252 HISTOLOGY OF MEDICINAL PLANTS 



In canella alba bark the periderm is composed of stone- 

 cell cork or stone cells arranged in superimposed rpws, 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 arc found rosette crystals; in 



