MECHANICAL CONSTRUCTION OF PLANT-BODY 177 



compared with that of the hollow steel columns used for supporting 

 shop-fronts. 



A more frequent, and mechanically a more effective type of strength- 

 ening 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 



Fig. hi. 



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. 



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

 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. 38). 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. 

 112, A, B.) The cells of Hemp are about 100 times as long as broad, in 

 linen the proportion is about 200 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. 



B.B. 



M 



