_A._ Cross-section of a portion of canella alba bark (_Canella alba_, Murr.). 1. Excretion cavity. _B._ Cross-section of a portion of klip buchu leaf. 1. Epidermal cells. 2. Secretion cavity. 3. Secretion cells.] [Illustration: PLATE 64 _A._ Cross-section of bitter orange peel (_Citrus aurantium_, _amara_, L.). 1, Internal secretion cavity formed by the dissolution of the walls of the central secreting cells; 2, Secretion cells. _B._ Cross-section of white pine leaf (_Pinus strobus_, L.). 1, Epidermal and hypodermal cells; 2, Parenchyma cells with protruding inner walls; 3, Endodermis; 4, Secretion cavity; 5, Secretion cells.] In calamus (Plate 62, Fig. A) the cavity is larger than the surrounding cells; it is rounded in outline, and it contains oleoresin. These cavities are in contact with the ordinary parenchyma cells, from which they are easily distinguished by their larger size and rounded form. The =unicellular oil cavity= of canella alba (Plate 63, Fig. A) is rounded or oval in cross-section and is many times larger than the surrounding cells. The wall, which is very thick, is of a yellowish color. Secretion cavities vary greatly in form, according to the part of the plant in which they are found. In flower petals and leaves they are spherical; in barks they are usually elliptical; in umbelliferous fruits they are elongated and tube-like. Mucilage cavities are not of common occurrence in medicinal plants. They occur, however, in the stem and root bark of sassafras, the stem bark of slippery elm, the root of althea, etc. CHAPTER VIII STORAGE TISSUE Most drug plants contain storage products because they are collected at a period of the year when the plant is storing, or has stored, reserve products. These products are stored in a number of characteristic ways and in different types of tissue. The most important of the different types of storage tissue that occurs in plants are the storage cells, the storage cavities, and the storage walls. STORAGE CELLS Several different types of cells function as storage tissue. These cells, which are given in the order of their importance, are parenchyma, crystal cells, medullary rays, stone cells, wood fibres, bast fibres, and epidermal and hypodermal cells. CORTICAL PARENCHYMA =Cortical parenchyma= of biennial rhizomes, bulbs, roots, and the parenchyma of the endosperm of seeds store most of the reserve economic food products of the higher plants. =Pith parenchyma= of sarsaparilla root (Plate 65, Fig. 4) and the pith parenchyma of the rhizome of memspermun, like the pith parenchyma of most plants, function as storage cells. WOOD PARENCHYMA