MECHANICAL TISSUES 109 
bark and the collenchyma of the stems of herbs, contain 
chlorophyll. 
The walls of collenchyma consist of cellulose. 
STONE CELLS 
Stone cells, like bast fibres, are branched or non-branched. 
Each group is then separated into subgroups according to wall 
structure (whether striated, or pitted and striated, etc.), thick- 
ness of wall and of cell cavity, color of wall and of cell contents, 
absence of color and of cell contents, etc. 
BRANCHED STONE CELLS 
Branched stone cells occur in a number of drugs. In witch- 
hazel bark (Plate 31, Fig. 2) the walls are thick, white, and very 
porous. In some cells the branches are of equal length; in 
others they are unequal. In the tea-leaf (Plate 31, Fig. 1) the 
walls are yellowish white and finely porous. When the lower 
wall is brought in focus, it shows numerous circular pits. These 
pits represent the pores viewed from the end. The branches 
frequently branch or fork. 
Branched stone cells also occur in coto bark, acer spicatum, 
staranise, witch-hazel leaf, hemlock, and wild-cherry barks. 
Non-branched stone cells are divided into two main groups, 
as follows: 
1. Porous and striated stone ceils, and, 
2. Porous and non-striated stone cells. 
POROUS AND STRIATED STONE CELLS 
Porous and striated walled stone cells occur in ruellia root, 
winter’s bark, bitter root, allspice, and aconite. These stone 
cells are shown in Plate 33, Figs. 1, 2, 3, 4, and 5. 
The stone cells of ruellia root (Plate 32, Fig. 1) are greatly 
elongated, rectangular in form, with thick, white, strongly 
porous walls. The central cavity is narrow and is marked with 
prominent pores and striations. 
The stone cells of winter’s bark (Plate 32, Fig. 2) vary from 
elongated to nearly isodiametric. The pores are very large, 
