ROOT STRUCTURES 155 



The individual cells are thin-walled and irregularly 

 circular in outline, when seen in the transverse section. 



Viewed in longitudinal section these cortical cells 

 often appear rectangular or polygonal. In powdered 

 materials the parenchymatic tissues may occur as 

 rectangular, polygonal or irregularly circular cells, 

 usually in mass and containing cell contents. 



Endodermis. This tissue separates the periblem 

 tissues from those of the plerom, and is the innermost 

 of the primary periblem tissues. The presence of 

 endodermal tissues in a mature root is an indication of 

 primary structure. The endodermis is very seldom 

 more than one layer of cells in thickness and may be 

 readily distinguished from the adjacent cellular ele- 

 ments. The individual endodermal cells are usually 

 slightly colored and rectangular or polygonal in out- 

 line when viewed in transverse section. The cell walls 

 may be uniformly thickened, or may show thickening 

 on all sides except that toward the cortex. Thickening 

 of these cell walls is due to deposition of suberin. In 

 longitudinal sections or in powdered materials, the 

 endodermal cells appear as rather long structures, 

 resembling fibers. They differ from the latter in that 

 one wall is thicker than the other and the end walls 

 are blunt or square. 



Phloem Bundles. These tissues are developed in 

 the parenchyma of the plerom zone and therefore 

 are located within the endodermis. The phloem 

 bundles occur as isolated groups of cells in the plerom 

 parenchyma and usually alternate with the xylem 

 bundles. Each phloem bundle consists of a trans- 

 porting element (sieve cells) and a mechanical or 

 supporting element (bast fibers). In transverse sec- 



