Bast fibres are classified as follows: 1. =Crystal bearing.= 2. =Non-crystal bearing.=
The crystal-bearing fibres are divided into two classes: 1. =Of leaves.= 2. =Of barks.=
The non-crystal bearing are divided into: 1. =Branched.= 2. =Non-branched.=
The branched and non-branched are divided into four classes: 1. =Non-porous and non-striated.= 2. =Porous and non-striated.= 3. =Striated and non-porous.= 4. =Porous and striated.=
The =crystal-bearing fibres= are composed (1) of groups of fibres, (2) of crystal cells, and (3) of crystals. In these cases the groups of fibres are large, and they are frequently completely covered by crystal cells, which may or may not contain a crystal. The crystals found on the fibres from the different plants vary considerably in size and form. As a rule, the fibres when separated are free of crystal cells and crystals. This is so because the crystal cells are exterior to the fibres, and in separating the fibres during the milling process the crystal cells are broken down and removed from the fibres. It is common, therefore, to find isolated fibres and crystals associated with the crystal-bearing fibres. The fibres which are crystal-bearing may be striated or porous, etc.; but owing to the fact that the grouping of the fibres and crystals is so characteristic, little or no attention is paid to the structure of the individual fibres.
1. Frangula (_Rhamnus frangula_, L.). 2. Cascara sagrada (_Rhamnus purshiana_, D.C.). 3. Spanish licorice (_Glycyrrhiza glabra_, L.). 4. Witch-hazel bark (_Hamamelis virginiana_, L.).]
=Crystal-bearing fibres= occur in the barks of frangula (Plate 19, Fig. 1); cascara sagrada (Plate 19, Fig. 2); witch-hazel (Plate 19, Fig. 4); in cocillana (Plate 20, Fig. 1); in white oak (Plate 20, Fig. 2); in quebracho (Plate 20, Fig. 3); and in Spanish licorice root (Plate 19, Fig. 3).
The crystal-bearing fibres of leaves are always associated with vessels or tracheids and with cells with chlorophyl. The presence or absence of crystal-bearing fibres in leaves should always be noted. The crystal-bearing fibres of leaves are composed of fragments of conducting cells, fibres, crystal cells, and crystals. The crystal-bearing fibres of leaves occur in larger fragments than the other parts of the leaf, because the fibres are more resistant to powdering. Having observed that a leaf has crystal-bearing fibres, in order to identify the powder it is necessary to locate one of the other diagnostic elements of the leaf--as the papillæ of coca (Plate 21, Fig. 1), or the hair of senna (Plate 21, Fig. 3), or the vessels in eucalyptus (Plate 21, Fig. 2).