92 THE UNIVERSITY SCIENCE BULLETIN. 



cells included in these areas of pericycle resemble pith cells, in 

 shape and nature of cell walls. 



The cell walls of the primary medullary rays are cellulose up- 

 ward from a little above region II, as shown in fig. 7, g, of region I, 

 and remain cellulose in certain sections of region IV (fig. 11, g). 

 The cell walls of all but a few of the innermost cells of the primary 

 medullary rays are lignified in regions II and III (figs. 8, g, and 9, 

 g), and in some sections of region IV. The primary medullary ray 

 cells, the walls of which are delicately pitted, are shown in longi- 

 tudinal sections of region III in fig. 27. 



The lignification of the cell walls in the pericycle areas e" and / 

 (fig. 9) and the primary medullary rays in regions of stem from 

 region IV up to a little above region II would seem to give strength 

 and elasticity to these regions of stem. Probably the anomalous 

 structure which forms a greater part of the woody cylinder of the 

 stem in region IV and below (as described later) strengthens these 

 regions of the stem sufficiently to make unnecessary the lignifica- 

 tion of the cell walls of the pericycle areas e" and /and the primary 

 medullary rays oftentimes in sections of region IV, and in all 

 sections of regions below. 



The pith cells are shown in longitudinal view in fig. 28. The 

 cells of the pith do not break down to form hollow spaces, but 

 persist in older sections of the stem. Generally the cell walls of 

 the pith stain a deep purple color in chloroiodide of zinc, but also 

 stain a delicate red with phloroglucin. 



THE PRIMARY VASCULAR BUNDLES. 



The primary vascular bundles are mapped out for region I in 

 fig. 2; region II, fig. 3; region III, fig. 4; and region IV, fig. 5. 

 Usually one or two of the primary vascular bundles of larger size 

 are situated opposite each of the main ribs of the stem in regions 

 I, II, and III. The course of the primary vascular bundles in the 

 stem is mapped out in fig. 29, j. In regions above region TV the 

 leaves remain on the stem, and the leaf traces traverse a portion 

 of the anomalous structure (fig. 29, t"). The course of the leaf- 

 trace bundles out of the stem up through the petiole into the leaf 

 is discussed later. The leaf traces are broken apart in region V 

 and below (fig. 29, u") by further development of the anomalous 

 structure. 



The primary vascular bundles are differentiated into the phloem 

 and xylem elements in region I (fig. 30, h' and j'). In cross sec- 

 tion some of the cells of the phloem elements have walls unusually 



