90 THE UNIVERSITY SCIENCE BULLETIN. 



The collenchyma cells in the outer part of the rib, then the cells 

 situated farther in, are differentiated from the ground meristem by 

 thickening of the cellulose walls as shown in regions I and II (figs. 

 7, b, and 8, h). The collenchyma, which is the sole strengthening 

 tissue in region I, contains chloroplasts. In cross section the cells 

 are usually as large as the epidermal cells or larger. A longitudinal 

 view of the collenchyma cells of region III is shown in fig. 18. In 

 region IV (fig. 10, b) we see the collenchyma has reached a greater 

 development than higher up, and the lumina of the cells are smaller 

 because of the thickening of the cell walls. 



In most ages of stem seven groups of collenchyma, correspond- 

 ing to the seven prominent ribs, are present. There are three 

 main ribs in region I (fig. 2), giving this region of the stem a tri- 

 angular shape. Ten ribs are present in region V (fig. 6). 



The chlorophyllous tissue between the ribs is made up of ordi- 

 nary parenchyma cells, the walls of which are thickly lined with 

 chloroplasts (fig. 7, c). There are no intercellular spaces among 

 these cells in region I (fig. 7, c) ; a few occur in the outer part of 

 the chlorophyllous tissue of region II (fig. 8, c); and quite a num- 

 ber are present in region III (fig. 9, c). 



On certain sides of the stem the cell walls of one to six rows of 

 parenchyma cells, just exterior to the starch sheath, in region 

 IV, (fig. 10, o) suberize their cell walls. These cells resemble 

 the cells of the starch sheath in shape and size, and have 

 thicker walls than the other parenchyma cells of the primary 

 cortex. A phellogen layer arises in the parenchyma of the primary 

 cortex on various sides of the stem in region V and below and 

 forms cork (figs. 6, b', and 19, b') and lenticels (figs. 6, a', and 

 20, /') . The epidermis and cells of the primary cortex exterior to 

 the newly formed cork are sloughed off. According to Georghieff 

 (fide Solereder, '08), the cork arises in the primary cortex in 

 Kochia also, an allied genus. Solereder ( '08) states that cork may 

 be formed externally or internally in the Chenopodiese; internally 

 it is generally formed in the pericycle. 



The starch sheath is well defined in all regions of stem by the 

 shape and size of its cells, most of which have a long tangential 

 dimension (fig. 9, d) and are lengthened longitudinally (fig. 21, d). 

 The radial and the outer tangential walls of the starch sheath cells 

 in portions of region V have become suberized. 



The ground tissue of the pericycle has just been formed from 

 the ground meristem in region I (fig. 7, e and /) . In this region of 

 stem two areas of the pericycle are distinguishable by the size of 



