EQUISETACEAE (HORSETAIL FAMILY) 51 



EQUISETACEAE (HORSETAIL FAMILY) 

 (REVISED BY A. A. EATON.) 



Rush-like, often branching plants, with jointed and mostly hollow stems from 

 running rootstocks, having sheaths at the joints, and, when fertile, terminated 

 by the conical or spike-like fructification composed of shield-shaped stalked 

 scales bearing the spore-cases beneath. A single genus. 



1. EQUISETUM [Tourn.] L. HORSETAIL 



Rootstocks perennial, jointed, branched, wide-creeping, dull and blackish, 

 felted or naked, often tuber-bearing, the nodes provided with toothed, often 

 felted sheaths ; roots in verticils from the nodes, annual, felted. Stems usually 

 erect, simple or branched, cylindrical, jointed, the surface regularly striated, 

 overlaid with teeth, dots, bands, rosettes, or a smooth coat of silex; the stomata 

 in the grooves in regular rows or broad bands ; the internodes (except in E. scir- 

 poides) bearing a large central air-cavity (centrum), a medium sized one (vallecu- 

 lar^nder each groove, with which the stomata connect, and a smaller one 

 (carinal) under each ridge. The nodes are closed and solid, each bearing a 

 whorl of reduced leaves joined by their edges into cylindrical sheaths, their tips 

 thinner and prolonged into persistent or deciduous teeth. Branches, when 

 present, mostly in whorls from the nodes. Fruit in a terminal cone formed of 

 regular verticils of stalked sporophylls, the 6 or 7 sporangia opening down the 

 inner side and discharging many loose green spores, each provided with four 

 elastic hygroscopic clavate bands. Prothallus in damp places, dioecious, green, 

 variously lobed. (The ancient name from equus, horse, and seta, bristle.) 



1. Stems annual ; spikes rounded ; stomata scattered in the grooves. 



Fertile stems branchless, soon perishing ; silex of sterile stems in 



dots ............. 1. E. arvense. 



Fertile stems becoming branched. 



Branches simple ; silex in 3 rows of broad spinules on the ridges . 2. E. pratense. 



Branches compound ; silex in 2 rows of hooked spinules . 8. E. sylvaticum. 



a. Fertile and sterile stems alike, branched or simple. 



Centrum one sixth of the total diameter of stem ; teeth grooved, 



black, with broad white margins ....... 4. E. palustre. 



Centrum half the total diameter or more. 

 Centrum not more than two thirds the diameter ; vallecular holes 



present ; sheaths loose ; fruit abortive ..... 5. E. litorale. 



Centrum four fifths the diameter ; vallecular holes mostly absent; 



sheaths tight .......... 6. E. fluviatile. 



2. Stems evergreen (except in E. latvigatum and E. variegatum, v. 

 Nelsoni), mostly simple ; spikes apiculate ; stomata in single regular 

 series. - HIPPOCHAETE Milde &. 



b. Silex in cross-bands on ridges and grooves c. 



c. Vallecular bast cutting the green parenchyma, carinal not doing so ; 



sheaths ampliated, green. 



Teeth deciduous, leaving black triangular bases ; centrum wide . 7. E. laevigatwm. 

 Teeth persistent, broadly white-bordered ; centrum 



sma11 ......... (9) E. variegatum, v. Nelsoni. 



c. Vallecular bast not cutting the parenchyma, the carinal larger; 



sheaths usually with black and white bands. 

 Sheaths much longer than broad, ampliated; plants 



similar to E. laevigatum in appearance . . (8) K hyemale, v. intermedium. 

 Sheaths little longer than broad, tight .... (8) E.hyemale, v. robustum. 

 b. Ridges biangulate (except in E. variegatum, v. Nelsoni) ; silex of the 

 grooves in rosulae d. 



d. Teeth deciduous ; ridges slightly biangulate ..... 8. E. hyemale. 

 d. Teeth persistent, white-bordered e. 



e. Centrum one third of the diameter of the stem. 



Ridges distinctly bianpulate ; bristle-tips of teeth deciduous . 9. E. variegatum. 



Ridges slightly biangulate ; tips of teeth persistent (9) E. variegatum, v. Jesupi. 



Ridges rounded ....... (9) E. variegatum, v. Nelsoni. 



e. Centrum absent ; stems 6-angled ....... 10. E. scirpoides. 



