EQUISETACE^. (HORSETAIL FAMILY.) 445 



Class HI EQUISETIN^E. 



Plants with a hollow, elongated, grooved or striate, and 

 jointed stem, bearing at each node a whorl of narrow united 

 leaves *vhich form a close sheath. The branches, arising from 

 the axils of these leaves, are therefore in whorls. 



Order 97. EQUISETACE^J. (Horsetail Family.) 



Stems arising from subterranean rootstocks. Sterile leaves resem- 

 bling a toothed sheath at the joints; the fertile ones shield-shaped, 

 bearing sporangia on the under side, and forming a terminal spike or 

 cone. 



1. EQUISETUM, L. Hoesetail. Scopeing Rttsb. 



Stems simple or branched, the joints having closed ends : leaves of the 

 fruiting cone 5 to 7-angled, and sporangia hood-like : spores round, furnished 

 with two slender filaments attached by the middle and clavate at the free 

 ends : prothallus above ground, green, usually dioecious. 



* Stems of two kinds; the fertile (in spring) soft, pale or brownish; the sterile 



appearing later, herbaceous and very different; neither surviving the winter: 

 stomata scattered. 



1. E. arvense, L. Fertile stems 4 to 10 inches high, with loose and 

 usually distant about 8 to 12-toothed sheaths, remaining simple and soon perish- 

 ing: sterile stems slender, at length 1 to 2 feet high, 10 to 14-furrowed, pro- 

 ducing long and simple or sparingly branched 4-angular branches ; their teeth 4. 



— Across the continent, but more common eastward; also far northward. 

 The " Common Horsetail." 



2. E. pratense, Ehrh. Sterile and finally also fertile stems producing sim- 

 ple straight 3-angled branches : sheaths of the stem with ovate-lanceolate short 

 teeth, those of the branches 3-toothed : stems more slender and branches shorter 

 than in the last. — Colorado to Michigan and northward. 



* * Stems all alike, evergreen, mostly unbranched : fruit produced in summer: 



stomata in regular rows. 



— Stems tall and stout (1| to 6 feet high), mostly simple, evenly 15 to 40-grooved: 



sheaths appressed. 



3. E. lsevigatum, Braun. Stems l£ to 4 feet high, sometimes with 

 numerous branches ; the ridges convex, obtuse, smooth or minutely roughish with 

 minute tubercles : sheaths elongated, with a narrow black limb and about 22 

 linear-awl-shaped caducous teeth, l-keeled below. — From Colorado to Oregon, 

 and eastward to Illinois and Louisiana. 



