This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
Everybody knows a Mint when he comes upon it, by reason of its pungent odour, well represented by Spear-mint (Mentha viridis), the cultivated herb of kitchen gardens. Spear-mint is held to be only a naturalized, not a native species, unless it be in one corner of our country - West Yorks. We have, however, seven species that may be set down as natives, but they are a rather troublesome group for the botanical student; there are so many varieties, hybrids, and sub-species, which tend to connect the species and make it difficult to determine the identity of some specimens. With the exception of the Corn-mint (M. arvensis), they are all inhabitants of wet and marshy wastes, flowering in August and September. They are Labiate plants, and therefore the reader will know what type of flower to expect (see pages 21 and 23 ante). These flowers are individually small, but rendered more conspicuous by being borne in dense whorls, the whorls being often so many and so close together as to form long spikes of bloom. They are all perennial herbs, with square stems and rootstocks, the latter creeping on or just below the surface of the ground, and giving off runners freely. Mentha rotundifolia has broadly ovate, wrinkled, stalkless leaves, the edges indented with rounded teeth, and woolly on the underside. Flower-spikes dense, though with slight intervals between the whorls. The colour of the flowers varies from pink to white. The other species are:
I. Horse-Mint (M. sylvestris). Leaves stalkless, more tapering to a point than in M. rotundifolia, smooth above, sharply toothed, whitish beneath. Stem covered with white woolly hairs. Flowers lilac, spike continuous. Rare.
II. Peppermint (M. piperata). Leaves stalked, margins with large teeth, smooth above, a few hairs along the nervures underneath. Flowers purplish in spikes.
III. Water-Mint (M. aquatica). A very common form in marshes and by riversides, covered with soft hairs. Stout spikes, lilac or purple. Leaves stalked.
IV. Marsh-mint (M. sativa). In this and the two following species the whorls are produced from the axils of the leaves instead of as a terminal spike. The leaves are stalked, with sharp teeth. Flowers purplish. The throat of calyx smooth, calyx-teeth lance-shaped, ending in a fine point.
V. Corn-mint (M. arvensis). Leaves with blunt teeth. Calyx very hairy, teeth shorter than in last, triangular. Corolla hairy, purplish. Cornfields and waste places.
Round leaved Mint.
Mentha rotundifolia. - Labiatae. -
VI. Pennyroyal (M. pulegium). Calyx two-lipped, downy or hairy, with hairy throat. Leaves small, with short stalks, slightly toothed, recurved. Stem much-branched. Odour powerful.