The pores are slanting or strongly oblique (Plate 28, Fig. 2), and they show for their entire length on the broadest part of the wall--_i.e._, the upper or the lower surface--while in the side wall they are oblique; but they are not so distinct as they are on the broad part of the wall. Frequently the pores appear crossed when the upper and the lower wall are in focus, because the pores are spirally arranged, and the pore on the under wall throws a shadow across the pore on the upper wall, or _vice versa_. Wood fibres always occur in a broken condition (Plate 28, Fig. 1) in powdered drugs. These broken fibres usually occur both singly and in groups in a given powder. The color of wood fibres varies greatly in the different medicinal woods. Fragments of wood are usually adhering to witch-hazel, black haw, and other medicinal barks. In each of these cases the wood fibres are nearly colorless. In barberry bark adhering fragments of wood and the individual fibres are greenish-yellow. The wood fibres of santalum album are whitish-brown; of quassia, whitish-yellow; of logwood and santalum rubrum, red. [Illustration: PLATE 28 WOOD FIBRES 1. White sandalwood (_Santalum album_, L.). 2. Quassia wood (_Picræna excelsa_, [Swartz] Lindl.). 3. Logwood with crystals (_Hæmatoxylon campechianum_, L.). 4. Black haw root (_Viburnum prunifolium_, L.).] Some wood fibres function as storage cells. In quassia the wood fibres frequently contain storage starch. The wood fibres of logwood and red saunders contain coloring substances, which are partially in the cell cavity and partially in the cell wall. The walls of wood are composed largely of lignin. COLLENCHYMA CELLS =Collenchyma cells= form the principal medicinal tissue of stems of herbs, petioles of leaves, etc. In certain herbs the collenchyma forms several of the outer layers of the cortex of the stem. In motherwort, horehound, and in catnip the collenchyma cells occur chiefly at the angles of the stem. In motherwort (Plate 29, Fig. B) there are twelve bundles, one large bundle at each of the four angles, and two small bundles, one on either side of the large bundle. In catnip (Plate 29, Fig. A) there are four large masses, one at each angle of the stem.