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 — 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 rilled 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. 



