26 EQUISETACEAE 



shaped, containing a single megaspore at the base and a few minute bodies above it, 

 the larger globose, producing many pedicelled sporanges, each containing several 

 masses of microspores. 



1. Azolla Carolintena Willd. Plants greenish, 2-4 mm. broad, deltoid or 

 triangular-ovate, pinnately branching: leaves with ovate lobes, their color varying 

 with the amount of sunlight, the lower usually reddish, the upper green with a red- 

 dish border: megaspores minutely granulate, with three accessory bodies; masses of 

 microspores armed with rigid septate processes. 



Floating on still water, Ontario to British Columbia, south to Florida and Arizona. 



Order 4. EQUISETALES. 



Rush-like perennial plants, with horizontal rootstocks and mostly hollow 

 jointed simple or often much-branched grooved stems, provided with a double 

 series of cavities and usually with a large central one, the branches whorled, the 

 nodes provided with diaphragms. Leaves reduced to toothed sheaths at the 

 joints. Sporanges 1-celled, clustered underneath the scales of terminal cone- 

 like spikes. Spores uniform, furnished with 2 narrow appendages (elaters) at- 

 tached at the middle, coiling around the spores when moist, and spreading, when 

 dry, in diverse ways. Prothallia terrestrial, green, usually dioecious. 



FAMILY 1. EQUISETACEAE Michx. HORSETAIL FAMILY. 

 Characters of the order. 



1. EQUISETUM L. 



The only genus. 



Stems annual of 2 kinds; fertile vernal, simple, soon withering; sterile much branched: stomata 



scattered. 1. E. arvense. 



Stems perennial: spikes rigid-pointed: stomata in regular rows. 

 Stems rough and tuberculate, prominently ridged. 



Stems stout: ridges with 1 line of tubercles: sheaths with 3-keeled ridges. 2. E. robustum. 

 Stems slender: ridges with 2 indistinct lines of tubercles: sheaths with ob- 

 scurely 4-keeled ridges. 3. E. hyemale. 

 Stems not tuberculate: sheaths enlarged upward. 4. E. laevigatum. 



1. Equisetum arvense L. Stems annual, with scattered stomata, the fertile 

 appearing in early spring before the sterile. Fertile stems 1-2.5 dm. high, not 

 branched, soon withering, light brown, their loose scarious sheaths mostly distant, 

 whitish, ending in about 12 brown acuminate teeth; sterile stems green, rather 

 slender, 0.5-6 dm. high, 6-19-furrowed, with numerous long mostly simple whorled 

 4-angled or rarely 3-angled solid branches, their sheaths 4-toothed, the stomata in 

 two rows in the furrows. 



In sandy soil, Newfoundland, Greenland and Alaska, south to North Carolina and Cali- 

 fornia. Also in Europe and Asia. E. arvense ser6tinum is an occasional form with a cone 

 terminating the normally sterile plant. Spring. HOESETAIL. 



2. Equisetum robustum A. Br. 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, cylindrie, 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 British Columbia, south to Louisiana, California and Mexico. 

 Also in Asia. 



3. Equisetum hyemkle L. Stems slender, rather stiff, evergreen, 6-12 dm. 

 high, with the stomata arranged in 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, cylindrie, marked with one or two black girdles, their 

 ridges obscurely 4-keeled; teeth brown, membranous, soon deciduous: stem rarely 

 producing branches which are usually short and occasionally fertile. 



In wet places, especially on river banks, widely distributed in North America, Europe 

 and Asia. SCOURING RUSH. 



4. Equisetum laevigktum A. Br. Stems 3-15 dm. high, simple or little 

 branched, pale green, persistent, 14-3 O'-furr owed, the ridges almost smooth. Sheaths 



