EQUISETUM. 
267 
deep furrows, the number being variable, from six to eight. 
The joints are invested with nearly cylindrical sheaths, 
Which are quite loose, being almost twice the diameter of 
the stem in the upper parts of the plant; the lower sheaths 
are smaller and rather more funnel-shaped. The sheaths 
terminate in as many acute wedge-shaped teeth as there 
are ridges on the stem ; they are pale-coloured, tipped with 
black or dark brown, and have membranous edges. 
The stems are usually, except at the base, furnished 
with whorls of numerous simple branches, the number of 
the branches generally corresponding with the furrows of 
the stem. These are slender, four or five-ribbed, and 
their sheaths set nearly close, and terminate in pale-brown 
lance-shaped teeth, having a membranous border. 
In this species, when a section of the stem is examined, 
it shows a series of prominent ridges on the outer face; 
just within these, and over against the furrows, occurs a 
circle of moderate-sized cavities; and alternating with 
these, and near the inner margin, is a series of much 
smaller circular cavities. The central cavity of the stem 
is comparatively very small, not very much larger than the 
series of openings near the outer surface. The resemblance 
is considerable between its section and that of E. arvense. 
