STORAGE TISSUE 179 
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—z.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 char- 
acteristically 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. 
