M'CRONE: HISTOLOGY OF MALVA. 263 



each phloem strand. As the stem grows older the original 

 bast fiber bundles are pushed outward and apart, and new 

 bast fibers are produced by the cambium, until in the older 

 parts one finds groups of bast separated by phloem tissue and 

 extending in radial rows (figs. 34 and 35). 



The starch of the stem is all found in the starch sheath. 

 This in the young parts of the stem forms a continuous layer 

 of cells composed of the innermost cells of the thin-wa'lled 

 parenchyma of the cortex. In the older parts the bast fibers 

 appear just inside the starch sheath. Then as the bast fiber 

 region is extended radially the starch sheath follows these 

 radial extensions of the bast fibers. Figures 33, b, to 35, b, 

 illustrate this. 



Wood fibers (fig. 18) are also formed and the older parts 

 of the stem become quite woody. As the xylem develops the 

 pith becomes smaller, the water-conducting area becomes much 

 greater, as may be seen by a comparison of figures 30 to 35, 

 the tracheal tubes becomes somewhat larger and their fre- 

 quency is greatly increased. Figures 34-35 and 41-42 show 

 details of water-conducting tissue. 



The number of vascular bundles varies greatly, ranging 

 from fifteen to twenty-three. 



Crystals of calcium oxalate (fig. 19) occur closely packed 

 together, sometimes in groups of cells and in other instances 

 in isolated cells, in the pith and in the parenchyma of the cor- 

 tex and pericycle. The crystals may be found any place in 

 the parenchyma, but are more common along the bast fiber 

 region. These crystals are all compound, and are often almost 

 large enough to fill the cells which hold them. Figure 17, a. 

 shows the distribution of crystals in the parenchyma cells of 

 an old part of the stem. 



Mucilage is found in large quantities in the stem (fig. 15). 

 In the parenchyma of the cortex the cells containing it break 

 down to form long tubes or passages filed with mucilage. 

 These cells (fig. 24, a) sometimes also break down in lateral 

 directions, making the mucilage cavities much larger in lat- 

 eral diameter than the ordinary parenchyma cells, but in other 

 instances the mucilage-filled cells (fig. 25) are not so much 

 larger than the other cells. As the cells enlarge, in the older 

 parts of the stem, the mucilage cells do not grow accordingly, 

 and even get considerably smaller (figs. 25 and 26, a) as the 



