STEM STRUCTURE 179 



become partially lignified. In powdered woods (Plate 

 60) the fibers and ducts are always apparent on longi- 

 tudinal view. Owing to the tenacity with which 

 they adhere, wood fibers in powdered materials usually 

 occur in masses. The cavities of the fibers occasionally 

 contain crystals, coloring materials, volatile oils and 

 rarely starch grains. 



Medullary Rays. — These structures extend from 

 the central region of the stem, through the xylem 

 elements, to the cambium and thence to the bark 

 tissues. They provide channels for the lateral trans- 

 portation of nutrients, and communicate with both 

 ducts and sieve tubes. The ray cells occur in groups 

 which, viewed in different aspects, show great differ- 

 ences in appearance. In transverse sections the rays 

 occur as bands composed of one to four rows of rather 

 thin-walled, nearly rectangular cells (Plate 60). On 

 this view the width of the entire ray and the width 

 and length of the individual cells may be ascertained. 

 In radial section the rays appear as broad bands 

 composed of several rows of thin-walled, rectangular 

 cells (Plate 61^.). The height of the entire ray and 

 the height and length of the ray cells are apparent 

 on this view. In tangential sections the rays occur 

 as oval or elliptical groups of circular or polygonal 

 cells (Plate 615). The width and height of the indi- 

 vidual cells of the entire ray are apparent on this view. 

 Medullary ray cells may or may not possess porous 

 walls. The ray cells may contain 'starch, crystals 

 or other cell contents. In powdared materials the 

 ray cells (Plate 60) are apparent on radial and occa- 

 sionally on transverse view. The masses of wood 

 fibers frequently show attached ray cells, and in such 



