FIBRES. 



81 



Fig. 48. Portion of cross-section 

 wood ring of Geranium stem, w, 

 wood fibres ; b, duct ; c, middle 

 lamella (Bastin). 



In the wood ring there are other cells besides those just 

 described, with much larger openings, the ducts or tracheids, 

 which will be described a little 

 later. Wood fibres do not oc- 

 cur in all fibro-vascular bun- 

 dles, but are nearly always 

 present in woody and herbace- 

 ous dicotyls. 



BAST FIBRES. Outside the 

 cambium zone is another cir- 

 cle of thick-walled cells, look- 

 ing in all respects very much 

 like the wood fibres just de- 

 scribed. These are the bast or 

 liber fibres. The description 

 of the wood fibres answers also 

 for the bast fibres. With 

 phloroglucin and hydrochloric 

 acid they stain red and the mid- 



die lamella deepest. Stratifi- F ' 8 ' 48 - portlon of * of 

 cation lines and pore canals 

 can also be seen with care. In 

 many cases bast fibres are not as strongly lignified as wood 

 fibres and take the stains less deeply. In such cases the ligni- 

 fication is chiefly confined to the outer layers of the cell-walls, 

 while the inner layers are more or less cellulose in nature; with 

 double stains, as methyl-green and eosin the outer layers will 

 stain green and the inner ones reddish. 



Bast fibres are confined not alone to the phloem or bast por- 

 tion of fibro-vascular bundles, but are often found in other por- 

 tions of the plant, for example, the strengthening fibres beneath 

 the epidermis of some leaves ; in the ground tissue of many vas- 

 cular cryptogams. As a rule, bast fibres are thicker-walled 

 than wood fibres, but both kinds may vary considerably in the 

 thickness of the walls, in their lengths as compared with their 

 diameters, in the number of pore canals. 



WOOD AND BAST FIBRES IN LONGITUDINAL SECTION. 



Treat several longitudinal radial sections of the Geranium 

 stem with Schulze's macerating fluid, as directed under this 

 reagent. Wash them in water and place them in very dilute 

 methyl-green solution for a long -time, say 24 hours or more, 

 which will stain only the lignified walls and the cork cells. 

 Mount in water and examine first with low power. On the 

 outer edge is a green layer the cork cells scarcely at all 

 affected by the macerating fluid. Next to this is a transparent 

 layer of unstained parenchyma cells more or less injured by the 



