MECHANICAL TISSUES 109 



bark and the coUenchyma 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. i) 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 cells, and, 



2. Porous and non-striated stone cells. 



POROUS AND STRIATED STONE CELLS 



Porous and striated walled stone cells occur in ruelKa root, 

 winter's bark, bitter root, allspice, and aconite. These stone 

 cells are shown in Plate ^t,. Figs, i, 2, 3, 4, and 5. 



The stone cells of ruellia root (Plate 32, Fig. i) 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. 



