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.
PLATE 62