=Secretion cavities= are divided into three groups, according to the nature of the origin of the cavity: first, schizogenous cavities, which originate by a separation of the walls of the secretion cells; secondly, lysigenous cavities, which arise by the dissolution of the walls of centrally located secretion cells; and thirdly, schizo-lysigenous cavities, which originate schizogenously, but later become lysigenous owing to the dissolution of the outer layers of the secretion cells. [Illustration: PLATE 61 STALKED GLANDULAR HAIRS 1. Belladonna leaf (_Atropa belladonna_, L.). 2. Geranium stem (_Geranium maculatum_, L.). 3. Henbane leaf (_Hyoscyamus niger_, L.). 4. Tobacco leaf (_Nicotiana tabacum_, L.).] SCHIZOGENOUS CAVITIES =Schizogenous cavities= occur in white pine bark (Plate 62, Fig. B). The cells lining the cavity are mostly tangentially elongated, and the wall extends into the cavity in the form of a papillate projection. Immediately back from these cells are two or three layers of cells which resemble cortical parenchyma cells, except that they are smaller and their walls are thinner. In white pine bark there is a single layer of thin-walled cells lining the cavity. Immediately surrounding the secretion cells is a single layer of thick-walled fibrous cells. In klip buchu (Plate 63, Fig. B), as in white pine leaf (Plate 64, Fig. B), there is a single layer of thin-walled secretion cells which are surrounded on three sides with parenchyma cells and on the outer side by epidermal cells. LYSIGENOUS CAVITIES =Lysigenous cavities= occur on the rind of citrus fruits--bitter and sweet orange, lemon, grapefruit, lime, etc., and in the leaves of garden rue, etc. In bitter orange peel, (Plate 64, Fig. A) the cavity is very large, and the cells bordering the cavity are broken and partially dissolved. The entire cells back of these are white, thin-walled, tangentially elongated cells. There is a great variation in the size of these cavities, the smaller cavities being the recently formed cavities. SCHIZO-LYSIGENOUS CAVITIES =Schizo-lysigenous= cavities are formed in white pine bark and many other plants owing to the increase in diameter of the stem. In such cases the walls of the secreting cells break down. The resulting cavity resembles lysigenous cavities. =Unicellular secretion cavities= occur in ginger, aloe, calamus, and in canella alba barb. [Illustration: PLATE 62 _A._ Cross-section of calamus rhizome (_Acorus calamus_, L.). 1, Intercellular space; 2, Parenchyma cells; 3, Secretion cavity. _B._ Cross-section of white pine bark (_Pinus strobus_, L.). 1, Parenchyma; 2, Secretion cavity; 3, Secretion cells.] [Illustration: PLATE 63