First, trichomes; secondly, reddish-brown and white-angled epidermal cells; thirdly, the long, white bast fibres. CHAPTER IV BARKS Barks are all obtained from dicotyledonous plants. In studying barks there should be ascertained the thickness, arrangement, form, structure, color, and cell contents of the cells occurring in the _outer_, _middle_, and _inner_ barks. The outer bark includes the cork cells and the phellogen layer. The middle bark includes all the cells occurring between the phellogen layer and the beginning of the medullary rays. The inner bark includes the medullary ray cells and all cells associated with them. The plan of structure of all barks is similar, but in each species of plant the structure of the bark is uniform and characteristic for the species. A great number of drugs consist of the bark of woody plants; for this reason the bark is considered in a separate chapter from the stem. WHITE PINE BARK The cross-section of white pine bark (Plate 103) has the following structure: =Outer Bark.= The periderm consists of several layers of reddish-brown cork cells (1) which are narrow, elongated, and with thin walls. =Middle Bark.= The cells forming the middle bark are parenchyma and secretion cells. The =parenchyma cells= vary greatly in size, form, and thickness of the walls. The cells beneath the cork cells and around the secretion cells are tangentially elongated and oval in shape, while the other parenchyma cells are more irregular in shape. The secretion cells are arranged around the schizogenous secretion cavities. The cells are tangentially elongated, and the walls, which are slightly papillate, are white. =Inner Bark.= The cells forming the inner bark are medullary rays, parenchyma, sieve cells, and storage cavities. [Illustration: PLATE 103 CROSS-SECTION OF UNROSSED WHITE PINE BARK (_Pinus strobus_, L.) 1. Cork cells of the epidermis. 2. Parenchyma cells filled with chlorophyl. 3. Intercellular space. 4. Secretion cavity with resin. 5. Secretion cells. 6, One or more circles of parenchyma filled with chlorophyl. 7. Parenchyma. 8. Medullary rays. 9. Sieve cells. 10. Storage cavities.] The =medullary rays= form wavy lines. The medullary ray cells are radially elongated, rectangular in shape, and they contain granular cell contents. The sieve cells are either square or rectangular in shape. The walls are thin and white. The storage cavities are either filled with starch or with prisms and tannin. POWDERED WHITE PINE BARK White pine bark (Plate 104) when powdered shows the following characteristic elements: