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toothed sheaths ; each joint in fact terminates above in one
of these sheaths, into which the base of the next joint fits.
The sheaths seem to represent abortive leaves. The fructification
consists of terminal cone-like heads, made up of
peltate, usually hexagonal scales, to the lower face of which
the spore-cases are attached in a series around the margin.
The stems consist chiefly of cellular matter, but towards
the circumference there is a layer of woody fibre, from
which a series of plates of a similar nature project towards
the centre. The centre, as already mentioned, presents a
hollow cavity; and between the outer and inner cuticle of
the cylinder-like stem, occur one or more circles of hollow
tubes, or air-cavities, differing in size and position, and in
fact affording, by their comparative size, number, and arrangement,
excellent auxiliary marks for the recognition of
the species. Around these cavities, especially towards the
exterior surface of the stem, occur numerous spiral vessels of
small size.
The cuticle abounds in siliceous particles secreted in the
form of more or less prominent little warts, which impart to
the surface a greater or less degree of roughness in proportion
to their prominence. In some species this deposit of siHceous
matter is so great, that it is said, the whole of the vegetable
substance may be destroyed by maceration, the form of the
plant being preserved entire in the flinty coating. I t has
been found that the ashes contain half their weight of silica.
Some very interesting observations of Dr. Brewster, on the
microscopic structure of this siliceous coating in E. hyemale,
first published by Dr. Greville, we may quote.
“ On subjecting a portion of the cuticle to the analysis of
polarized light under a high magnifying power,” writes Dr.
Greville, “ 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 particles 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 beauty.