PTERIDOPHYTA. 23 



6. Equisetum fluviatile L. (I. F. f. 82.) Stems annual, all alike, 6-12 dm. 

 high, 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 other- 

 wise much like the stems, short or elongated ; rootstocks hollow. In swamps and 

 along the borders of ponds, N. S. to Alaska, south to Va., Neb. and Wash. Also in 

 Europe and Asia. 



7. Equisetum robiistum A. Br. (I. F. f. 83.) Stems perennial, stout, 

 tall, evergreen, 1-2.5 m - high? sometimes 2 cm. in diameter, 20-48 -furrowed, 

 simple or little branched. Ridges roughened with a single series of transversely 

 oblong siliceous tubercles ; sheaths short, 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, O to La. and Mex., west to Br. Col. and Cal. Also 

 in Asia. May-June. 



8. Equisetum hyemale L. Scouring-rush. (I. F. f. 84.) Stems slender, 

 rather stiff, evergreen, 6-12 dm. high, with the stomata arranged in rows, rough, 

 8-34-fur rowed, 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 N. Am., Europe and Asia. 



9. Equisetum laevigatum A. Br. (I. F. f. 85.) Stems 3-15 dm. high, 

 simple or little branched, pale green, persistent, 14-30 furrowed, the ridges almost 

 smooth. Sheaths elongated and enlarged upward, 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 R. in N. J. and E. Perm, to N. Car. and La., west to Br. 

 Col. and the Mexican border. 



10. Equisetum variegatum Schleich. (I. F. f. &6.) Stems slender, peren- 

 nial, evergreen, 1.5-4.5 dm. long, usually simple from a branched base, tufted, 

 5-10-furrowed, the stomata borne in regular rows. Sheaths companulate, dis- 

 tinctly 4-carinate, variegated with black above, the median furrow deep and 

 excurrent to the teeth and downward to the ridges of the stem, the teeth 5-10, each 

 tipped with a deciduous bristle ; central cavity small, rarely wanting. Lab. and 

 Greenland to N. Terr., south to N. H., W. N. Y., Neb. and Nev. Also in Europe 

 and Asia. May-June. 



11. Equisetum scirpoides Michx. (I. F. f. 87.) Stems perennial, ever- 

 green, filiform, 4-15 cm. long, somewhat rough, flexuous and curving, growing in 

 slender tufts, mostly 6-furrowed with acute ridges, simple or branching from near 

 the base. Sheaths 3 -toothed, distinctly 4-carinate, the central furrow broad, the 

 lateral narrow, the bristly teeth rather persistent ; central cavity entirely wanting. 

 On moist or wet wooded banks. Lab. to Alaska, south to Penn., 111. and Br. Col. 

 Also in Europe and A^ia. 



Orders LYCOPODIALES. 



Spores produced in sporanges, which are borne in the axils of scale- 

 like or elongated leaves. 



Spores all of one sort and size. Fam. 1. Lxcotodiaceae. 



Spores of two sizes. 



Leaves scale-like, 4-many-ranked, on branching stems. 



Fam. 2. Selaginellaceae. 

 Leaves elongated, clustered on a corm-like trunk ; aquatic or mud plants. 



Fam. 3. Isoetaceae, 



