CELLS AND TISSUES Z3 



The Fibers. — Fibers occur in the xylem, phloem, pericyclic, 

 and cortical regions of the axis. On the basis of wall structure, 

 they may be divided into non-sclerotic and sclerenchymatous types, 

 the latter occurring chiefly in the xylem, although the fibers in 

 any zone may become lignified. The non-sclerotic fibers cannot 

 readily be distinguished from one another structurally, and are 

 therefore designated on the basis of their position in the axis and 

 their relation to other tissues. They are sometimes referred to as 

 "bast" fibers in contrast to the wood fibers of the xylem, but the 

 term has been so loosely used that it has little value in descriptive 

 anatomy. It is more accurate to indicate the fiber type by pre- 

 fixing the name of the region in which it is differentiated, i.e., 

 cortical fiber, phloem fiber, and pericyclic fiber. 



NoN-scLEROTic FiBERs. — Frequently in stems, and not uncom- 

 monly in roots, non-sclerotic fibers are differentiated in the peri- 

 cyclic zone. These may form a continuous cylinder of mechanical 

 tissue, a series of pericyclic masses lying outside the phloem of 

 the vascular bundles, or occur as strands in a discontinuous cylinder. 

 Scattered groups of fibers may also be differentiated in the phloem 

 region and less frequently in the cortex as in Pisum. In this type, 

 the walls are non-lignified, or very slightly so, and consist chiefly of 

 cellulose. Where the proportion of cellulose is high (Linum) the 

 fiber has remarkable tensile strength and durability coupled with 

 flexibility and elasticity; but, in fibers where some lignification 

 occurs (Cannabis) these latter properties are less pronounced and 

 they are more brittle. 



In ontogeny, the fiber initial grows rapidly in the axial direction, 

 attaining a length which may be several to many times the original 

 dimension of the cell. In hemp, the average is 3.5 to 4 cm., and 

 fibers 10 cm. in length have been reported; while, in flax, the aver- 

 age is from 1.5 to 3 cm. The extreme length of the fiber is due to 

 the manner of its development. The cell grows without under- 

 going transverse division as frequently as the adjacent cells, and 

 elongation may continue without cross-wall formation as long as 

 the portion of the axis in which the cell is located continues to 

 grow in length. In addition to this method of fiber elongation, 

 there is probably some sliding action by which the tips of the elon- 

 gating cells push past one another, thus adding greatly to the 

 mechanical strength of the tissue. This has been termed " gliding 

 growth"; and, although the mechanism of the process is not 



