38 EQUISETACEAE. 
7. Equisetum robistum A. Br. Stout Scouring-rush. 
Equisetum robustum A. Br.; Engelm. Amer. Journ, 
Sci. 46:88. 1844. 
Stems perennial, stout, tall, evergreen, 3°-11° 
high, sometimes nearly 1’ in diameter, 20-48-fur- 
rowed, simple or little branched. Ridges of the 
stem roughened with a single series of transversely 
oblong siliceous tubercles; sheaths short, cylin- 
dric, appressed, marked with black girdles at the 
base, and at the bases of the dark caducous 
tecth ; ridges of the sheath 3-carinate; branches 
when present occasionally fertile; spikes tipped with 
(Fig. 83.) 
a rigid point. 
In wet places, Ohio to Louisiana and Mexico, west 
to British Columbia and California. i 
May-June, 
Also in Asia. 
8. Equisetum hyemale L. Common 
Scouring-rush. (Fig. 84.) 
Equiselum hyemale I,. Sp. Pl. 1062. 1753. 
Stems slender, rather stiff, evergreen, 2°-4° high, 
with the stomata arranged in regular rows, rough, 
8-34-furrowed, the ridges with two indistinct lines 
of tubercles, the central cavity large, from one-half 
to two-thirds the diameter; sheaths rather long, 
cylindric, marked with one or two black girdles, 
their ridges obscurely 4-carinate; teeth brown, 
membranous, soon deciduous; spikes pointed ; 
stem rarely producing branches which are usually 
short and occasionally fertile ; forms are sometimes 
found with longer sterile branches. 
In wet places and on banks, especially along rivers 
and lakes, throughout nearly the whole of North 
June. 
g. Equisetum laevigatum A. Br. Smooth 
Scouring-rush. (Fig. 85. ) 
Equisetum laevigatum A. Br.; Engelm, Amer. Journ. 
Sci. 46:87. 1844. 
Stems 1°-5° high, simple or little branched, pale 
green, persistent, ‘14—30-furrowed, the ridges al- 
most smooth, Sheaths elongated and enlarged 
upward, marked with a black girdle at the base of 
the mostly deciduous, white-margined teeth and 
rarely also at their bases; ridges of the sheath with 
a faint central carina and sometimes with faint 
short lateral ones; stomata arranged in single 
series; central cavity very large, the wall of the 
stem very thin; spikes pointed. 
Along streams and rivers, especially in clay soil, 
valley of the Delaware River in New Jersey and eastern 
Pennsylvania to North Carolina and Louisiana, west to 
British Columbia and the Mexican border. May-June. 
America, Europe and Asia. : 
and related species are used for scouring floors. 
The rough stems of this 
