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. EQUISÈTUM, L. Horsetail. Scouring Rush. ([Pl. 21])
Spore-cases (sporangia, thecæ) 6 or 7, adhering to the under side of the angled shield-shaped scales of the spike, 1-celled, opening down the inner side and discharging the numerous loose spores. To the base of each spore are attached 4 thread-like and club-shaped elastic filaments, which roll up closely around the spore when moist, and uncoil when dry.—Rootstocks perennial, wide-creeping, hard and blackish, jointed, often branched and sometimes bearing small tubers. Stems erect, cylindrical, hollow, jointed; the surface striated or grooved with alternate ridges and furrows, the cuticle in most species containing silica in the form of minute granules, rosettes, or tubercles; the joints containing besides the central air-cavity a circle of smaller hollows beneath the furrows and a set of still smaller ones beneath the ridges; the nodes closed and solid, each bearing instead of leaves a sheath which is divided into teeth corresponding in number and position to the principal ridges of the stem; stomata in the furrows, each with two pairs of guard-cells, of which the outer pair is marked with radiating lines of silica. Branches, when present, in whorls from the base of the sheath, like the stem, but without the central air-cavity. Prothallus green, formed upon the ground, often variously lobed, usually diœcious. (The ancient name, from equus, horse, and seta, bristle.)
§ 1. Annual-stemmed, not surviving the winter.
[*] Fruiting in spring from soft and rather succulent pale or brownish fertile stems, the sterile stems or branches appearing later, herbaceous and very different.
[+] Fertile stems unbranched, destitute of chlorophyll and soon perishing; the sterile branching copiously.
1. E. arvénse, L. (Common H.) Fertile stems (4–10´ high) with loose and usually distant about 8–12-toothed sheaths; the sterile slender (at length 1–2° high), 10–14-furrowed, producing long and simple or sparingly branched 4-angular branches, their teeth 4, herbaceous, lanceolate.—Moist, especially gravelly soil; very common. March–May. Rootstocks often bearing little tubers.—Var. campéstre, Milde, is a not uncommon state, in which the sterile stem bears a small fruiting spike at the summit. (Eu.)
[+][+] Fertile stems when older producing herbaceous 3-sided branches, and lasting through the summer, except the naked top which perishes after fructification.
2. E. praténse, Ehrh. Sterile and finally also the fertile stems producing simple straight branches; sheaths of the stem with ovate-lanceolate short teeth, those of the branches 3-toothed; stems more slender and the branches shorter than in the last.—Mich. to Minn., and northward. April, May. (Eu.)
3. E. sylváticum, L. Sterile and fertile stems (about 12-furrowed) producing compound racemed branches; sheaths loose, with 8–14 rather blunt teeth, those of the branches bearing 4 or 5, of the branchlets 3, lance-pointed divergent teeth.—Wet shady places; common northward. May. (Eu.)