This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", 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, either leafy stemmed or stemless; petaliferous flowers mostly in early spring, succeeded, except in V. pedata, by cleistogamous flowers, that are usually without petals and never expand, but bear abundant seed; stamens five in the petaliferous flower, the two lowest with appendages that project into the spur or nectar sac of the odd petal, these two stamens alone developed in the cleistogamous flower. [The Latin name.]
Allied species freely hybridize when growing together; the hybrids commonly display characters more or less intermediate to those o- the parent species, and show marked vegetative vigor, but impaired fertility; their offspring are often much unlike the mother plant and unlike each other, reverting variously to the characters of the two original species.
* Written by Dr. Ezra Brainerd.
Some 200 species, widely distributed. Besides the following, about 35 others occur in the southern and western parts of North America. Viola villosa Walt. (V. Carolina Greene), of the Southern States, may be looked for in southeastern Virginia. Type species: Viola odorata L.
* Stemless, the leaves and scapes directly from a rootstock or from runners.
† Flowers without marked fragrance; indigenous.
Cleistogamous flowers wanting; petals all beardless.
Cleistogamous flowers present.
A. Rootstock thick, often stout, without stolons; lateral petals bearded.
1. Cleistogamous flowers ovoid on short prostrate peduncles; their capsules mostly purplish.
Leaves (except rarely the earliest) palmately 5-11-lobed or -parted.
Plants villous-pubescent; seeds brown.
Plants nearly or quite glabrous; seeds buff.
Early and late leaves uncut; others 3-7-lobed or -parted, villous-pubescent.
Leaves all uncut; blades ovate to reniform, cordate, crenate-serratc.
Plants nearly or quite glabrous.
Petals violet-purple; seeds brown.
Petioles smooth; plants of moist soil.
Petioles glandular roughened; plants of dry soil.
Petals pale violet, or nearly white; seeds buff.
Leaves villous, especially beneath and on petioles; seeds dark-brown.
Leaves hirsutulous above, otherwise glabrous; seeds buff.
2. Geistogamous flowers ovoid, on ascending peduncles soon elongate, their capsules purplish;
leaves cordate, none cut.
Leaves pubescent beneath and on petioles.
Sepals and their auricles ciliolate; blades broadly ovate, cordate.
Sepals and auricles not ciliate; blades at flowering time narrowly ovate
Leaves glabrous beneath and on petioles.
3. Cleistogamous flowers on erect peduncles, their capsules green.
Leaves ovate to reniform, cordate, glabrous, uniformly crenate-serrate.
Cleistogamous flowers ovoid; spurred petal villous.
Mature leaves rounded at apex or bluntly pointed; sepals obtuse.
Mature leaves abruptly acuminate; sepals acute.
Cleistogamous flowers long and slender, spurred petal glabrous.
Leaves lobed, or the margins sharply incised or toothed toward the subcordate or truncate base.
Spurred petal glabrous, lateral with clavate beard; leaves lobed.
Spurred petal villous, lateral with capillary beard.
Blades of mature leaves ovate-oblong, ciliate, finely pubescent.
Blades of mature leaves lanceolate, usually glabrous; petioles long.
Blades of mature leaves broadly ovate or deltoid.
Margin coarsely toothed near base; blades sometimes lobed.
Margin sharply toothed toward base or pectinately incised.
Blades of mature leaves primarily 3-lobed or 3-parted, segments variously cleft.
Segments 2-3-cleft into linear or oblanceolate lobes; eastern.
Segments 3-cleft, the subdivisions often 2-4-lobed; western.
Middle segment uncut, the outer usually 2-4-cleft; southern.
B. Rootstocks slender (thicker and scaly with age); plants usually from stolons.
Petals lilac or pale violet.
Leaves minutely hairy on the upper surface; spur large, 3" long.
Leaves glabrous throughout; spur short, 1" long.
Petals white, with dark purple lines on the three lower.
Cleistogamous capsules ovoid, usually purplish; woodland plants.
Leaves reniform, lateral petals beardless; stolons short.
Leaves broadly ovate, acute, lateral petals bearded; seeds obtuse at base.
Leaves ovate, acute or acuminate; lateral petals beardless, seeds acute at base.
Cleistogamous capsules ellipsoid, always green, peduncles erect; bogs and wet meadows.
Leaves broadly ovate or orbicular, cordate, obtuse.
Leaves oblong to ovate; base slightly cordate to tapering.
Leaves lanceolate or elliptical.
‡‡ Petals bright yellow.
†† Flowers very fragrant; introduced.
** Leafy stemmed; the flowers axillary.
† Style capitate, beakless, bearded near the summit; spur short; stipules nearly entire, soon scarious.
Stems at first short, flowers and leaves, from near the base; later elongating.
Stems not leafly below; peduncles from axils of upper leaves.
Rootstock short, woody, brown, bearing coarse fibrous roots.
Rootstock long, brittle, whitish, bearing crisp capillary roots.
Sparingly pubescent; root-leaves usually 1-3
Markedly pubescent; root-leaves usually wanting.
Winner face of petals white with yellow base, outer face usually violet.
Root-leaves and lower stem-leaves reniform, densely hirsutulous beneath.
Root-leaves and lower stem-leaves broadly ovate, acuminate, subglabrous.
†† Style not capitate; spur long; stipules bristly toothed, herbaceous.
Spur 2"-4" long; lateral petals bearded; style bent at tip, with short beard.
Petals white or cream-colored.
Herbage glabrous or nearly so.
Leaves orbicular or suborbicular.
Stipules ovate-lanceolate, bristly serrate; leaves often 1 3/4' wide,
Stipules linear, entire except at base; leaves not over 3/4' wide.
Leaves ovate; Canada and far west.
Stems ascending; blades mostly ovate; Canada and bordering states,
Stems prostrate; blades orbicular; Kentucky and southward.
Spur 4"-6" long, lateral petals beardless; style straight and smooth.
††† Style much enlarged upward into a globose, hollow summit; stipules large, leaflike, pectinate at base.
Upper leaves and middle lobe of stipules entire or nearly so; indigenous.
Upper leaves and middle lobe of stipules plainly crenate; introduced from Europe.
Petals large, 2-3 times as long as the sepals.
Petals usually shorter than the sepals.