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= 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 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.
_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.]
_A._ Cross-section of a portion of canella alba bark (_Canella alba_, Murr.). 1. Excretion cavity. _B._ Cross-section of a portion of klip buchu leaf. 1. Epidermal cells. 2. Secretion cavity. 3. Secretion cells.]
_A._ Cross-section of bitter orange peel (_Citrus aurantium_, _amara_, L.). 1, Internal secretion cavity formed by the dissolution of the walls of the central secreting cells; 2, Secretion cells. _B._ Cross-section of white pine leaf (_Pinus strobus_, L.). 1, Epidermal and hypodermal cells; 2, Parenchyma cells with protruding inner walls; 3, Endodermis; 4, Secretion cavity; 5, Secretion cells.]
In calamus (Plate 62, Fig. A) the cavity is larger than the surrounding cells; it is rounded in outline, and it contains oleoresin. These cavities are in contact with the ordinary parenchyma cells, from which they are easily distinguished by their larger size and rounded form.
The =unicellular oil cavity= of canella alba (Plate 63, Fig. A) is rounded or oval in cross-section and is many times larger than the surrounding cells. The wall, which is very thick, is of a yellowish color.