* Calyx-throat naked.
M. sylvestris: stems 1-2 feet high, erect, slightly branched, and, as well as the whole plant, hoary with short down; leaves closely sessile, broadly lanceolate or narrow-ovate; flowers small, numerous, pinkish, in dense cylindrical spikes, usually several together, forming an oblong terminal panicle. - Horse Mint. - Wet pastures, sides of ditches, etc. Fl. August, September.
M. rotundifolia is a greener, and more hairy plant, with broadly ovate or orbicular leaves, and terminal cylindrical spikes of small pale pink flowers; and M. viridis, has glabrous green stems and leaves, the latter acutely lance-shaped, and lax cylindrical terminal flower spikes. This last is the common Mint of our gardens.
M. piperita: stems 1-1 1/2 foot high, erect, glabrous, or nearly so; leaves stalked, ovate-oblong, serrated; flowers lilac, in blunt rather dense spikes, the lower whorls often distant. - Peppermint. - Wet places, rather rare. Fl. July, August.
M. aquatica: stems 1-1 1/2 feet high, much branched, softly hairy, or sometimes nearly glabrous; leaves stalked, ovate or slightly heart-shaped; flowers in dense, terminal, globular or oblong heads, pale lilac. - Wet ditches, marshes, and edges of streams. Fl. July, August.
†† Flowers in distant axillary whorls.
M. sativa: stems more or less spreading or ascending; leaves stalked, ovate or elliptical serrate; flowers all in distinct axillary whorls, without any terminal head or spike, the calyx tubular, with spreading teeth. - Wet places. Fl. July, August.
M. arvensis: stems low, spreading, branched, hairy, 1/2-1 foot long, rarely erect; leaves stalked, ovate, toothed; flowers all in axillary whorls, mostly shorter than the leafstalks, the calyx campanulate. - Fields and moist places. Fl. July to September.
** Calyx-throat closed with hairs.
M. Pulegium: stems prostrate, much branched, rooting; leaves small, elliptical, entire or slightly crenate, the floral ones still smaller; flowers in dense axillary whorls. - Pennyroyal. - Wet ditches, and marshy places. Fl. August, September.