TISSUB-SYSTEMS. 175 



300. Fig. 230 sliows a section of an exogenous stem 

 somewhat older than that shown in Fig. 228. Here new 

 bundles have been formed between the earlier ones, so 

 that the whole centre of the stem, except the pith and the 

 lines radiating from it, is occupied by 

 the wood. This cylinder of wood is 

 now encircled by a ring of cambium, 

 beyond which are the tissues of the 

 phloem. 



301. The appearance presented by 

 the cross-section of an exogenous stem 

 Tig. 230. is that of a series of concentric rings, 



each ring showing the limit of a year's growth. The 

 portions of wood formed late in the summer are more 

 compressed by the outlying tissue than those formed in 

 spring, and hence the outer part of each year's ring appears 

 denser, and is sharply marked off from the ring of the 

 following year. 'So growth of the cambium takes place 

 in winter. The rays which intersect these rings as fine 

 lines consist of portions of the ground or fundamental 

 tissue which have been squeezed into their present form 

 by the increasing fibro-vascular bundles on each side of 

 them ; they are called medullary rays, and, as the stem 

 grows, new ones are formed from the cambium. Only the 

 yrimary ones, however, extend from the pith to the bark; 

 those formed later are shorter. 



302. In roots a special arrangement of the tissues of 

 the bundles prevails, the xylem and phloem forming 

 alternate rays. This is the radial arrangement. 



303. The fimdamental or ground tissue com- 

 prises all the parts of the plant not already included in 



Figf. 230.— Section of an older dicotyledon, tiie bundles now forming a ring. 



