STRUCTURE AND CHEMISTRY OK CELL WALL 75 



acuminate or, as in certain bast fibers, variously "lobed" or 

 "branched" (Mansfield, p. 22, Fig. 2). In short fibers, the 

 ratio of the diameter of the cell to its length may average 1 :10 

 to 1 :20 while in extreme cases (e.g., in the Urticaceae) the ratio 

 may reach or exceed 1:4000. These data, taken from De Bary 

 (p. 13), emphasize the fact that certain bast fibers may represent 

 the largest of all cells in higher plants. According to Hay ward's 

 (p. 241) discussion of the literature, hemp (Cannabis sativa) 

 fibers vary in length from 1-10 em. In flax (Linnm) the length 

 of the fibers likewise varies from 2.5 cm. to as high as 12 cm. 

 Apparently the longest bast fibers which have been accurately 

 measured occur in the stem of Boehmeria nivea, a member of 

 the Urticaceae. In this species, Aldaba (1927) succeeded, by 

 means of a special maceration technique, in isolating individual 

 fibers, the five longest of M'hich measured ' ' respectively 400, 500, 

 520, 540, and 550 mm." 



3. Structure and chemistry of the cell wall. Mature fibers 

 characteristically possess a well-defined secondary wall which 

 is often so thick that the cell lumen may be almost or entirely 

 occluded at various points. The thick secondary wall exhibits 

 typically slit-like vestigial pits which in bast fibers are disposed 

 obliquely in a left-handed spiral series. Haberlandt (p. 154) 

 contends that this arrangement of pits indicates a "corresponding 

 arrangement of the micellar rows" and that "an obliquely pitted 

 bast-fiber may therefore be regarded as an aggregate of exceed- 

 ingly numerous and delicate fibrillae twisted together into a spiral 

 coil of many turns which surrounds a longitudinal canal consist- 

 ing of the cell cavity. ' ' Because of the great economic importance 

 of certain fibers, many studies have been devoted to the chemical 

 composition of their walls. Ilayward (p. 23), while admitting 

 that the degree of lignification of the cell wall may vary even 

 within the same zone of fibers, distinguishes between (1) non- 

 sclerotic fibers, which occur commonly in the pericycle of stems 

 and w'hich possess secondary walls with a relativelj^ high propor- 

 tion of cellulose (e.g., Linum), and (2) sclcrenchymatoiis fibers, 

 which are part of the xylem and which exhibit highly lignified 

 secondary walls. According to Ilayward, lignification tends to 

 render fibers rather brittle while the high cellulose content of 



