106 HISTOLOGY OF MEDICINAL PLANTS 
wood fibres frequently contain storage starch. The wood fibres 
of logwood and red saunders contain coloring substances, which 
are partially in the cell cavity and partially in the cell wall. 
The walls of wood are composed largely of lignin. 
COLLENCHYMA CELLS 
Collenchyma cells form the principal medicinal tissue of 
stems of herbs, petioles of leaves, etc. In certain herbs the 
collenchyma forms several of the outer layers of the cortex of 
the stem. In motherwort, horehound, and in catnip the col- 
lenchyma cells occur chiefly at the angles of the stem. In 
motherwort (Plate 29, Fig. B) there are twelve bundles, one 
large bundle at each of the four angles, and two small bundles, 
one on either side of the large bundle. In catnip (Plate 29, 
Fig. A) there are four large masses, one at each angle of 
the stem. 
Collenchyma cells differ from parenchyma cells in a number 
of ways: first, the cell cavity is smaller; secondly, the walls 
are thicker, the greater amount of thickening being at the angles 
of the cells—that is, the part of the cell wall which is opposite 
the usual intercellular space of parenchyma cells, while the wall 
common to two adjoining cells usually remains unthickened. 
In horehound stem (Plate 30, Fig. 2) the thickening is so great 
at the angles that no intercellular space remains. In the side 
column of motherwort stem (Plate 30, Fig. 1) the thickening 
between the cells has taken place to such an extent that the 
cell cavities become greatly separated and arranged in parallel 
concentric rows. 
The collenchyma of the outer angle of motherwort stem 
(Plate 30, Fig. 3) is greatly thickened at the angles. There 
are no intercellular spaces between the cells, and cell cavity 
is usually angled in outline instead of circular, as in the cells 
of horehound. In certain plants intercellular spaces occur be- 
tween the cells, and the walls are striated instead of being non- 
striated, as in the stems of horehound, motherwort, and 
catnip. 
Collenchyma cells retain their living contents at maturity. 
Many collenchyma cells, particularly of the outer layers of 
