CHAPTER IV 
BARKS 
Barks are all obtained from dicotyledonous plants. In 
studying barks there should be ascertained the thickness, ar- 
rangement, form, structure, color, and cell contents of the cells 
occurring in the outer, middle, and inner barks. 
The outer bark includes the cork cells and the phellogen 
layer. The middle bark includes all the cells occurring between 
the phellogen layer and the beginning of the medullary rays. 
The inner bark includes the medullary ray cells and all cells 
associated, with them. The plan of structure of all barks is 
similar, but in each species of plant the structure of the bark’ 
is uniform and characteristic for the species. 
A great number of drugs consist of the bark of woody plants; 
for this reason the bark is considered in a separate chapter from 
the stem. 
WHITE PINE BARK 
The cross-section of white pine bark (Plate 103) has the 
following structure: 
Outer Bark. The periderm consists of several layers of 
reddish-brown cork cells (1) which are narrow, elongated, and 
with thin walls. 
Middle Bark. The cells forming the middle bark are paren- 
chyma and secretion cells. 
The parenchyma cells vary greatly in size, form, and thick- 
ness of the walls. The cells beneath the cork cells and around 
the secretion cells are tangentially elongated and oval in shape, 
while the other parenchyma cells are more irregular in shape. 
The secretion cells are arranged around the schizogenous 
secretion cavities. The cells are tangentially elongated, and the 
walls, which are slightly papillate, are white. 
Inner Bark. The cells forming the inner bark are medullary 
rays, parenchyma, sieve cells, and storage cavities. 
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