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and the plants having hollow stalks, it is of great use, 

 and is exceedingly strong, and in the microscope seems 

 composed of a kind of glassy net- work, which is prin- 

 cipally siliceous earth. 



This is the case in wheat, in the oat, in different 

 species of equisetum, and, above all, in the rattan, the 

 epidermis of which contains a sufficient quantity of 

 flint to give light when struck by steel ; or two pieces 

 rubbed together produce sparks. This fact first oc- 

 curred to me in 1798, and it led to experiments, by 

 which I ascertained that siliceous earth existed gener- 

 ally in the epidermis of the hollow plants. 



The siliceous epidermis serves as a support, pro- 

 tects the bark from the action of insects, and seems 

 to perform a part in the economy of these feeble ve- 

 getable tribes, similar to that performed in the animal 

 kingdom by the shell of the crustaceous insects. 



Immediately beneath the epidermis is the faren- 

 chyma. It is a soft substance consisting of cells filled 

 with fluid, having almost always a greenish tint. The 

 cells in the parenchymatous part, when examined by 

 the microscope, appear hexagonal. This form, in- 

 deed, is that usually affected by the cellular mem- 

 branes in vegetables, and it seems to be the result of 

 the general re-action of the solid parts, similar to that 

 which takes place in the honey-comb. This arrange- 

 ment, which has usually been ascribed to the skill and 

 artifice of the bee, seems, as Dr. Wollaston has ob- 

 served, to be merely the result of the mechanical laws 

 which influence the pressure of cylinders composed of 

 soft materials, the nests of solitary bees being uni- 

 formly circular. 



