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..
Sedge Family. Grass-like or rush-like herbs. Stems (culms) slender, solid (rarely hollow), triangular, quadrangular, terete or flattened. Roots fibrous (many species perennial by long rootstocks). Leaves usually with closed sheaths. Flowers perfect or imperfect, arranged in spikelets, one (rarely 2) in the axil of each scale (glume, bract), the spikelets solitary or clustered, 1-many-flowered. Scales 2-ranked or spirally imbricated, persistent or deciduous. Perianth hypogynous, composed of bristles, or interior scales, rarely calyx-like, or entirely wanting. Stamens 1-3, rarely more. Filaments slender or filiform. Anthers 2-celled. Ovary 1-celled, sessile or stipitate. Ovule 1, anatropous, erect. Style 2-3-cleft or rarely simple or minutely 2-toothed. Fruit an achene. Endosperm mealy. Embryo minute.
About 75 genera and 3200 species, widely distributed. The dates give time of perfecting fruit.
* Fertile flowers perfect.
t Basal empty scales of the spikelets none, or not more than 2 (.except in Eriophorum).
Scales of the spikelets 2-ranked; bristles none.
Spikelets with only 1 perfect flower.
Spikelets with 2 to many perfect flowers.
Scales of the spikelets spirally imbricated.
Base of the style persistent as a tubercle on the achene.
Spikelet 1; culm leafless; bristles usually present.
Spikelets several or numerous; culms leaf-bearing; bristles none.
Base of the style not persistent as a tubercle.
Flowers without any inner scales.
Base of the style swollen; bristles none.
Base of the style not swollen; bristles usually present.
Bristles 6 to many, silky, much elongated.
Bristles short, or little elongated, smooth or barbed.
Flowers with 1 or more inner scales.
Flowers with 3 broad, stalked scales alternating with barbed bristles.
Flowers with 1 or 2 hyaline scales; bristles none.
Flowers with 2 convolute inner scales.
Flowers with a single minute inner scale.
++ Basal empty scales of the spikelets 3 or more.
Spikelets breaking up into 1-fruited joints; bristles present; scales 2-ranked.
Rachis of the spikelets not jointed, persistent; scales spirally imbricated.
Spikelets flattened, clustered in a single involucrate head; bristles none.
Spikelets not flattened, variously clustered.
Spikelets few-flowered; bristles usually present.
Spikelets many-flowered; bristle none.
Style 3-cleft; bristles none.
** All the flowers imperfect.
Pistillate flower subtended by a flat scale; achene bony.
Pistillate flower enclosed in a perigynium or enwrapped by a concave or convolute scale.
Pistillate flower partly enwrapped by a scale.
Pistillate flower wholly enclosed by a perigynium.
Leaves more than one, with sheath, ligule and midvein.
Leaf one, without sheath, ligule or midvein.