissue lying between the vascular bundles as interfascicular cambium cb formed by subsequent division of 

 large parenchymatous cells; bb are bast-fibres, y y phloem (sieve-tubes, parenchyma, etc), ^/vessels with narrow 

 P'ts, g e vessels with broad pits, between them wood-parenchyma, m pith, r primary cortex. 



number of these rays increases, and the later layers of wood are traversed by an ever- 

 increasing number of rays. The rays are thin plates formed by one or more layers 

 of cells thinning out like wedges above and below, and showing as radial ribbon-like 

 formations on the longitudinal section, the ' silver-grain ' of the wood ; the tangential 

 section shows how the vascular bundles and the elements as they run longitudinally 

 are curved outwards by the rays and form a network of long meshes, as may be 

 well seen in decaying cabbage-stalks, etc. The rays are formed, like the vascular 

 bundles, from the cambium-ring outwardly and inwardly, and as the latter increases in 

 circumference, it produces new rays between those already formed. 



Where the increase in the thickness of the stem ceases periodically and begins 



