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.