96 THE UNIVERSITY SCIENCE BULLETIN. 



a xylem zone around the stem (figs. 29, q", and 50, p) and a 

 phloem zone located just exterior to the xylem zone (figs. 29, q", 

 and 50, q) have been formed in regions of stem a little farther 

 down. The outer and inner borders of the two zones are very 

 irregular in boundary (figs. 29, q, and 50, p and q), due to the 

 varied activity of the first ring of secondary meristematic arcs, 

 which is described in the following paragraph. 



As seen in cross section, the formation of the xylem zone, and 

 the narrow zone just exterior to the xylem zone, is as follows: 

 Various arcs of the secondary meristem form radial strips of 

 xylem on their inner side, and small, isolated groups of phloem 

 on their outer side (fig. 51, n") ; some secondary arcs form radially 

 longer strips of xylem on their inner side and do not form phloem 

 on their outer side (fig. 51, o"); and other secondary arcs form 

 radial strips of xylem, and secondary medullary ray tissue be- 

 tween the phloem groups (fig. 51, p")- What else can this area 

 of tissue (figs. 50, r, and 51, r) located between the groups of 

 secondary phloem be called other than a secondary medullary 

 ray? The xylem-phloem zone is mapped out in fig. 50, p and q. 



The outer two rows of cells of the phloem zone, shown in figs. 

 50, t, and 51, t, are pericycle cells; the rest of the narrow zone is 

 made up of the phloem groups (figs. 50, s, and 51, s) and the 

 secondary medullary rays (figs. 50, r, and 51, r) ; and the phloem 

 zone is interrupted by certain radial, anastomosing strips of xylem 

 (figs. 50, u, and 51, u). 



As seen in cross section, the newly formed anomalous xylem 

 zone (figs. 50, p, and 51, p) coalesces with the outer part of the 

 lignified primary pericycle (figs. 50, e" and 51, e"). The anoma- 

 lous xylem stains a light yellow color in chloroiodide of zinc (figs. 

 50, p, and 51, p); the part of the woody cylinder formed from 

 lignified pericycle and primary medullary rays, a yellowish-brown 

 (figs. 50, e", f, and g, and 51, e", f, and g). The anomalous xylem 

 and the primary tissue included in the woody cylinder stain uni- 

 formly with phloroglucin. 



In region IV (figs. 5, and 29, r"; and 10) more xylem zones, and 

 the phloem zones alternating with the xylem zones in a radial 

 direction, have been formed by the successive rings of arcs of 

 secondary meristem, arising in the pericycle. The arcs in the 

 rings of secondary meristematic arcs, arising later than the first 

 ring of secondary arcs, do not have their origin directly exterior 

 to the phloem groups previously formed by the next older ring of 

 secondary arcs, as illustrated in fig. 48, n, where a secondary arc 



