luckan: abutilon theophrasti. 223 



readily than if it were to diffuse through the thickened walls of 

 the collenchyma. The second layer of thin-walled parenchyma 

 of the cortex is only one or two cells deep and is often crushed up 

 against the collenchyma. The starch sheath is apparent, but the 

 cells do not contain starch. 



The bast strands of the pericycle are in groups, separated by 

 thin- walled cells. The cell walls of the bast are lignified. Besides 

 the bast of the pericycle there are several layers of secondary bast 

 strands formed by the cambium and interspersed in alternate 

 layers with the secondary leptome elements (figs. 14 and 15). 



This growth develops a wedge tapering outward, so that in 

 cross section the stem has wedges of bast and phloem alternating 

 with wedges of medullary ray, which taper in the opposite direc- 

 tion (fig. 14). 



The medullary rays are only one to two cells wide in the xylem 

 region of the stem, but where they extend out through the phloem 

 portion of the vascular bundle they broaden radially, so that here 

 they are from four to six cells wide (fig. 15). The medullary rays 

 of the stem do not contain large amounts of starch. 



The phloem is conspicuous, for there are between eight and ten 

 layers of cells between the inner bast strands and the cambium. 



Although this plant is an annual, the xylem of the stem seems 

 to be made up of rings of growth due to alternate layers of tracheal 

 tubes and xylem parenchyma (fig. 15). 



The pith is permanent in the stem, increasing with the other 

 tissues, so that in mature stems the pith has a diameter of one 

 centimeter. In the younger pith the outer cells are filled with 

 starch grains, and among these cells filled with starch are mucilage 

 channels, or sometimes mucilage cells (fig. 14). These channels 

 are formed by the cell walls becoming mucilaginous and some of 

 them breaking down. They may be made of only one cell in the 

 cross section, or several cells adjoining may break down so that 

 the channels or pockets are of different sizes. The pith cells con- 

 tain many calcium oxalate crystals, usually compound, which are 

 very noticeable in the young cells, where they fill the cell cavity. 



The Root. 



The root is a long taproot with few secondary roots. The root 

 resembles the stem in having large wedges of bast. The medul- 

 lary rays, especially in the region of the xylem, are well filled with 

 starch. Cork is present as an outer covering of the older portionr 

 of the root (fig. 16). 



