EQUISETUM. 
275 
underground stems. It has been suggested that our own 
sandy sea-coasts might be profitably planted with it. 
The peculiarity which gives it its commercial value, is the 
presence of a very hard coating of silex, which is deposited 
in the form of little crystals, rendering the surface rough 
like a rasp or file, and hence not only woods, but metals 
and stones may be polished by it. This siliceous coating 
is so entire, and of such density, that it is stated the whole 
of the vegetable matter may be removed by maceration, 
or, according to others, by burning, without destroying the 
form of the plant. The minute crystals of silex, of which 
the flinty coating consists, are arranged with a degree of 
regularity which, under a microscope, has a very beautiful 
appearance; they form a series of longitudinal elevated 
points, and in the furrows between them are -cup-shaped 
depressions, at the bottom of each of which is placed a 
stomate or pore. 
All the species of Equisetum have a flinty coating to 
their stems, and may be, and are, more or less employed 
in polishing ; but the stems of the E. hyemale are much 
preferable to those of the other kinds, in consequence of 
their rougher and more hardened surface. 
