158 HISTOLOGY OF MEDICINAL PLANTS 
plants. In acer spicatium they are brown; in witch-hazel they 
are gray; in xanthoxylium they are yellowish; and lastly, the 
number of lenticels occurring in a given surface of the bark 
should always be considered. 
On cross-sections the lenticel (Plate a7 Fig. 2) is seen to 
have a central depressed portion made up of loosely arranged 
cells. Bordering the cavity are typical cork cells. The cork 
cells immediately surrounding the lenticels are usually darker 
in color, and many of the cells are partly broken down. 
The size of lenticels will vary according to the type of the 
lenticel. In studying sections more attention should be paid 
to the character of the cells forming the lenticels than to the 
size of the lenticel. 
On cross-section the intercellular spaces (Plate 58) are tri- 
angular, quadrangular, or irregular. The spaces between equal 
diameter parenchyma cells is triangular if three cells surround - 
the space, and quadrangular if four cells surround the space, 
etc. These spaces are in direct contact with similar spaces that 
traverse the tissue at right angles to its long axis. 
The branched mesophyll cells of the leaf and aquatic plant 
parenchyma (Plate 59) are arranged around irregular cavities. 
In leaves and aquatic plants these spaces run parallel to the 
long axis of the organ. 
In each of the above cases the cavity is formed by the sepa- 
ration of the cell walls. There is still another type of irregular 
cavities which is formed by the dissolution or tearing apart of 
the cell walls. Such cavities are found in the stems and roots 
of many herbs. 
The pith cells in the stems of many herbs become torn 
apart during the growth of the stem, with the result that large 
irregular cavities are formed. These cavities are usually filled 
with circulatory air. 
In the stems of conium, cicuta, angelica, and other larger 
herbaceous stems the pith separates into layers. When a 
longitudinal section is made of such a stem it is seen to be com- 
posed of alternating air spaces and masses of pith parenchyma. 
The intercellular spaces are very large in leaves where 
enormous quantities of carbon dioxide are vitalized in photo- 
synthesis. 
