HISTOLOGY OP ANGHOSPERM STEM, ROOT, ETC. 113 



(4) The pericycle, a layer of cells some of which (espe- 

 cially those outside the xylem strands) show division by a 

 tangential wall (so that there is a double layer at these 

 points). 



(5) The radiating primary xylem bundles, usually 

 five (sometimes 4 or 6) in number, each roughly triangular 

 with the narrowest (protoxylem) vessels at the apex of the 

 triangle, which points outwards and lies immediately within 

 the pericycle the development of the wood is centripetal, 

 and new vessels may be seen in course of formation on the 

 inner side of the bundle. 



(6) The primary phloem bundles, alternating with the 

 primary xylems the outer part of each phloem bundle 

 consists of thick- walled tissue, while the inner thin- walled 

 portion is not readily distinguished from 



(7) The parenchymaof the conjunctive tissue or ground 

 tissue of the vascular cylinder. 



131. Secondary Thickening of Bean Boot. Cut a 



series of sections across the older parts of the root avoid 

 the oldest part near the seed itself, where the hypocotyl tran- 

 sition region between root and stein begins. Note that the 

 ground tissue lying within the phloem bundles shows 

 repeated division by tangential walls and is therefore 

 arranged in radial rows, forming cambium bands which 

 produce secondary xylem internally and secondary phloem 

 externally. Then the portions of pericycle lying outside 

 the primary xylem bundles also become meristematic, so 

 that the cambium now forms a continuous zone. The 

 piliferous layer dies off and the hypodermal cortex layer 

 becomes cutinised, forming the exodermis. 



Cut transverse sections from the oldest part of the root 

 of a large seedling or an adult plant, in which considerable 

 secondary thickening has taken place. Note (1) the 

 primary xylem bundles, still in their original position at 

 the outside of the central ground tissue ; (2) the broad rays 

 of parenchyma, on the same radii as the primary xylems ; 

 (3) the secondary xylem, in masses alternating with the 

 rays ; (4) the cambium, forming a continuous zone ; (5) the 

 secondary phloem, lying outside the cambium ; (6) the dis- 

 P. B. 8 



