Genus i. HORSETAIL FAMILY. 41 



6. Equisetum fluviatile L. Swamp Horsetail. Fig. 94. 



Equisetum limosum L. Sp. PI. 1062. 1753. 



Equisetum limosum L. Sp. PI. 1062. 1753. 



Stems annual, all alike, 2°-4° high, slightly 10-30- 

 furrowed, very smooth, usually producing upright 

 branches after the spores are formed, the stomata 

 scattered. Sheaths appressed with about 18 dark 

 brown short acute rigid teeth, air cavities wanting 

 under the grooves, small under the ridges; central 

 cavity very large ; branches hollow, slender, smaller 

 but otherwise much like the stems, short or elon- 

 gated; rootstocks hollow. 



In swamps and along the borders of ponds, Nova 

 Scotia to Alaska, south to Virginia, Nebraska and 

 Washington. Also in Europe and Asia. Water Horse- 

 tail, Paddock-pipes. May-June. 



7. Equisetum robustum A. Br. Stout Scouring-rush. Fig. 95. 



Equisetum robustum A. Br. ; Engelm. Amer. Journ. 

 Sci. 46: 88. 1844. 



Equisetum hyemale robustum A. A. Eaton, Fern Bull. 

 n : 75- 1903- 



Stems perennial, stout, tall, evergreen, 3°-n° high, 

 sometimes nearly 1' in diameter, 20-48-furrowed, 

 simple or little branched. Ridges of the stem rough- 

 ened with a single series of transversely oblong 

 siliceous tubercles ; sheaths short, nearly as broad 

 as long, cylindric, appressed, marked with black 

 girdles at the base, and at the bases of the dark 

 caducous teeth; ridges of the sheath 3-carinate; 

 branches when present occasionally fertile ; spikes 

 tipped with a rigid point. 



In wet places, Ohio to Louisiana and Mexico, west 

 to British Columbia and California. Also in Asia. 

 May-June. 



8. Equisetum hyemale L. Common Scouring-rush. Fig. 96. 



Equisetum hyemale L. Sp. PI. 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, mem- 

 branous, 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 

 America, Europe and Asia. The rough stems of this 

 and related species are used for scouring floors. The 

 species consists of numerous races. Called also Horse- 

 pipe, Mare's-tail, Shave-grass, Shave-weed, Pewter- 

 wort, Rough Horsetail, Dutch-rush, Gun-bright. May- 

 June. 



