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..
Erect or diffuse odorous herbs, with simple sessile or petioled mostly punctate leaves, and small whorled purple pink or white flowers, the whorls axillary or in terminal dense or interrupted spikes. Calyx campanulate to tubular, 10-nerved, regular, or slightly 2-lipped, 5-toothed. Corolla-tube shorter than the calyx, the limb 4-cleft, somewhat irregular, the posterior lobe usually somewhat broader than the others, entire or emarginate. Stamens 4, equal, erect, included or exserted, sometimes imperfect; filaments glabrous; anthers 2-celled, the sacs parallel. Ovary 4-parted; style 2-cleft at the summit. Nutlets ovoid, smooth. [Name used by Theophrastus; from the nymph Minthe.]
About 30 species, natives of the north temperate zone. The more or less characteristic odors of the species change during the progress of the life of the plant. Type species: Mentha spicata L.
* Whorls of flowers in terminal spikes, or some in the upper axils. Plants glabrous or very nearly so.
Spikes slim, narrow, mostly interrupted; leaves sessile, or nearly so.
1. M. spicata.
Spikes thick, mostly dense, at first short; leaves petioled. Leaves lanceolate or oblone, acute.
2. M. piperita.
Leaves ovate, obtuse, or the upper acute, subcordate.
3. M. citrata.
Plants villous, hirsute or canescent, at least at the nodes. Spikes slim or narrow, often interrupted.
Leaves lanceolate or ovate-lanceolate, acute.
4. M. longifolia.
Leaves elliptic or ovate-oblong, obtuse, reticulated beneath.
5. M. rotundifolia.
Spikes thick (6"), dense, elongated or short.
Leaves sessile; spikes 1'-3' long; plant canescent.
6. M. alopecuroides.
Leaves distinctly petioled, or the uppermost sessile; spikes short. Leaves simply serrate.
7. M. aquatica.
Leaves mostly incised, the margins crisped and wavy.
8. M. cripsa.
** Whorls of flowers all axillary. Upper leaves much smaller than the lower.
9. M. Cardiaca.
Upper leaves not conspicuously reduced. Stem pubescent.
Leaves rounded or obtuse at the base.
10. M. arvensis.
Leaves narrowed, mostly cuneate at the base.
11. M. canadensis.
Stem glabrous or nearly so.
12. M. gentilis.
Mentha spicata L. Sp. Pl. 576. 1753. Mentha spicata var. viridis L. loc. cit. 1753. Mentha viridis L. Sp. Pl. Ed. 2, 804. 1763.
Glabrous, perennial by leafy stolons; stem erect, branched, 1°-1 1/2° high. Leaves lanceolate, sessile or short-petioled, sharply serrate, acute or acuminate at the apex, narrowed at the base, the largest about 2 1/2' long; whorls of flowers in terminal narrow acute usually interrupted spikes, which become 2'-4' long in fruit, the one terminating the stem surpassing the lateral ones; bracts subulate-lanceolate, ciliate, some of them usually longer than the flowers; calyx cam-panulate, its teeth hirsute or glabrate, subulate, nearly as long as the tube; corolla glabrous.
In moist fields or waste places, Nova Scotia to Ontario, Minnesota, Washington, Florida, Texas and California. Naturalized from Europe. Also in Bermuda. Native also of Asia. Garden-, brown- or mackerel-mint. Sage-of-bethlehem. July-Sept.