80 VEGETABLE HISTOLOGY. 



are the xylem portions of the bundles, forming the cylinder of 

 wood. 



The term bast was originally given to the inner bark of the 

 Linden tree, which was called Bass-wood tree. It is now ap- 

 plied to the inner layer of any bark. The term liber, which is 

 also given to the inner or bast layer, was applied in a more gen- 

 eral way to any smooth, inner bark upon which one could 

 write (liber is the Latin for book) . 



The peculiar, long, tough, thick-walled cells which impart 

 toughness to the inner bark, making it valuable in the arts, are 

 hence known as bast or liber fibres. It is in the bark of dicoty- 

 ledons that liber fibres occur most abundantly in the phloem 

 portions of the bundles. 



Some plants, for example, the monocotyls, have no true bark, 

 consequently the bast or phloem portions of the bundles do not 

 lie in the inner or bast layer of the bark, and the bast fibres of 

 such bundles are apparently misnamed. However, the term 

 liber or bast fibres has been extended to embrace all those fibres 

 that occur in the phloem portions of the bundles, whether they 

 occur in the inner bark or elsewhere in the plant and whether 

 they occur in gymnosperms, dicotyls or monocotyls. 



The term libriform has reference to the general resemblance 

 of the wood fibres to bast or liber fibres. Wood fibres usually 

 differ from liber fibres in being relatively less elongated, less 

 tough and flexible and less strongly lignified at maturity, but 

 there are many exceptions, especially in monocotyls, where the 

 two tissues are often indistinguishable by structure alone. The 

 fibres vary often in length. 



Wood and bast fibres may be conveniently studied in a mod- 

 erately stout Geranium stem, in which they have already been 

 noted superficially in Chapter IX. Make thin cross-sections 

 of a Geranium stem and study without staining and also by 

 staining with phloroglucin and hydrochloric acid (red). 



The wood circle lies within the cambium zone and surrounds 

 a central area of parenchyma cells, the pith. It is composed of 

 the xylem portions of numerous bundles which have grown 

 together into a solid ring of woody tissue. The wood fibres, in 

 cross section, are seen to be very thick-walled, compactly 

 arranged, more or less compressed laterally by mutual pressure, 

 so as to appear angular. They lie next to the cambium zone. 

 The cells are separated by a distinct line, the middle lamella. 

 The walls are lignified, as shown by the red stain, the middle 

 lamella being deeper red than the rest of the walls. There are 

 no intercellular spaces. The cells are unequal in size and 

 irregularly arranged. There are delicate stratification lines 

 and occasionally pore-canals in the walls, though these are seen 

 with difficulty. 



