56 
PHARMACEUTICAL BOTANY 
parenchyme cells in which they occur and giving these a grayish- 
black appearance. 
Crystal fibers are longitudinal rows of superimposed parenchyme 
cells each of which contains a single monoclinic prism or rosette 
aggregate. Crystal fibers are found adjacent to sclerenchyme fibers 
such as bast or woody fibers. 
Membrane crystals are monoclinic prisms, each of which is sur- 
rounded by a wall or membrane. In the process of formation a 
crystal first is formed in the cell sap and then numerous oil globules 
make their appearance in the protoplasm surrounding it; later some 
of the walls of the cell grow around the crystal and completely 
envelop it. 
13. Cystoliths. — Cystoliths are clustered bodies formed by the 
thickening of the cell wall at a certain point and subsequent in- 
growth which latter forms a cellulose skeleton consisting of a stalk 
and body. Silica is subsequently deposited on the stalk while 
calcium carbonate is piled up on the body in layers forming an irreg- 
ular spheroidal or ellipsoidal deposit. These structures are abun- 
dantly found in the plants of the Nettle Family and constitute a 
leading peculiarity of the same. 
Hair cystoliths differ from the average type in that they are 
devoid of a stalk. Such are seen in the non-glandular hairs of 
Cannabis saliva. 
The calcium carbonate incrustation of a cystolith dissolves with 
effervescence on the addition of a mineral or organic acid. 
14. Silica. — Silica (Si 0 2 ) occurs in a number of plants either as 
an incrustation in the cell wall as in Diatoms, the Equisetinea and 
Graminece or more rarely in the form “silica bodies” such as are 
found in certain Palms, Orchids and Tristicha. It is insoluble in 
all the acids except hydroflouric. It may be obtained in pure form 
by placing tissue containing it in a drop or two of concentrated 
sulphuric acid and after a time treating with successively stronger 
solutions of chromic acid (starting with 25 per cent.) and then wash- 
ing with water and alcohol. 
15. Tannins. Tannins are amorphous substances occurring in 
plants having an astringent taste, and turning dark blue or green 
with iron salts. They occur in greatest quantity in the bark of 
