In white oak, cascara sagrada and prickly ash are groups of stone cells; in the cinnamon barks are bands of stone cells; in cinchona bark are isolated stone cells. In cramp bark, mezerum, elm, and white pine bark no stone cells occur. In frangula, cascara sagrada, cocillina, cinnamon, cinchona, sassafras, and wild cherry barks the bast fibres occur in groups. In frangula, cascara sagrada, and cocillina the bast fibres are surrounded by crystal cells with crystals. In sassafras bark mucilage cells occur. In canella alba, white pine, and sassafras barks secretion cells occur; but in most barks no secretion cells occur. In sassafras bark the medullary ray cells are nearly as broad as long; in cramp bark they are elongated and oval in shape. In cascara sagrada, as in most barks, the cells are longer than broad and rectangular in shape. In cascara sagrada the sieve cells are very large; in granatum bark the sieve cells are very small. In cassia cinnamon and in canella alba bark the walls of the sieve cells have collapsed, with the result that the sieve cells have become partly obliterated. In witch-hazel, mountain maple, willow, and black walnut are found prisms; in cramp bark, black haw, wahoo, pomegranate, and cotton root bark are found rosette crystals; in the cinnamon barks are found raphides; in cinchona bark, micro-crystals. In cocillina, frangula, cascara sagrada, white oak, poplar and Jamaica dogwood barks are found crystal-bearing fibres (Plates 19 and 20). When studying barks we must consider the kind, structure, and amount of the periderm; the nature of the phellogen; the nature and amount of the cortical parenchyma; the occurrence, distribution, and amount of stone cells, when present; the occurrence and structure of the bast fibres; the presence or absence of secretion cells; the width, distribution, and structure of the medullary rays. CHAPTER V WOODS Quite a number of drugs consist of the =wood= of woody plants; such drugs are quassia, red saunders, white sandalwood, and guaiac. When studying woods it is necessary to observe the cross, tangential, and radial sections. Such sections of quassia are shown in Plates 105, 106, and 107. When studying these sections it should be remembered that while the types of cells forming quassia wood are similar to the cells forming other woods, still their structure, arrangement, and amount will vary in a recognizable way in the different woods. CROSS-SECTION QUASSIA Plate 105 is a cross-section of quassia. It has the following structure: =Vessels.= The vessels occur singly or in groups of two to eight cells. The cells are variable in size and shape. The walls are yellowish white and porous. =Medullary Rays.= The medullary rays vary from one to five cells in width. The =medullary ray cells= are radially elongated and the walls are strongly porous.