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. [Illustration: PLATE 67 CROSS-SECTION OF SKUNK-CABBAGE LEAF (_Symplocarpus fœtidus_, [L.] Nutt.) 1. Crystal cavity. 2. Bundle of raphides.] OIL CAVITY Canella alba contains an =oil cavity= resembling in form the mucilage cavity of elm bark. =Secretion cavities= occur in most of the umbelliferous fruits. For each fruit there is a more or less constant number of cavities. Anise has twenty or more, fennel usually has six cavities, and parsley has six cavities. In poison hemlock fruits there are no secretion cavities. In certain cases, however, the number of secretion cavities can be made to vary. This was proved by the author in the case of celery seed. He found that cultivated celery seed, from which stalks are grown, contains six oil cavities (Plate 122, Fig. 2), while wild celery seed (Plate 102, Fig. 1), grown for its medicinal value, always contains more than six cavities. Most of the wild celery seeds contain twelve cavities. Many leaves contain cavities for storing secreted products. Such storage cavities occur in fragrant goldenrod, buchu, thyme, savary, etc. The leaves in which such cavities occur are designated as pellucid-punctate leaves. Such leaves will, when held between the eye and the source of light, exhibit numerous rounded translucent spots, or storage cavities. GLANDULAR HAIRS The =glandular hair of peppermint= (Plate 60, Fig. 3) and other mints consists of eight secretion cells, arranged around a central cavity and an outer wall which is free from the secretion cells. This outer wall becomes greatly distended when the secretion cells are active, and the space between the secretion cells and the wall serves as the storage place for the oil. When the mints are collected and dried, the oil remains in the storage cavity for a long time. STONE CELLS The =stone cells= of the different cinnamons (Plate 65, Fig. 1) store starch grains; these grains often completely fill the stone cells. The yellow stone cells of calumba root (Plate 65, Fig. 2) usually contain four prisms of calcium oxalate, which may be nearly uniform or very unequal in size. BAST FIBRES