THE HORSETAIL. 45 



ridges, which give so peculiar and distinctive a cha- 

 racter to the whole stem ; and so accurately is every 

 cell fitted to the others, that if the plant be treated 

 with nitric acid, the flinty skeleton will be found to 

 remain entire ; indeed, it is said that by a careful 

 maceration in water, a similar result may be ob- 

 tained, and an object of most wonderful microscopic 

 beauty be thus produced. Each individual fragment 

 as has been shewn by Sir David Brewster, is pos- 

 sessed of an " axis of double refraction/' It will be 

 seen, under the microscope, that the silicified cells 

 form a coat over those longitudinal ridges, which 

 cover the surface of the whole plant, giving it that 

 unpleasant roughness to the touch which must be 

 familiar to every one. In the depressions lying 

 between the ridges are situated a large number 

 of stomata, so that the whole anatomy is one of 

 peculiar interest. The proportions of silica in the 

 ashes of several different species of the Equise- 

 tums, are thus given by Professor Balfour : 



Ashes. Silica. 



E. arvense . . . 13'84 . . 6'38 



E. limosum . . . 15'50 . . 6'50 



E. hyemale . . .11'81. . 875 



E. telmateia . . . 23'61 . . 12*00* 



an analysis, which at a glance shews us, that of 

 our British species the E. hyemale is certainly that 

 best adapted to the purposes of the polisher, whether 

 or not it be identical with the " Dutch rushes/' 

 This prevalence of silicic acid in the Equisetums, is 

 apparently the result of a combination of a silicate 



* "Manual of Botany." 



