MECHANICAL CONSTRUCTION OF PLANT-BODY 145 



compared with that of the hollow steel columns used for supporting 

 shop-fronts. 



A more frequent, and mechanically a more effective form of 

 strengthening cell is seen in the elongated sclerenchyma- fibre. Such cells 

 are commonly associated in masses, often forming strands which run 

 continuously for long distances. These strands of the Flax supply 

 the material for linen, of the Jute for sacking, of Hemp, New Zealand 

 Flax, etc., materials for cordage, while similar strands of the COCO- 



FIG. 104. 



Part of a transverse section of the xerophytic leaf of Hakea, showing a stoma 

 greatly depressed below the well-developed epidermis, which is propped out by 

 thick-walled sclerotic cells. ( x 150.) F. O. B. 



Nut, and other Palms are worked up into mats, brushes, etc. In the 

 plant they frequently accompany the vascular strands, and are often 

 associated with the phloem as bast-fibres, or with the xylem as wood- 

 fibres (compare Fig. 37). But there is no necessary association with 

 vascular tissues, and the sclerenchyma is often quite independent of 

 them. The mechanical cells themselves are elongated, with ends pointed 

 and sides flattened, so that they fit closely together. (Fig. 105, A, B.) 

 The cells of Hemp are about 100 times as long as broad, in linen 

 the proportion is about 2OO to I ; in the extreme case of the Rhea 

 fibre (Boehmerid) the length has. been estimated at 1500 times the 

 breadth. The lignified walls may be so thickened that the cell-cavity 

 is obliterated. They are thus practically rods of resistant material. 



