172 HISTOLOGY OF MEDICINAL PLANTS 
In calamus (Plate 62, Fig. A) the cavity is larger than the 
surrounding cells; it is rounded in outline, and it contains 
oleoresin. These cavities are in contact with the ordinary 
parenchyma cells, from which they are easily distinguished by 
their larger size and rounded form. 
The unicellular oil cavity of canella alba (Plate 63, Fig. A) 
is rounded or oval in cross-section and is many times larger 
than the surrounding cells. The wall, which is very thick, is 
of a yellowish color. 
Secretion cavities vary greatly in form, according to the 
part of the plant in which they are found. In flower petals and 
leaves they are spherical; in barks they are usually elliptical; 
in umbelliferous fruits they are elongated and tube-like. 
Mucilage cavities are not of common occurrence in medicinal 
plants. They occur, however, in the stem and root bark of 
sassafras, the stem bark of slippery elm, the root of althea, etc. 
