168 HISTOLOGY OF MEDICINAL PLANTS 
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 schizogen- 
ously, but later become lysigenous owing to the dissolution of 
the outer layers of the secretion cells. 
SCHIZOGENOUS CAVITIES 
Schizogenous cavities occur in white pine bark (Plate 62, 
Fig. B). The cells lining the cavity are mostly tangentially elon- 
gated, 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. 
