=Wood Parenchyma.= The wood parenchyma cells have thin, yellowish-white, angled walls. =Wood Fibres.= The wood fibres have thick, yellowish-white, angled walls. These cells are smaller in diameter than the wood parenchyma cells. RADIAL SECTION QUASSIA The radial section of quassia (Plate 107) is as follows: =Vessels.= The vessels appear as in the tangential section. =Medullary rays.= The medullary rays vary from ten to twenty cells in height according to the part of the medullary ray bundle cut across. [Illustration: PLATE 105 CROSS-SECTION OF QUASSIA WOOD (_Picræna excelsa_ [Sw.], Lindl.) 1. Vessels. 2. Medullary rays. 3. Wood parenchyma. 4. Wood fibres.] [Illustration: PLATE 106 TANGENTIAL SECTION OF QUASSIA WOOD (_Picræna excelsa_ [Sw.], Lindl.) 1. Vessel. 2. Wood parenchyma. 3. Wood fibre. 4. End wall of medullary ray cell. 5. Medullary ray bundle.] [Illustration: PLATE 107 RADIAL SECTION OF QUASSIA WOOD (_Picræna excelsa_ [Sw.], Lindl.) 1. Showing the height and length of the medullary rays and cells. 2. Cells with starch. 3. Wood parenchyma and wood fibres.] The =medullary ray cells= exhibit their height and length. The walls of the cells are yellowish white and strongly porous. =Wood Parenchyma.= The wood parenchyma cells have yellowish, thin walls and blunt end walls. =Wood Fibres.= The wood fibres have thick, yellowish-white walls, and the end of the cells are strongly tapering. TANGENTIAL SECTION QUASSIA The tangential section of quassia (Plate 106) shows the following structure: =Vessels.= The vessels are very long and broad and the yellow walls are marked with clearly defined pits. =Medullary Rays.= The tangential section shows the cross-section of the medullary ray bundle and the cross-section of the medullary ray cell. The =medullary ray bundle= varies in width from one to five cells. The ends of the bundles are always one cell in width, while the central part of the bundle is frequently five cells in width. The =medullary ray cell= varies in size, structure, and shape according to the part of the cell cut across. The cells cut across the centre show hollow spaces, but the cells cut just above or below the end wall show a strongly pitted surface. The cells forming the end of the bundle are larger than the cells forming the centre of the bundle. =Wood Parenchyma.= The wood parenchyma cells are greatly elongated and the walls are thin and yellowish white. The ends of the cells are blunt. =Wood Fibres.= The wood fibres are elongated, the walls are thick and the cells are strongly tapering. In quassia, white sandalwood, red sandalwood, and guaiac wood are characteristic crystals. In quassia the vessels are finely pitted, yellowish, and distinct; in white sandalwood the vessels are coarsely and sparingly pitted and white translucent; in red saunders the vessels are coarsely pitted, bright red and distinct.