quently opposite to tlie external ridges, occurs an annular
series of small circular cavities, which are placed near the
inner surface of the tube.
This plant is not, as far as we are aware, applied to any
use ; and the harshness of its stems renders it by no means
agreeable to cattle, although it often occurs abundantly
among their pasturage ; and in cultivated ground becomes
a troublesome weed.
E q u ise tum h y em a l e , Linnoeus. — The Great Eough
Horsetail. (Plate XX. fig. 1.)
The underground stems of this species of Horsetail are
branched, and creeping to a considerable extent ; they are
black, and furnished with whorls of branched, black, fibrous
roots. The aerial stems are in this species all alike in
structure, those which bear fructification differing in no
other particular from those which do not. They grow upright,
and are scarcely ever branched : when this does occur
a solitary branch is produced, and this protrudes from below
the base of one of the sheaths of the stem. Their colour is
a deep glaucous green.
These stems, which grow from two to tliree feet high, are
-cylindrical, tapering off at the apex, and marked on the
thicker parts with from fourteen to twenty ridges, formed