EQUISETUM. 
253 
in number, equalling the ribs on the sheath. These fertile 
stems are very slightly striated. 
The barren stems grow erect to the height of eighteen 
inches or more, and have their surface disposed in about 
twenty sharp ridges, with corresponding furrows, the ridges 
being coated with prominent siliceous warty particles, so 
that the stems become very rough. The few lower joints 
are without branches, but in all the upper part of the stem 
they produce whorls of from ten to sixteen branches, which 
are simple, and at first drooping, but eventually take a 
spreading or slightly ascending direction. The sheaths of 
these barren stems are much smaller than those of the 
fertile, less funnel-shaped, and more closely set to the stem, 
and their teeth are also fewer, shorter and blunter; but 
in respect of colour they do not materially differ. The 
branches, which are slender, and about four inches long, are 
three or four-ribbed, and have loose sheaths, which termi¬ 
nate in three or four short, acute, membranous-edged, 
faintly brown-tipped teeth; the ribs of the stem extend 
upwards into the teeth, one entering each, but they do not 
quite reach the apex. 
The fructification forms a moderate-sized, terminal, oval, 
cone-like head ; at first sessile in the uppermost sheath, but 
