290 



STRUCTURE OF STEMS AND ROOTS 



493. In roots, the phloem and xylem are not definitely 

 arranged in bundles, but in alternating radial strands or 

 plates. This plan is typical in young roots and rootlets, 

 but is more or less obscured in older ones as seen in Fig. 467. 



460. Microphotograph of cross-section of grape cane of a single season's growth; a 

 cambium; a-b, phloem; a-c, xylem; b-d, periderm layer, derived from phellogen, 

 which cuts off the cortex, d-e, with its primary bast bundles. Note large medullary 

 rays, m, and the large ducts for water conduction. Compare with structure of 

 pine wood, Fig. 461. 



