This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", 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..
Annual, biennial or perennial herbs, shrubs, or some tropical species trees. Leaves alternate, very rarely opposite or verticillate, exstipulate, mostly entire and hispid, pubescent, scabrous or setose. Flowers perfect, usually regular, mostly blue, in one-sided scorpioid spikes, racemes, cymes, or sometimes scattered. Calyx inferior, mostly 5-lobed, 5-cleft, or 5-parted, usually persistent, its lobes valvate. Corolla gamopetalous, mostly regular and 5-lobed, sometimes crested or append-aged in the throat, rarely irregular, its lobes imbricated, convolute, plicate or induplicate in the bud. Stamens as many as the corolla-lobes and alternate with them, inserted on the tube or throat; filaments slender or short; anthers 2-celled, the sacs longitudinally dehiscent. Disk annular, entire, or 5-lobed, or none, commonly inconspicuous. Ovary superior, of 2 2-ovuled carpels, entire, or the carpels commonly deeply 2-lobed, making it appear as of 4 1- ovuled carpels; style simple, entire or 2-cleft in our genera; ovules anatropous or amphitropous. Fruit mostly of 4 1-seeded nutlets, or of 2 2-seeded carpels. Endosperm none; embryo straight or curved; cotyledons mostly flat or plano-convex; radicle short.
About 85 genera and 1500 species, of wide geographic distribution.
* Ovary entire or 2-4-grooved; style terminal. ** Ovary 4-divided or deeply 4- lobed, the style arising from the center.
Nutlets armed with barbed prickles.
Nutlets spreading or divergent, covered by the prickles.
Nutlets erect or incurved, the prickles on their backs or margins.
Nutlets attached laterally to the receptacle, sometimes just above their bases. Fruiting calyx not greatly enlarged nor membranous.
Corolla small, usually white; receptacle conic or elongated.
Annuals; calyx nearly closed in fruit; inflorescence naked or bracteolate. Lowest leaves mostly opposite: calyx persistent.
Leaves all alternate; calyx at length deciduous.
Perennials or biennials; calyx-segments more or less spreading in fruit; inflorescence leafy.
Corolla tubular-funnelform; receptacle flat or convex.
Corolla blue, rarely white; nutlets attached just above their bases. Maritime: nutlets fleshy, smooth and shining.
Not maritime; nutlets wrinkled when mature and dry.
Corolla yellow; nutlets laterally attached.
Fruiting calyx much enlarged, membranous, veiny.
Nutlets attached to the receptacle by their very bases. Scar of attachment small, flat.
Corolla salverform or funnelform, its lobes rounded, spreading. Racemes not bracted: corolla-tube short.
Racemes bracted: corolla-tube cylindric, usually slender.
Corolla tubular, its lobes erect, acute.
Scar of attachment large, concave. Corolla tubular, 5-toothed.
Corolla rotate: anthers erect in a cone.
Stamens included; throat of the corolla closed by scales.
Stamens exserted; throat of the corolla dilated, open.