This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Usually perennial plants, principally of swamp habitat, with glabrous herbage, stems leaf-bearing or scapose, leaf-sheaths with free margins, and leaf blades terete, gladiate, grasslike, or channeled. Inflorescence paniculate or corymbose, often unilateral, sometimes congested, bearing its flowers either singly and with 2 bractlets (prophylla), or in heads and without bractlets, but each in the axil of a bract; bractlets almost always entire; stamens 6 to 3; ovary i-celled or by the intrusion of the placentae 3-celled, the placentae correspondingly parietal or axial; seeds several-many, usually distinctly reticulated or ribbed, often tailed.
About 215 species, most abundant in the north temperate zone. Type species: Juncus acutus L. The plants bloom in summer. [Latin, from jungo, to bind, in allusion to the use of these plants for withes.]
A. Lowest leaf of the inflorescence terete, not conspicuously channeled, erect, appearing like a continuation of the stem, the inflorescence therefore appearing lateral; stem leaves none.
Flowers bracteolate, inserted singly on the branches of the inflorescence. Genuini.
Perianth-parts green, or in age straw-colored.
Perianth-parts equalling or exceeding the capsule, all acute.
Stamens 3; leaf of the inflorescence much shorter than the stem.
Capsule without a distinct apical papilla.
Capsule with a distinct apical papilla.
Stamens 6; leaf of inflorescence about equalling the stem, or longer.
Perianth-parts reaching only the middle of the capsule, inner obtuse.
Perianth-parts with a chestnut-brown stripe down either side of the midrib.
Flowers not bracteolate, inserted in heads on the branches of the inflorescence. Thalassici.
Perianth-parts pale brown; seed tailless.
Perianth-parts green, or in age straw-colored; seed tailed.
B. Lowest leaf of the inflorescence not appearing like a continuation of the stem, or if so, conspicuously channeled along the upper side, the inflorescence usually appearing terminal.
1. Leaf-blade transversely flattened (inserted with its flat surface facing the stem), or terete and channeled, not provided with septa.
* Flowers bracteolate, inserted singly on the branches of the inflorescence, sometimes clustered or congested, but never in true heads. Poiophylli.
Annual; inflorescence, exclusive of its leaves, more than one-third the height of the plant.
Perennial; inflorescence, excluding leaves, not one-third the height of the plant.
Leaf-blade flat, but sometimes involute in drying.
Inflorescence 1-3-flowered; leaves with fimbriate auricles.
Inflorescence, except in depauperate specimens, several-many-flowered; leaves with entire auricles.
Cauline leaves 1 or 2, rarely wanting; perianth-parts obtuse.
Cauline leaves none; perianth-parts acute or acuminate.
Auricles at top of leaf-sheath cartilaginous, yellow when dry.
Auricles at top of leaf-sheath membranous, whitish or brownish.
Inflorescence exceeded by its lowest leaf: flowers not conspicuously secund.
Auricles of the upper leaves usually 1/2"-i 1/2" long, thin, membranous; peri-
anth parts widely spreading.
Auricles less than 1/2" long, thin only at the margin; perianth parts appressed
to the capsule.
Inflorescence not exceeded by its lowest leaf; flowers conspicuously secund.
* Text contributed by Mr. Frederick V. Coville.
Leaf-blade terete, channeled along the upper side.
Lowest leaf of inflorescence not four lengths of the panicle; capsule oblong to obovoid.
Capsule as long as the perianth or longer.
Capsule much shorter than the perianth.
Seed not tailed.
Perianth i 1/4"-1 1/2" long, plainly exceeded by the capsule.
Perianth 13/4"-2" long, not exceeded by the capsule.
Lowest leaf of the inflorescence rarely less than four times as long as the panicle;
** Flowers not bracteolate, in true heads on branches of the inflorescence. Graminifolii.
Stem erect; capsule oblong or obovoid, obtuse at the apex.
Stamens 3, with red-brown anthers; capsule not mucronate.
Inner perianth-parts obtuse or mucronate.
Heads few, commonly 5-10-flowered.
Heads numerous, commonly 2-5-flowered.
Inner perianth parts setiform-acuminate.
Stamens 6, with yellow anthers; capsule mucronate.
Stem creeping, floating, or ascending; capsule subulate.
J. re pens.
2. Leaf-blade not transversely flattened, commonly terete, hollow, provided with septa.
* Leaf-blade usually channeled along the upper side; septa usually imperfect, not externally evident;
inflorescence of 1-4 heads; plants of arctic or alpine range. Alpini.
Body of the seed 1/2" in length or more.
Leaf-sheath not auriculate.
Body of the seed less than 1/2" in length.
Lowest leaf of inflorescence foliose, erect; capsule deeply retuse at apex.
Lowest leaf of inflorescence membranous, spreading; capsule obtuse and mucronate at the
** Leaf-blade not channeled along the upper side (except in j. bulbosus), the septa perfect (except in j. polycephalus), and usually externally evident; inflorescence, except in depauperate specimens, of several to many heads; plants not of arctic-alpine range. Septati.
+ Stamens 6, one opposite each perianth-part.
Heads reduced to one, or rarely two flowers.
Plant erect; flowers several-many, paniculate.
Plant creeping or floating; flowers 1 or 2, peduncled or sessile.
Epidermis not roughened.
Plants with two kinds of leaves, one normal, the other basal, submersed, and capillary.
Plant low, less than 10' high.
Plant tall, more than 10' high.
Plants without submersed capillary leaves.
Capsule oblong, either abruptly acuminate or bluntly acute.
Branches widely spreading; capsule sharply acute, tapering into a conspicuous point.
Branches usually strict; capsule broadly acute, or obtuse, with a short point.
Leaf-blades erect; inner perianth-parts longer than the outer.
Leaf-blades abruptly divergent from the stem; outer perianth-parts longer than the
Epidermis of the leaves roughened with minute tubercles.
++ Stamens 3, none opposite the inner perianth-parts.
Capsule less than three-fourths as long as the perianth.
Capsule more than three-fourths as long as the perianth.
Capsule tapering evenly into a prominent subulate beak.
Leaf-blade vertically flattened and with incomplete septa, only rarely compressed and with
Leaf-blade terete or nearly so, the septa complete.
Capsule obtuse to acute at the apex, sometimes mucronate, but not prolonged into a beak.
Seed 1/3"-1" long.
Perianth about 1" long, the fruiting head not more than 2" high.
Perianth 1 1/2"-2" long, or if shorter, the fruiting head 2" high or more.
Inflorescence broad; capsule about as long as the perianth.
Inflorescence narrow; capsule much longer than the perianth.
Seed 1/5"-1/4" long.
Perianth and mature capsule 1"-2" long.
Perianth 1 1/4"-2" long, or if less the whole plant not 20' high.
Perianth equaling or longer than the capsule; heads several-many-flowered.
Perianth shorter than the capsule; heads 2-7-flowered.
Perianth i"-i 1/4" long, the whole plant more than 20' high.
Perianth and mature capsules 2"-3" long.