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 between the cells, and the walls are striated instead of being non-striated, as in the stems of horehound, motherwort, and catnip. [Illustration: PLATE 29 _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.] [Illustration: PLATE 30 COLLENCHYMA CELLS 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 =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 =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.