246 
HISTOHY OF BRITISH FERNS. 
beauty. From these data Dr. Brewster concludes that 
the crystalline portions of silex and other earths which 
are found in vegetable films are not foreign substances of 
accidental occurrence, but are integral parts of the plant 
itself, and probably perform some important function in 
the processes of vegetable life."' 
Beyond their employment in the arts, the Equisetums 
are of little importance in an economical point of view. 
They are useless as fodder, and exploded as physic, though 
they have had some reputed astringent virtues. The 
under- ground stems, however, contain in winter, when the 
plants are inactive, a considerable quantity of starch, and 
they may be occasionally eaten by animals. In the cells 
of these underground stems, during the month of October, 
the particles of starch may be seen in active motion, 
passing up one side and down the other, as is observed in 
the stems of Chara, Dr. Bindley mentions having often 
noticed this phenomenon in the stems of the great Water 
Horsetail. 
The Horsetails consist of the one genus Equisetum^ of 
which some nine or ten species are recognized as British. 
