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 fonned 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. 



