MINUTE ANATOMY OF FLOWERING PLANTS 219 



changed in shape. Cells of the external layer are in 

 many instances flattened, in accordance with their protec- 

 tive function. Cells of strengthening and conducting 

 tissues, on the other hand, are frequently greatly elon- 

 gated. In the conducting tissues, elongated cells placed 

 end to end in rows become united into tubes or ducts, 

 the end walls being absorbed, wholly or in part, to allow 

 the passage of liquids. 



506. Several of the principal modifications of cells should 

 now be described. We may begin with 



wood fibers. 



507. Wood, whether occurring in so- 

 called woody stems, or in succulent herba- 

 ceous stems, consists largely of fibrous 

 cells, associated, in most cases, with ducts, 

 or vessels. The fibrous cells are of a great 

 variety of form and appearance in differ- 

 ent plants ; but those which are termed, 

 in rather an indefinite way, wood fibers, 

 are pointed cells, several times longer 

 than broad, having thickened and lignified 

 walls, and characteristically showing in 

 these walls numerous pits, i.e. spots where 

 the walls have remained thin or become 

 perforated in such a way as to allow com- 

 munication between the cells (Fig. 366). 



508. Bast fibers. These are found in 

 strands in the bark. They are generally 



of considerable length, compared with their diameters. 

 Their walls are generally much thickened, so that the 

 internal space, or lumen, is small, as seen in cross section 

 (Fig. 367). Bast fibers give strength to the inner, stringy 

 bark of the Basswood, the Grapevine, the Leatherwood, 

 and so on. They constitute the fiber of Flax, from which 

 linen fabric is woven. 



509. Collenchyma. The name collenchyma is given to 

 masses of cylindrical or prismatic cells, having walls 

 thickened at the corners in a peculiar manner (Figs. 368, 



