EQUISETUM. 
273 
aerial stems are of a deep glaucous green, and all alike in 
structure, those which bear fructification, differing in no 
other particular from those which do not. They grow 
upright, from two to three feet high, 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 ; they are cylindrical, tapering 
off at the apex, and marked on the thicker parts with 
from fourteen to twenty ridges, formed of a double row of 
elevated points, consisting of crystallized siliceous parti- 
cles ; hence the stems are very rough. In this species the 
sheaths fit closely around the stems, so that they are nearly 
cylindrical ; they are marked by the same number of ridges 
as the stem, but they are less prominent, and terminate in 
a series of black, membranous, bristle-shaped teeth, which 
soon fall off, and leave the margin crenated. The sheath 
immediately below the cone of fructification has, however, 
its teeth persistent, and it is somewhat funnel-shaped. 
The sheaths are at first pale green, with a black margin ; 
from this they change to be entirely black ; and finally 
they become whitish in the middle, leaving a narrow ring 
of black at the base and margin. 
In this species a section of the stem shows on the 
T 
