55 
of flint to give light when struck by steel ; or two 
pieces rubbed together produce sparks. This fact 
first occurred to me in 1798, and it led to experi- 
ments, by which I ascertained that siliceous earth 
existed generally 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 
vegetable tribes, similar to that performed in the 
animal kingdom by the shell of the crustaceous 
insects. 
Immediately beneath the epidermis is the paren- 
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, indeed, is that usually affected by the 
cellular membranes 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 
honeycomb. This arrangement, which has usually 
been ascribed to the skill and artifice of the bee, 
seems, as Dr. Wollaston has observed, to be merely 
the result of the mechanical laws which influence 
the pressure of cylinders composed of soft mate- 
rials, the nests of solitary bees being uniformly 
circular. 
The innermost part of the bark is constituted by 
the cortical layers , and their numbers vary with the 
age of the tree. On cutting the bark of a tree of 
several years standing, the productions of different 
periods may be distinctly seen, though the layer 
e 4 
