MEMBRANE. 
11 
decomposition before being silicified. Two preparations 
are described in the Histological Catalogue of the 
Museum of the College of Surgeons, H 58-59, in both 
of which the membrane has been destroyed. In the 
one a series of oval holes, and in the other only an 
irregular hexagonal network, indicate the original porous 
type of the vessels. 
Membrane may also be thickened by a deposit so 
dense as nearly to resemble bone. We have examples 
of this in the stones of the plum, peach, cherry, &c., 
and more especially in a nut known as vegetable ivory. 
This hard material called sclerogen by Turpin may 
occur in a homogeneous form, or in concentric strata, 
but the deposit being rarely if ever sufficient to fill the 
cell entirely, leaves a central cavity with radiating canals 
or pores, which, upon section, resembles one of the 
lacunae of bone ; but in these sclerous vegetable tissues 
the radiating canals never pass beyond the walls of 
their proper cells, and never anastomose with those of 
neighbouring cells. The thickening of the walls of cells 
is well shown in the vertical and horizontal sections of 
one of the scales of the cone of Pinus Webbiana , 
represented in Fig. 4, G H, in both of which the pores 
radiating from the central cavity, and proceeding as 
far as the cell-wall, are distinctly seen ; in Fig. 4, d, 
is shown a large ligneous cell from the Snake- wood, 
in which the deposit has taken place in concentric 
strata. 
The specimens already described are instances of 
