178 THE ELEMENTS OF VEGETABLE HISTOLOGY 

 CHARACTER OF WOOD STRUCTURES 



Xylem Elements. — The predominant structures 

 in woody stems are wood fibers and ducts. A ligni- 

 fied type of parenchyma may be associated with these 

 xylem elements. The wood fibers immediately adja- 

 cent to the cambium are living cells; but those nearer 

 the center of the stem are to all intents lifeless. Each 

 fiber is a typical prosenchymatic structure and, in 

 longitudinal section, appears as a long, thick-walled 

 cell tapering toward each end. The ends of fiber cells 

 are usually pointed, and interlock or fit into corre- 

 sponding ends of fibers below and above. Occasionally 

 the ends of a fiber are branched. In transverse 

 section, the fibers appear as angled cells with fairly 

 thick walls and small central cavities (Plate 60). 

 Pores penetrating the fiber wall may be apparent upon 

 longitudinal view, but are rarely seen in transverse 

 sections of a fiber. The ducts of woody stems are 

 of the pitted or finely reticulate types. In transverse 

 sections they appear as large, thick-walled cavities 

 or cells, frequently with porous walls (Plate 60). 

 The ducts usually occur in groups and are surrounded 

 by wood fibers or woody parenchyma. It is in longi- 

 tudinal section that the true nature of the duct is 

 apparent, as its length and the character of its walls 

 can only be seen in this view. Wood fibers and ducts 

 extend parallel to each other and both are at right 

 angles to the medullary rays. Woody parenchyma 

 differs from fibrous tissue in that the cells are much 

 shorter and thinner-walled (Plate 61). In form the 

 woody parenchyma cells appear similar to true par- 

 enchyma; but the cellulose walls of the latter have 



