TISSUES AND SIMPLE ORGANS. 113 



of thick-walled supporting layers. Suberized walls without thick- 

 ening very frequently assist the mechanical function. In the first 

 place it is a mistake to suppose that completely suberized cell-walls 

 are in general very extensible; the thin-walled corky layer of 

 birch-bark proves the contrary. (Actual tests in regard to the 

 elasticity of suberized protective sheath-cells are wanting.) In the 

 second place we may ascribe considerable absolute firmness to the 

 suberized cell- walls. (Based upon actual observation.) 



The relative impermeability of suberized cell-walls to water- 

 solutions and water-vapors is known from what has already been 

 stated. 



It will not be difficult to illustrate what has been said of these 

 anatomical relations by studying a dicotyledonous and a monocoty- 

 ledonous root. In passing, it may be recalled that the above 

 mechanical function is immediately related to the utilization of 

 thick- walled cells for the purpose of " local protection," as was 

 mentioned in the discussion of the mechanical septum. 



The root of A Ilium ascalonicum (monocotyledon) (Fig. 62) 



Fig. 62. Central vascular system of the root of Allium ascalonicum. 



g, Large vessel; s, bundle-sheath with passage-cells (transit-cells), d; p, pericambium. (After 



Haberlandt.) 



shows the protective sheath between the parenchyma and pericam- 

 bium. The thick- walled and the thin- walled cells of the protective 



