1911 Encyclopædia Britannica/Labiatae

LABIATAE (i.e. “lipped,” Lat. labium, lip), in botany, a natural order of seed-plants belonging to the series Tubiflorae of the dicotyledons, and containing about 150 genera with 2800 species. The majority are annual or perennial herbs inhabiting the temperate zone, becoming shrubby in warmer climates. The stem is generally square in section and the simple exstipulate leaves are arranged in decussating pairs (i.e. each pair is in a plane at right angles to that of the pairs immediately above and below it); the blade is entire, or toothed, lobed or more or less deeply cut. The plant is often hairy, and the hairs are frequently glandular, the secretion containing a scent characteristic of the genus or species. The flowers are borne in the axils of the leaves or bracts; they are rarely solitary as in Scutellaria (skull-cap), and generally form an apparent whorl (verticillaster) at the node, consisting of a pair of cymose inflorescences each of which is a simple three-flowered dichasium as in Brunella, Salvia, &c., or more generally a dichasium passing over into a pair of monochasial cymes as in Lamium (fig. 1), Ballota, Nepeta, &c. A number of whorls may be crowded at the apex of the stem and the subtending leaves reduced to small bracts, the whole forming a raceme- or spike-like inflorescence as in Mentha (fig. 2, 5) Brunella, &c.; the bracts are sometimes large and coloured as in Monarda, species of Salvia, &c., in the latter the apex of the stem is sometimes occupied with a cluster of sterile coloured bracts. The plan of the flower is remarkably uniform (fig. 1, 3); it is bisexual, and zygomorphic in the median plane, with 5 sepals united to form a persistent cup-like calyx, 5 petals united to form a two-lipped gaping corolla, 4 stamens inserted on the corolla-tube, two of which, generally the anterior pair, are longer than the other two (didynamous arrangement)—sometimes as in Salvia, the posterior pair is aborted—and two superior median carpels, each very early divided by a constriction in a vertical plane, the pistil consisting of four cells each containing one erect anatropous ovule attached to the base of an axile placenta; the style springs from the centre of the pistil between the four segments (gynobasic), and is simple with a bifid apex. The fruit comprises four one-seeded nutlets included in the persistent calyx; the seed has a thin testa and the embryo almost or completely fills it. Although the general form and plan of arrangement of the flower is very uniform, there are wide variations in detail. Thus the calyx may be tubular, bell-shaped, or almost spherical, or straight or bent, and the length and form of the teeth or lobes varies also; it may be equally toothed as in mint (Mentha) (fig. 2, 3), and marjoram (Origanum), or two-lipped as in thyme (Thymus), Lamium (fig. 1) and Salvia (fig. 2, 1); the number of nerves affords useful characters for distinction of genera, there are normally five main nerves between which simple or forked secondary nerves are more or less developed. The shape of the corolla varies widely, the differences being doubtless intimately associated with the pollination of the flowers by insect-agency. The tube is straight or variously bent and often widens towards the mouth. Occasionally the limb is equally five-toothed, or forms, as in Mentha (fig. 2, 3, 4) an almost regular four-toothed corolla by union of the two posterior teeth. Usually it is two-lipped, the upper lip being formed by the two posterior, the lower lip by the three anterior petals (see fig. 1, and fig. 2, 1, 6); the median lobe of the lower lip is generally most developed and forms a resting-place for the bee or other insect when probing the flower for honey, the upper lip shows great variety in form, often, as in Lamium (fig. 1), Stachys, &c., it is arched forming a protection from rain for the stamens, or it may be flat as in thyme. In the tribe Ocimoideae the four upper petals form the upper lip, and the single anterior one the lower lip, and in Teucrium the upper lip is absent, all five lobes being pushed forward to form the lower. The posterior stamen is sometimes present as a staminode, but generally suppressed; the upper pair are often reduced to staminodes or more or less completely suppressed as in Salvia (fig. 2, 2, 6); rarely are these developed and the anterior pair reduced. In Coleus the stamens are monadelphous. In Nepeta and allied genera the posterior pair are the longer, but this is rare, the didynamous character being generally the result of the anterior pair being the longer. The anthers are two-celled, each cell splitting lengthwise; the connective may be more or less developed between the cells; an extreme case is seen in Salvia (fig. 2, 2), where the connective is filiform and jointed to the filament, while the anterior anther-cell is reduced to a sterile appendage. Honey is secreted by a hypogynous disk. In the more general type of flower the anthers and stigmas are protected by the arching upper lip as in dead-nettle (fig. 1) and many other British genera; the lower lip affords a resting-place for the insect which in probing the flower for the honey, secreted on the lower side of the disk, collects pollen on its back. Numerous variations in detail are found in the different genera; in Salvia (fig. 2), for instance, there is a lever mechanism, the barren half of each anther forming a knob at the end of a short arm which when touched by the head of an insect causes the anther at the end of the longer arm to descend on the insect’s back. In the less common type, where the anterior part of the flower is more developed, as in the Ocimoideae, the stamens and style lie on the under lip and honey is secreted on the upper side of the hypogynous disk; the insect in probing the flower gets smeared with pollen on its belly and legs. Both types include brightly-coloured flowers with longer tubes adapted to the visits of butterflies and moths, as species of Salvia, Stachys, Monarda, &c.; some South American species of Salvia are pollinated by humming-birds. In Mentha (fig. 2, 3), thyme, marjoram (Origanum), and allied genera, the flowers are nearly regular and the stamens spread beyond the corolla.

Fig. 1.—Flowering Shoot of Dead-nettle (Lamium album). 1, Flower cut lengthwise, enlarged; 2 calyx, enlarged; 3, floral diagram.

Fig. 2.—1, Flower of Sage (Salvia officinalis); 2, Corolla of same cut open showing the two stamens; 3, flower of spearmint (Mentha viridis); 4, corolla of same cut open showing stamens; 5, flowering shoot of same, reduced; 6, floral diagram of Salvia.

The persistent calyx encloses the ripe nutlets, and aids in their distribution in various ways, by means of winged spiny or hairy lobes or teeth; sometimes it forms a swollen bladder. A scanty endosperm is sometimes present in the seed; the embryo is generally parallel to the fruit axis with a short inferior radicle and generally flat cotyledons.

The order occurs in all warm and temperate regions; its chief centre is the Mediterranean region, where some genera such as Lavandula, Thymus, Rosmarinus and others form an important feature in the vegetation. The tribe Ocimoideae is exclusively tropical and subtropical and occurs in both hemispheres. The order is well represented in Britain by seventeen native genera; Mentha (mint) including also M. piperita (peppermint) and M. Pulegium (pennyroyal); Origanum vulgare (marjoram); Thymus Serpyllum (thyme); Calamintha (calamint), including also C. Clinopodium (wild basil) and C. Acinos (basil thyme); Salvia (sage), including S. Verbenaca (clary); Nepeta Cataria (catmint), N. Glechoma (ground-ivy); Brunella (self-heal); Scutellaria (skull-cap); Stachys (woundwort); S. Betonica is wood betony; Galeopsis (hemp-nettle); Lamium (dead-nettle); Ballota (black horehound); Teucrium (germander); and Ajuga (bugle).

Labiatae are readily distinguished from all other orders of the series excepting Verbenaceae, in which, however, the style is terminal; but several genera, e.g. Ajuga, Teucrium and Rosmarinus, approach Verbenaceae in this respect, and in some genera of that order the style is more or less sunk between the ovary lobes. The fruit-character indicates an affinity with Boraginaceae from which, however, they differ in habit and by characters of ovule and embryo.

The presence of volatile oil renders many genera of economic use, such are thyme, marjoram (Origanum), sage (Salvia), lavender (Lavandula), rosemary (Rosmarinus), patchouli (Pogostemon). The tubers of Stachys Sieboldi are eaten in France.