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 between the cells, and the walls are striated instead of being non-striated, as in the stems of horehound, motherwort, and catnip.
_A._ Diagrammatic sketch of the cross-section of catnip stem (_Nepeta cataria_, L.). 1. Collenchyma occurring at the four angles of the stem.
_B._ Diagrammatic sketch of the cross-section of motherwort stem (_Leonurus cardiaca_, L.). 1, 2, 3. Twelve masses of collenchyma tissue occurring at the four sides of the stem.]
1. Cross-section of a side column of the collenchyma of motherwort stem (_Leonurus cardiaca_, L.).
2. Cross-section of the collenchyma of horehound stem (_Marrubium vulgare_, L.).
3. Cross-section of the collenchyma of the outer angle of motherwort stem.]
Collenchyma cells retain their living contents at maturity. Many collenchyma cells, particularly of the outer layers of bark and the collenchyma of the stems of herbs, contain chlorophyll.
The walls of collenchyma consist of cellulose.
=Stone cells=, like bast fibres, are branched or non-branched. Each group is then separated into subgroups according to wall structure (whether striated, or pitted and striated, etc.), thickness of wall and of cell cavity, color of wall and of cell contents, absence of color and of cell contents, etc.
=Branched stone cells= occur in a number of drugs. In witch-hazel bark (Plate 31, Fig. 2) the walls are thick, white, and very porous. In some cells the branches are of equal length; in others they are unequal. In the tea-leaf (Plate 31, Fig. 1) the walls are yellowish white and finely porous. When the lower wall is brought in focus, it shows numerous circular pits. These pits represent the pores viewed from the end. The branches frequently branch or fork.
Branched stone cells also occur in coto bark, acer spicatum, star-anise, witch-hazel leaf, hemlock, and wild-cherry barks.
Non-branched stone cells are divided into two main groups, as follows:
1. Porous and striated stone cells, and, 2. Porous and non-striated stone cells.