MECHANICAL TISSUES 



133 



v 



^ 



'' Fig. 42.— Powdered althaea root, sbowine, h, 

 bast fibres, sji. Pitted vessel, starch etc. (Vogl). 



narrow sense.) 

 the longest and 

 plant They are 



These are in general 



strongest cells of the 

 usually much elongated 



Fig 43.— Wood fibres or Libriforin. (Tschirch). 



and thick-walled, and have sharpened 

 points and small, spirally arranged, slit- 

 like pores. As a rule, the bast cells 

 have uo contents; the wall is generally 

 lignified and the lumen filled with air. 



The length varies greatly from =^ mm. 

 to an inch or more, the proportion of 

 length to breadth being about 10 to 1; 

 often however, bast fibers are from 2,000 

 to 4,000 times as long as wide. (Fig. 41.) 

 The thickening of the cell wall varies 

 greatly, being generally very unequal. 

 In some plants the bast fibers are but 

 slightly thickened, and have a wide lu- 

 lurn resembling wood fibers; at other 

 times the lumen is entirely gone. 



On cross section the bast fibers when 

 a lone a re 



when 



more or less rounded; but 

 collected into bundles, mutual 

 pressure tends to make them angular, 

 polyhedronal, or even apparently iso- 

 dia metric. 



Fig-. 44.~Longitudmal section of root of Inula 

 Helenium, showing Ih. libriform fibres with pores, 

 g. vessels, (Tschirch). 



The ends vary greatly; typically the^ 

 are sharp and somewhat rounded; occa- 

 sionally they are squarish, and some- 

 times branched, in which case, especial- 

 ly when short, they are hard to difi^er- 

 entiate from some stone elements. 



The slit-shaped pores are characteris- 

 tic, especially in their left-handed spiral 

 arrangement. (DeBary, 1. c.) 



The strength and toughness of the bast 



fibers are great. Experiments have 

 shown that some of the stronger fibers 

 will stand proportionately the same bend- 

 ing as steel without breaking, and that 

 they can bear from one-quarter to one- 

 lialf us great a pull as this metal, while 

 they are from five to ten times as elas- 

 tic, that is,- they will allow of so much 



