EQUISETUM. 275 



underoTOund stems. It has Ijcen sn!?-2;csted 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. hjemale are much 

 preferable to those of the other kinds, in consequence of 

 their rougher and more hardened surface. 



T 2 



