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). =Branched bast fibres= occur in only a few of the medicinal plants, notable examples being tonga root and sassafras root. Occasionally one is found in mezereum bark. The bast fibre of tonga root (Plate 22, Fig. 2) often has seven branches, but four- and five-branched forms are more common. The walls are non-porous, non-striated, and nearly white. The bast fibre of sassafras (Plate 22, Fig. 1) has thick, non-porous, and non-striated walls, and the branching occurs usually at one end only of the fibre. Most of the bast fibres of sassafras root are non-branched. POROUS AND STRIATED BAST FIBRES =Porous and striated= walled bast fibres occur in blackberry bark of root, wild-cherry bark, and in cinchona bark. The fibres of blackberry root bark (Plate 23, Fig. 1) have distinctly porous and striated walls; the cavity, which is usually greater than the diameter of the wall, contains starch. These fibres usually occur as fragments. In wild-cherry bark (Plate 23, Fig. 2) the fibre has short, thick, unequally thickened walls, which are porous and striated. Most of the fibres are unbroken. [Illustration: PLATE 20 CRYSTAL-BEARING FIBRES OF BARKS 1. Cocillana (_Guarea rusbyi_, [Britton] Rusby). 2. White oak (_Quercus alba_, L.) 3. Quebracho (_Aspidosperma quebracho-blanco_, Schlechtendal).] [Illustration: PLATE 21 CRYSTAL-BEARING FIBRES OF LEAVES 1. Coca leaf (_Erythroxylon coca_, Lam.). 2. Eucalyptus leaf (_Eucalyptus globulus_, Labill). 3. Senna leaf (_Cassia angustifolia_, Vahl.).] [Illustration: PLATE 22 BRANCHED BAST FIBRES 1. Sassafras root bark (_Sassafras variifolium_, [Salisb.] Kuntze). 2. Tonga root.] Yellow cinchona bark (Plate 23, Fig. 3) has very thick, prominently striated porous-walled fibres, with either blunt or pointed ends. The cavity is narrow, and the pores are simple or branched. POROUS AND NON-STRIATED BAST FIBRES =Porous and non-striated= bast fibres occur in marshmallow root and echinacea root.