An Illustrated Flora of the Northern United States, Canada and the British Possessions/Equisetaceae
Rush-like perennial plants, with 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 verticillate, the nodes provided with diaphragms. Rootstocks subterranean. Leaves reduced to sheaths at the joints, the sheaths toothed. Sporanges 1-celled, clustered underneath the scales of terminal cone-like spikes. Spores all of the same size and shape, furnished with 2 narrow strap-like appendages attached at the middle, coiling around the spore when moist and spreading when dry and mature, in the form of a cross (elaters). Epidermis impregnated with silica, rough. Prothallium on the surface of the ground, green, usually dioecious.
The family consists of the following genus:
1. Equisètum [Tourn.] L. Sp. Pl. 1061. 1753.
Characters of the family. [Name ancient, signifying horse-tail, in allusion to the copious branching of several species.] Called also Toad-pipe, Tad-pipe.
About 25 species, of very wide geographic distribution. Type species: Equisetum fluviatile L.
|•||Stems annual; stomata scattered.|
|•||Stems of two kinds, the fertile appearing in early spring before the sterile.|
|•||Fertile stems simple, soon withering; sheaths of branches of sterile stems 4-toothed.||1.||E. arvense.|
|•||Fertile stems branched when old, only the apex withering.|
|•||Branches of the stem simple, their sheaths 3-toothed.||2.||E. pratense.|
|•||Branches compound.||3.||E. sylvaticum.|
|•||Stems all alike; spores mature in summer; branches simple or none.|
|•||Sheaths rather loose; branches usually long; stems bushy below, attenuate upwards.|
|•||Central cavity very small; spike long.||4.||E. palustre.|
|•||Central cavity about one-half the diameter of stem; spike short.||5.||E. littorale.|
|•||Sheaths appressed; branches usually short.||6.||E. fluviatile.|
|•||Stems mostly perennial, evergreen; spikes tipped with a rigid point; stomata in regular rows.|
|•||Stems tall, usually many-grooved.|
|•||Stems rough and tuberculate, prominently ridged.|
|•||Ridges with 1 line of tubercles; ridges of sheath tricarinate; stem stout.||7.||E. robustum.|
|•||Ridges of the stem with 2 indistinct lines of tubercles; ridges of sheath obscurely 4-carinate; stem slender.||8.||E. hyemale.|
|•||Stems not tuberculate; sheaths enlarged upward.||9.||E. laevigatum.|
|•||Stems low, slender, tufted, usually 5-10-grooved.|
|•||Central cavity small; sheaths 5-10-toothed.||10.||E. variegatum.|
|•||Central cavity none; sheaths 3-toothed.||11.||E. scirpoides.|