STEM STRUCTURE 177 



cells is often so reduced as to appear as a mere dot 

 or mark in the center of the cell. Pores traversing 

 the wall of the fiber are rarely apparent in sectional 

 views. The sieve elements in barks are usually 

 collapsed, and appear as very small, irregular, thin- 

 walled cells adjacent to the bast fibers. In powdered 

 materials the bast fibers are usually prominent, whereas 

 sieve elements are rarely apparent. The fibers occur 

 singly and in masses, frequently combined with par- 

 enchyma or ray tissues (Plate 59). 



CELL CONTENTS 



The cell contents in barks are stored in the cortical 

 parenchyma, medullary rays and occasionally within 

 stone cells. Resins and volatile oils are contained 

 in special secretion cells or cavities. The stored materi- 

 als include starch, crystals, tannins, resins, gums 

 and volatile oils. 



WOOD STRUCTURE 



From a histological standpoint wood is the material 

 remaining after removal of the bark. According 

 to this statement, the wood includes all struc- 

 tures internal to the cambium. True wood is never 

 present in the monocotyledonous stems which retain 

 their primary structures through life. The structures 

 usually present in woods, in the order of their 

 arrangement, beginning with the outermost, are as 

 follows: 



1. Xylem elements, 



2. Medullary rays, 



3. Pith parenchyma. 



