One of the most important characters to be kept in mind in studying bast fibres is the structure of the wall. In fact, the author’s classification of bast fibres is based largely on wall structure. Such a classification is logical and accurate, because it is based upon permanent characters. Another character used in classifying bast fibres is the nature of the cell, whether branched or non-branched. In fact, this latter character is used to separate all bast fibres into two fundamental groups--namely, branched bast fibres and non-branched bast fibres. The third important character utilized in classifying fibres is the presence or absence of crystals. 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.= CRYSTAL-BEARING BAST FIBRES 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. [Illustration: PLATE 19 CRYSTAL-BEARING FIBRES OF BARKS 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).