EQUISETUM. 
213 
branched from a joint near the base, but there is nothing 
approaching to a whorl. The very scales of the cone are 
minutely striated. 
The most remarkable feature in this Horsetail is its 
extreme roughness, caused by 
the presence of closely-set 
rows of flint crystals, which 
beset it everywhere. So pre- 
valent are these, that when 
the vegetable part of the stem 
is destroyed the form remains 
in crystal. Dr. Greville thus 
writes of it : — “ On subjecting 
a portion of the cuticle to the 
analysis of polarized light un- 
der a high magnifying power, 
Dr. Brewster detected a beau- 
tiful arrangement of the sili- 
ceous particles, which are dis- 
tributed 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 ova 
forms, connected together, like the jewel of a neck- 
lace, by a chain of particles forming a sort of curvi- 
linear 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 phe- 
nomena, but the particles were arranged in a different 
