HORSETAILS. 
245 
by maceration, tbe 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 
interesting observations of Dr. Brewster, on the micro¬ 
scopic structure of this siliceous coating in E. Jiyemale, 
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 
straw 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 
