RAPHANUS SATIVUS 301 



placed tangentially and widely separated from one another radially 

 by the continued growth and division of the parenchyma. (Fig. 

 151.) The limited development of lignified vascular elements and 

 the differentiation of abundant thin-walled parenchyma from the 

 cambial derivatives accounts for the succulence of the edible root- 

 hypocotyledonary axis. The vessels are relatively large with walls 

 that are reticulately thickened or pitted. Reticulate thickening is 

 more common where the vessels adjoin parenchymatous cells, and 

 pits commonly occur between adjacent vessels. The vessel seg- 

 ments vary in length, ranging from two to five or six times as long 

 as broad; and the end walls, which are resorbed as the vessel 

 develops, are transverse or slightly oblique. There is great varia- 

 tion in the width of the vessel, some being much narrower than 

 others. Each vessel is surrounded by parenchyma consisting of 

 rows of thin-walled, elongated, prismatic cells which have pointed 

 or transverse end walls. There are also radially arranged, isodia- 

 metric, parenchymatous cells which are produced in progressively 

 larger numbers as the diameter of the axis increases. The vessels 

 immediately centrad to the cambial ring in a nearly mature radish 

 are widely separated tangentially by rays of parenchyma. 



Further activity of the xylem parenchyma results in the develop- 

 ment of crescentic or circular zones of secondary cambium which 

 give rise to tertiary xylem and phloem elements. (Fig. 151.) In 

 these regions, vascular strands may be differentiated having a cen- 

 tral phloem, a type frequently found in the Cruciferae. Tertiary 

 thickening may be extensive, and this activity, together with a 

 general division of the secondary xylem parenchyma, supplements 

 the function of the primary cambium, and with the latter accounts 

 for the fleshiness of the axis. (Fig. 151, £.) The amount of 

 secondary phloem produced is relatively small as compared with 

 the xylem, and the pericyclic zone is narrow. The vascular strands 

 which are differentiated in interrupted radial rows are not vertical 

 but form a reticulate system by branching and anastomosing. 



The secondary and tertiary thickening of the hypocotyl is 

 identical to that described for the root, except that there is a 

 central pith in the upper transitional portion of the former. The 

 development of secondary xylem proceeds as in the root and two 

 broad sectors develop separated by wedge-shaped parenchymatous 

 rays outside the protoxylem points. The cortex is shed as in the 

 root region, but this occurs later in ontogeny. 



