ROOT STRUCTURES 161 



Cambium. — This tissue is meristematic, possessing 

 the power of cell division or reproduction, and is placed 

 between the xylem and phloem elements. The indi- 

 vidual cells, as seen in transverse section, appear as 

 light-colored, thin-walled, rectangular, tangentially 

 elongated cells. The cambium zone comprises one 

 or more layers of cells, and, although first existing 

 in the form of detached strips attached to the pri- 

 mary bundles, soon forms a complete ring or circle 

 in the root. In this manner the cambium not only 

 bisects the bundles but also crosses the medullary rays 

 or strips of tissue between the fibro-vascular bundles. 



Medullary Rays. — The original medullary rays 

 are wedge-shaped strips of plerom parenchyma between 

 the fibro-vascular bundles. At first these strips are 

 broad; but, owing to the development of new fibro- 

 vascular bundles within them, they ultimately appear 

 as narrow strips of tissue extending from the center 

 of the root to the cortical region. The rays may be 

 from one to five cells in width and as much as twelve 

 cells in height. Seen on transverse section (Plate 

 60), the ray cells are long and rectangular with slightly 

 thickened, porous or non-porous walls. On radial 

 section (Plate 61 A), they appear as groups of regularly 

 arranged rectangular cells extending at right angles 

 to the other tissues. On tangential section the rays 

 appear as elliptical or oval patches, each of which 

 contains several ray cells (Plate 61-B). The dimen- 

 sions of a medullary ray can only be determined by 

 examining sections made in different directions. Trans- 

 verse sections show the width and length, radial 

 sections the height, and tangential sections the width 

 and height of the medullary rays. 



