1. Cross-section of elm bark (_Ulmus fulva_, Michaux) showing two cavities filled with partially swollen mucilage. 2. Mucilage mass from sassafras stem bark (_Sassafras variifolium_, L.). 3. Mucilage mass from elm bark. 4. Resin mass from white pine bark (_Pinus strobus_, L.).]
In many plants, however, the parenchyma cells contain crystals. The parenchyma cells of rhubarb contain rosette crystals, while the parenchyma cells of the cortex of sarsaparilla and false unicorn root contain bundles of raphides. In every case observed the raphides are surrounded by mucilage. This is true of squills, sarsaparilla, false unicorn, etc. When cells with raphides and mucilage are mounted in a mixture of alcohol, glycerine, and water, the mucilage first swells and finally disappears.
Particular attention should be given to =storage cavities= whenever they occur in plants, for the reason that they are usually filled with storage products, and for the added reason that storage cavities are not common to all plants. Storage cavities occur in roots, stems, leaves, flowers, fruits, and seeds.
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).
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.
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.