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..
Herbs, shrubs or trees, with opposite or alternate exstipulate leaves, and perfect mostly complete and irregular flowers (corolla wanting in one species of Synthyris). Calyx inferior, persistent, 4-5-toothed, -cleft, or -divided, or sometimes split on the lower side, or on both sides, the lobes or segments valvate, imbricate or distinct in the bud. Corolla gamopetalous, the limb 2- lipped, or nearly regular. Stamens 2, 4 or 5, didynamous, or nearly equal, inserted on the corolla and alternate with its lobes; anthers 2-celled, the sacs equal, or unequal, or sometimes confluent into one. Disk present or obsolete. Pistil 1, entire or 2-lobed; ovary superior, 2-celled, or rarely 1-celled; ovules mostly numerous, rarely few, anatropous or amphitropous, borne on axile placentae; style slender, simple; stigma entire, 2-lobed or 2-lamellate. Fruit mostly capsular and septicidally or loculicidally dehiscent. Seeds mostly numerous, the testa reticulated, pitted, striate, ribbed, or nearly smooth; endosperm fleshy; embryo small, straight or slightly curved; cotyledons little broader than the radicle.
About 165 genera and 2700 species, widely distributed, most abundant in temperate regions. 1. Anther-bearing stamens 5; corolla rotate; leaves alternate. (Verbasceae.)
2. Anther-bearing stamens 2 or 4; leaves opposite, verticillate or alternate.
* Corolla spurred, saccate or gibbous on the lower side at the base. (Antirrhineae.) Corolla spurred at the base.
Leaves palmately 3-5-veined.
Leaves pinnately veined.
Flowers solitary in the axils.
Throat of the corolla closed by the palate; leaves broad.
Throat of the corolla not closed by the palate; leaves narrow.
Flowers in terminal racemes.
Corolla saccate or gibous at the base.
** Corolla neither spurred, saccate nor gibbous on the lower side.
† Anther-bearing stamens 4, the fifth sterile or rudimentary. (Cheloneae.) Sterile stamen a scale adnate to the upper side of the corolla.
Sterile stamen elongated, longer or shorter than the others. Corolla tubular, 2-lipped, the lobes of the lower lip flat. Sterile stamens shorter than the others: seeds winged.
Sterile stamen about equalling the others; seeds wingless.
Corolla 2-cleft, declined; middle lobe of the lower lip conduplicate.
†† Stamens 4, all anther-bearing, or 2 sterile, or 2 only. a. Stamens 4, all anther-bearing; large Asiatic tree. (Paulownieae.)
b. Stamens 4, all anther-bearing; herbs; corolla 2-lipped; stamens not enclosed in upper lip of corolla. (Mimuleae.)
Calyx prismatic, 5-angled, 5-toothed.
Calyx 5-parted, not prismatic.
Calyx-segments equal; leaves pinnatifid in our species.
Calyx-segments unequal, the upper one the largest. Corolla nearly regular, about equally 5-lobed.
Corolla manifestly 2-lipped.
Leaves palmately nerved; flower with an hypogynous disk.
Leaves pinnately veined; no hypogynous disk.
C. Stamens 4, 2 anther-bearing and 2 sterile, or 2 only; corolla obviously 2-lipped. (Gratioleae.)
Calyx 5-parted; upper lip of corolla present. Sterile filaments short or none.
Anther-sacs transverse, separated.
Anther-sacs parallel, contiguous.
Sterile filaments slender, 2-lobed.
Calyx 4-toothed; upper lip of corolla obsolete; low mud plant.
d. Stamens 4, all anther-bearing; corolla nearly regular; flowers on scapes. (Limoselleae.)
e. Stamens 2 only (rarely 4 in Synthyris); corolla rotate, salvertorm, tubular, or none. (Veroniceae.) Leaves alternate; flowers spicate; corolla 2-3-lobed or none.
Leaves, at least the lower, opposite or verticillate; corolla 4-lobed.
Corolla rotate or salverform; capsule obcordate or emarginate, compressed.
Corolla tubular-funnelform; capsule ovoid, not compressed.
f. Stamens 4, all anther-bearing; corolla campanulate, salverform or funnelform, scarcely 2-lipped. Leaves alternate: flowers in 1-sided racemes. (Digitaleae.)
Leaves, at least the lower, opposite. (Buchnereae.) Corolla salverform; flowers in a long spike.
Corolla campanulate or funnelform.
Stamens nearly equal; calyx-lobes as long as the tube.
Key to Genera.
Stamens strongly didynamous, unequal; calyx-teeth shorter than the tube. Anthers awned at the base; corolla yellow.
Anthers all alike: flowers pedicelled; leaves not auricled.
Anthers of the shorter stamens smaller; leaves auricled at base.
g. Stamens 4, all anther-bearing, ascending under the upper lip of the corolla. (Euphrasieae.) Ovules several or numerous; capsule many-seeded.
Anther-sacs dissimilar, the inner one pendulous by its apex; leaves mostly alternate. Upper lip of the corolla much longer than the lower.
Upper lip of the corolla scarcely longer than the lower.
Anther-sacs similar and parallel; leaves mostly opposite. Calyx 2-bracteolate at the base, oblique, 5-toothed.
Calyx not bracteolate, 4-5-toothed, or cleft or split.
Upper lip of the corolla 2-lobed, its margins recurved; calyx 4-cleft.
Margins of the upper lip of the corolla not recurved.
Calyx scarcely or not at all inflated in fruit; galea entire. Calyx 4-toothed or 4-cleft; capsule straight. Seeds spreading, numerous.
Seeds pendulous, few.
Calyx split on the lower side or on both sides; capsule oblique. Galea short-beaked or beakless.
Calyx ovoid, much inflated and veiny in fruit.
Ovules only 1 or 2 in each cell of the ovary; capsule 1-4-seeded; leaves opposite.