CRYSTAL CAVITIES
Characteristic crystal cavities occur in many plants. Such a cavity containing a bundle of raphides is shown in the cross-section of skunk cabbage leaf (Plate 67).
SECRETION CAVITIES
In white pine bark there are a great number of secretion cavities which are partially or completely filled with oleoresin. In the cross-sections of white pine bark the secretion cavities are very conspicuous, and they vary greatly in size. This variation is due, first, to the age of the cavity, the more recently formed cavities being smaller; and secondly, to the nature of the section, which will be longer in longitudinal section, which will be through the length of the secretion cavity, and shorter on transverse section. Such a section shows the width of the secretion cavity.
Characteristic mucilage cavities occur in sassafras root, stem bark, elm bark (Plate 66, Fig. 1), marshmallow root, etc. These cavities form a conspicuous feature of the cross-section of these plants. The presence or absence of mucilage cavities in a bark should be carefully noted.
LATEX CAVITIES
The latex tube cavities are characteristic in the plants in which they occur. These cavities as explained under latex tubes are very irregular in outline.
PLATE 67
Cross-Section of Skunk-cabbage Leaf (Symplocarpus fœtidus,
[L.] Nutt.)
1. Crystal cavity.
2. Bundle of raphides.