In each of the above fibres the length and width of the fibre are shown. The fibres also have pores of variable length. Such a variation is common to most fibres with pores. That part of the wall immediately over or below the cell cavity shows the end view or diameter of the pore, as in the fibre of marshmallow root (Plate 24, Fig. 3). As a rule, however, the pores show indistinctly on the upper and lower wall. OCCURRENCE IN POWDERED DRUGS In powdered drugs bast fibres occur singly or in groups. The individual fibres may be broken, as in mezereum and elm bark, or they may be entire, as in Ceylon cinnamon and in sassafras bark (Plate 26, Figs. 2 and 3). The lignified walls of bast fibres are colored red by a solution of phlorogucin and hydrochloric acid, and the walls are stained yellow by aniline chloride. In fact, few of the fibres found in individual plants occur in a broken condition. Isolated bast fibres are circular in outline. Bast fibres, when forming part of a bundle, have angled outlines when they are completely surrounded by other bast fibres; but when they occur on the outer part of the bundle, and when in contact with parenchyma or other cortical cells, they are partly angled and partly undulated in outline. In the bast fibres the pores are placed at right angles to the length of the fibre. The side walls show the length of the pore (Plate 24, Fig. 3); while the upper or lower wall shows the outline, which is circular, and the pore, which is very minute. Most bast fibres have no cell contents. In some cases, however, starch occurs, as in the bast fibres of rubus. The color of the bast fibres varies, being colorless, as in Ceylon cinnamon; or yellowish-white, as in echinacea; or bright yellow, as in bayberry bark. Bast fibres retain their living-cell contents until fully developed; then they die and function largely in a mechanical way. The walls of bast fibres are composed of cellulose or of lignin. Most of the bast fibres occurring in the medicinal plants give a strong lignin reaction. WOOD FIBRES =Wood fibres= always occur in cross-sections associated with vessels and wood parenchyma, from which they are distinguished by their thicker walls, smaller diameter, and by the nature of the pores, which are usually oblique and fewer in number than the pores in the walls of wood parenchyma, and different in form from the pores of vessels. The wood fibre on cross-section (Plate 105, Fig. 4) shows an angled outline, except in the case of the fibres bordering the pith-parenchyma, etc., in which case they are rounded on their outer surface, but angled at the points in contact with other fibres. The pore of wood fibres is one of the main characteristics which enable one to distinguish the wood fibres from bast fibres.