6o2. THE STRUCTURE OF ECONOMIC PLANTS 



phloem forms a zone on the centrad face of each bundle, but is 

 interrupted by the four rays. The outer phloem is continuous 

 with that of the root, having maintained its original position. 

 There is often a differentiation of phloem across the rays connect- 

 ing the adjacent areas of outer phloem; also a development of con- 

 nective phloem in the ray, which establishes continuity between 

 the inner and outer phloem groups of each bundle. (Fig. 3ii, F.) 

 The diameter of the axis increases in the region of the peg, 

 the pith becomes larger, the bundles are separated by wider rays, 

 and their number is increased from four to eight. (Fig. 313, 6^, H.) 

 These are transitional, since the metaxylem is still tangentially 

 oriented in relation to the protoxylem, except that a few large 

 pitted vessels are located on the outer face of the first-formed 

 elements in an endarch relationship. (Fig. 314, A.^ The inner 

 phloem is separated from the xylem by one or two layers of paren- 

 chymatous cells which may or may not initiate cambial activity, 

 and this situation also obtains in the zone between the lateral 

 phloem and the vascular tissue. The pericyclic zone is irregularly 

 multi-layered, the endodermis persists as a continuous uniseriate 

 layer, and root hairs develop approximately to the peg. 



In the upper portion of the peg, the primary xylem develops 

 in an endarch relationship rather than tangentially. (Fig. 313, /.) 

 This is accompanied by a change in the metaxylem elements, 

 which, instead of being scalariform and reticulate, as in the root, 

 are chiefly spiral or loosely scalariform and somewhat larger in 

 diameter. There is also a greater continuity of the phloem, the 

 outer strand forming a broad mass capping each bundle, while 

 the inner one maintains its centrad position. 



The Peg. — Although the transition occurs in the axis at the 

 point where the peg develops, there seems to be no constant 

 relationship between the transition and the structure of the peg. 

 Whiting points out that in some cases the greatest dimension of 

 the peg occurs at the level where there are four tangential transition 

 bundles; while, in other instances, the maximum diameter of the 

 peg is at the point where there are eight or more endarch bundles. 

 The pattern of transition is determined in the embryo at a time 

 when there is no evidence of an enlargement to form the peg, and 

 this structure does not begin to develop as a lateral ridge until 

 about the third day after planting. 



The anatomy of the peg is simple, and its growth results from 



