Mentha L.

J. R. Timberlake, E. S. Martins (2013). Flora Zambesiaca, Vol 8: part 8. Royal Botanic Gardens, Kew

Morphology General Habit

Perennial, rarely annual herbs, subshrubs or shrubs, usually gynodioecious, strongly aromatic with sessile glands

Morphology Stem

Stems often rhizomatous, quadrangular

Morphology Leaves

Leaves opposite, entire or toothed, petiolate or sessile

Morphology Reproductive morphology Inflorescences

Inflorescence of few to many-flowered axillary cymes, pedunculate or not, when many-flowered either forming dense verticils along stem in axils of leaf-like bract, or congested into terminal spiciform thyrses in axils of reduced bract; bracteoles inconspicuous

Morphology Reproductive morphology Flowers

Flowers pedicellate

Morphology Reproductive morphology Flowers Calyx

Calyx either subactinomorphic, (4)5-lobed, lobes subequal, triangular to subulate or acicular, or slightly 2-lipped with posterior lobe 3-lobed and anterior lip 2-lobed, anterior lobes often longer; tube cylindrical or infundibuliform, 10–15-veined, throat hairy or glabrous

Morphology Reproductive morphology Flowers Corolla

Corolla lilac, pink or white, subactinomorphic, 4-lobed, lobes spreading, subequal or posterior lip often slightly broader, weakly 2-lobed or emarginate, rarely entire; corolla tube ± cylindrical or rarely gibbous on anterior side, usually included within calyx tube

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 4, subequal, exserted, divergent (included in male-sterile plants); anther ellipsoidal, thecae parallel, distinct

Disc

Disk ± regular

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary deeply 4-lobed; style gynobasic with stigma lobes subequal, shortly spreading

Morphology Reproductive morphology Fruits

Nutlets pale to dark brown, ovoid, smooth to rugose, not mucilaginous.

Distribution

Genus of c.20 species in damp, open habitats, often by water, especially Mediterranean and Australasian (one species in New Zealand) but extending through Europe, Asia and Africa to the Cape, with one species extending to North America.

Recognition

In Africa one can expect to find most frequently the Garden Mint, Mentha spicata L. (M. viridis L.) and some of its hybrids. These are easily recognised by the characteristic spearmint scent and glabrous or subglabrous lanceolate or ovate- lanceolate foliage. M. spicata has a slender spiciform inflorescence, as does its sterile hybrid with M. suaveolens Erhardt, Mentha × villosa Huds. In Europe the wild, hairy forms occasionally arise from seed, but these are very unlikely to occur in the Flora area. M. spicata and M. × villosa are very difficult to separate using morphological characters, although the latter often has broader and more rugose leaves. Another hybrid, M × piperita L., the cultivated Peppermint (M. aquatica × spicata), is also recorded as a cultivated plant. This too has a very distinctive, strong scent, and its oil, which contains a very high proportion of the terpenoid menthol, is used for flavouring confectionary and toothpaste. It has a broader spike of flowers intermediate between that of its parents, and the leaves are distinctly petiolate. In many parts of the tropics this has been replaced by the Japanese mint, M. canadensis L. cv. piperascens, which grows more vigorously under tropical conditions and is widely used commercially. A somewhat hairy mint, it is easily recognised by its verticillate inflorescence, the verticils arising from the axils of leaf-like bracts. The only other mint likely to be encountered in cultivation is the Pennyroyal, M. pulegium L. This is a plant of lower stature, with smaller, ± elliptic leaves, entire or slightly toothed, with a pungent scent. The calyx is weakly 2-lipped and is distinctly hairy on the throat. It is often procumbent in habit with shortly erect flowering stems, though entirely erect forms are also known. There is evidence that some mints from the north temperate zone are shy of flowering due to differences in day length.

Harley, R.M. et al. (2004). Labiatae, in The Families and Genera of Vascular Plants (K. Kubitzki, ed. in chief) VI: 167-275. Springer-Verlag, Berlin, Heidelberg, New York.

Morphology General Habit

Perennial, rarely annual herbs, subshrubs or shrubs, usually gynodioecious, strongly aromatic

Morphology Stem

Stems often rhizomatous

Morphology Reproductive morphology Inflorescences

Inflorescence of few- to many-flowered axillary cymes, pedunculate or not, often forming either dense verticillasters along stem, in axils of leaf-like bracts, or congested into terminal spiciform thyrses in axils of reduced bracts

Morphology Reproductive morphology Inflorescences Bracts

Bracts leaf-like or reduced

Morphology Reproductive morphology Inflorescences Bracteoles

Bracteoles inconspicuous, rarely broader or digitately lobed

Morphology Reproductive morphology Flowers Calyx

Calyx ± actinomorphic, 5-(rarely 4-)lobed, lobes subequal, triangular to subulate or acicular, or slightly 2-lipped, (3/2), anterior lobes often longer, tube cylindrical to infundibuliform, 10-15-nerved, throat hairy or glabrous

Morphology Reproductive morphology Flowers Corolla

Corolla ± actinomorphic, 4-lobed, lilac, pink or white, lobes spreading, subequal, or posterior lobe slightly broader, often emarginate, corolla-tube ± cylindrical or rarely gibbous on anterior side, usually included within calyx-tube

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 4, subequal, exserted, divergent (included in male-sterile plants), anthers ellipsoidal, thecae parallel, distinct

Morphology Reproductive morphology Flowers Gynoecium Stigma

Stigma-lobes subequal, shortly spreading

Morphology Reproductive morphology Flowers Disc

Disc ± symmetrical

Morphology Reproductive morphology Fruits

Nutlets ovoid, smooth to rugose, pale to dark brown, not mucilaginous;

Note

2n = 18, 20, 24, 36, 40, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96, 108, 120, 132. About 20 species. Hybrids are of frequent occurrence in sect. Mentha (Harley and Brighton 1977). These can persist and spread widely by vegetative means. Mentha spicata L. and

Ecology

In damp, open habitats, often by water

Distribution

Especially Mediterranean and Australasian (one species in New Zealand), but extending through Europe, Asia and Africa to the Cape, and with one species in North America and some cultivated species now introduced worldwide, including the Central and Wester

Use

Some species cultivated since antiquity as potherbs.