252 HISTOLOGY OF MEDICINAL PLANTS 
In canella alba bark the periderm is composed of stone- 
cell cork or stone cells arranged in superimposed rows, which 
form the outer layers of the bark. 
In white oak and most barks from woody trees the periderm 
consists of lifeless parenchyma, medullary rays, sieve cells, 
bast fibres, and in some cases stone cells and of phellogen 
cells. 
In young wild cherry, cascara sagrada, and frangula are 
several layers of tangentially elongated collenchyma cells with 
chlorophyll. In the older barks of the above and in many 
other barks no collenchyma cells occur. 
In cramp bark and in tulip tree bark the outer layers of the 
cortical parenchyma cells are beaded. In most barks there is. 
no beaded walled parenchyma. The outer layers of most 
cortical parenchyma cells are tangentially elongated while the 
inner parenchyma cells are mostly circular in outline. 
In white oak, cascara sagrada and prickly ash are groups 
of stone cells; in the cinnamon barks are bands of stone cells; 
in cinchona bark are isolated stone cells. In cramp bark, 
mezerum, elm, and white pine bark no stone cells occur. 
In frangula, cascara sagrada, cocillina, cinnamon, cinchona, 
sassafras, and wild cherry barks the bast fibres occur in groups. 
In frangula, cascara sagrada, and cocillina the bast fibres are 
surrounded by crystal cells with crystals. 
In sassafras bark mucilage cells occur. In canella alba, 
white pine, and sassafras barks secretion cells occur; but in 
most barks no secretion cells occur. 
In sassafras bark the medullary ray cells are nearly as broad 
as long; in cramp bark they are elongated and oval in shape. 
In cascara sagrada, as in most barks, the cells are longer than 
broad and rectangular in shape. 
In cascara sagrada the sieve cells are very large; in granatum 
bark the sieve cells are very small. 
In cassia cinnamon and in canella alba bark the walls of the 
sieve cells have collapsed, with the result that the sieve cells. 
have become partly obliterated. 
In witch-hazel, mountain maple, willow, and black walnut 
are found prisms; in cramp bark, black haw, wahoo, pome- 
granate, and cotton root bark are found rosette crystals; in 
