The yellow stone cells of calumba root (Plate 65, Fig. 2) usually contain four prisms of calcium oxalate, which may be nearly uniform or very unequal in size.
BAST FIBRES
The bast fibres of the different rubus species (Plate 65, Fig. 7) contain starch. The medullary rays of quassia (Plate 107, Fig. 2) contain starch; while the medullary rays of canella alba contain rosette crystals. In a cross-section of canella alba (Plate 81, Fig. 3) the crystals form parallel radiating lines which, upon closer examination, are seen to be medullary rays, in each cell of which a crystal usually occurs.
The epidermal and hypodermal cells of leaves serve as water-storage tissue. These cells usually appear empty in a section.
The barks of many plants—i.e., quebracho, witch-hazel, cascara, frangula, the leaves of senna and coca, and the root of licorice—contain numerous crystals. These crystals occur in special storage cells—crystal cells (Plate 65, Fig. 6)—which usually form a completely enveloping layer around the bast fibres. These cells are usually the smallest cells of the plant in which they occur, and with but few exceptions each cell contains but a single crystal.
The epidermal cells of senna leaves and the epidermal cells of mustard are filled with mucilage; the walls even consist of mucilage. Such cells are always diagnostic in powders.
STORAGE WALLS
Storage walls (Plates 68 and 69) occur in colchicum seed, saw palmetto seed, areca nut, nux vomica, and Saint Ignatius’s bean. In each of these seeds the walls are strongly and characteristically thickened and pitted. In no two plants are they alike, and in each plant they are important diagnostic characters.
Storage cell walls consist of reserve cellulose, a form of cellulose which is rendered soluble by ferments, and utilized as food during the growth of the seed. Reserve cellulose is hard, bony, and of a waxy lustre when dry. Upon boiling in water the walls swell and become soft.
The structure of the reserve cellulose varies greatly in the different seeds in which it occurs in the thickness of the walls and in the number and character of the pores.