HORSETAILS. 245 



by maceration, the form of the plant being preserved entire 

 in the flinty coating. It has been found that the ashes 

 contain half their weight of silica. We quote some very 

 interestino- observations of Dr. Brewster, on the micro- 

 scopic structure of this siliceous coating in E. hyemale, 

 first published by Dr. Greville — 



" On subjecting a portion of the cuticle to the analysis 

 of polarized light under a high magnifying power, Dr. 

 Brewster detected a beautiful arrangement of the siliceous 

 particles, which are distributed in two lines parallel to the 

 axis of the stem, and extending over the whole surface. 

 The greater number of the particles form simple straight 

 lines, but the rest are grouped into oval forms, connected 

 together like the jewels of a necklace by a chain of parti- 

 cles forming a sort of curvilinear quadrangle ; these rows 

 of oval combinations being arranged in pairs. Many of 

 those particles which form the straight lines do not exceed 

 the five-hundredth part of an inch in diameter. Dr. 

 Brewster also observed the remarkable fact, that each 

 particle has a regular axis of double refraction. In the 

 Btraw and chaff of wheat, barley, oats, and rye, he noticed 

 analogous phenomena ; but the particles were arranged in 

 a different manner, and displayed figures of singular 



