Acanthaceae

The family Acanthaceae comprises about 2,600 species belonging to approximately 250 genera. Mostly these are perennial herbaceous plants and subshrubs, widely distributed in the tropics and subtropics of both hemispheres. About 10 genera, including most of the large ones, are represented in both hemispheres.

Small genera with limited ranges predominate in the family. Within the extensive range of Acanthaceae there are four centers of greatest generic concentration: South and Southeast Asia, tropical Africa and Madagascar, South America, and Central America with the Caribbean islands. A few species grow in the southeastern United States; three species of Acanthus (Acanthus) occur in southern Europe. Alongside large herbs, Acanthaceae include nearly stemless plants with a basal rosette of leaves, and shrubs 2–4 m high are also common.

In the subfamilies Thunbergioideae and Mendoncioideae lianas with more or less woody stems are represented. The Cuba-occurring Oplonia nannophylla (Oplonia nannophylla) is a small shrub 4–8 cm tall, while one variety of Oplonia armata on Jamaica grows into a small tree. Besides these, only two small South American genera — Trichanthera (Trichanthera) and Bravaisia (Bravaisia) — are represented by trees. Bravaisia integerrima (B. integerrima), growing in swamp forests and wet meadows, is an 18-meter tree with stilt-like adventitious roots at the base of the trunk. Many Acanthaceae are characterized by simple or glandular pubescence and by the presence of cystoliths in the aerial vegetative organs. Leaves are simple, opposite, sometimes very large as in some species of Acanthus. Flowers are bisexual, zygomorphic, and arranged in various types of inflorescences, which are always characterized by large and sometimes brightly colored bracts. Thyrse-type inflorescences and various modifications of them predominate. Due to reduction of the inflorescence axis, some Acanthaceae have solitary flowers in the leaf axils. Sometimes shortening of the inflorescence axis leads to the formation of capitulum-like heads. The calyx consists of 5 (rarely 4 or 3) lobes fused at the base; in species of Thunbergia (Thunbergia) it is reduced and its role is taken by large bracts that completely enclose the bud. Petals are fused into a tube of various shapes, ending in a usually irregular 5-lobed limb, or the corolla is two-lipped, with a two-lobed upper lip and a three-lobed lower lip. Sometimes one of the lips is underdeveloped. Thus, in species of Acanthus, Blepharis (Blepharis) and other related genera the corolla consists only of the lower lip. Stamens are usually 4 (rarely 5), arranged in two pairs; one pair often has longer filaments. Sometimes only two fertile stamens develop, and in such flowers one can find 1–3 staminodes. Anthers dehisce by a longitudinal slit, often surrounded by pubescence that retains the pollen. Sometimes the anthers bear spur-like appendages at the base. The connective is often expanded, and the anthers may be positioned asymmetrically at different heights. In some species the lower anther is smaller or absent; such stamens with a single anther occur, for example, in species of Acanthus. The gynoecium consists of two carpels. The ovary is surrounded by a nectariferous disk, superior, two-locular, with 2–10 or numerous anatropous or amphitropous ovules in each locule. The style is simple, filiform, usually bearing a two-lobed stigma. The shape and size of the lobes vary; often one lobe is situated higher than the other, and sometimes one is undeveloped. Heterostyly is observed in species of Oplonia.

Acanthaceae are predominantly protandrous and are typically insect-pollinated. Pollinators are attracted by the showy, variably colored corolla and the bracts of the inflorescence, abundant nectar, and sometimes extrafloral nectaries on bracteoles, bracts, leaf petioles and stems. Bees appear to be of greatest importance in pollinating Acanthaceae. Anthers, oriented toward one another with the pubescent dehiscent side, hold pollen in a brush until a bee, probing for nectar, parts the stamens; then the insect’s body becomes heavily dusted with pollen. Some Acanthaceae show high specialization to particular pollinator species.

In species of Aphelandra (Aphelandra), Jacobinia (Jacobinia) and Sanchezia (Sanchezia) from tropical American forests, hummingbirds participate in pollination. The peculiar flowers with a sack-like inflated tube of Louteridium donnell-smithii (Louteridium donnell-smithii), a shrub from Central America, are visited and apparently pollinated by bats. Cleistogamy is known in some species of Barleria (Barleria), Ruellia (Ruellia), Blechum (Blechum), Stenandrium (Stenandrium) and other genera. Apomixis has been established in species of Blepharis. The characteristic fruit type for Acanthaceae is a two-locular capsule that opens with two valves, leaving half of the splitting septum in each valve. An exception are representatives of the subfamily Mendoncioideae, in which two ovary locules are distinguishable during flower development, but one of them degenerates and a one-locular, indehiscent drupe-like fruit is formed. In Justicia heterocarpa (Justicia heterocarpa) from East Africa, alongside the usual two-locular capsules, indehiscent fruits with four toothed wing-like appendages are known. These fruits can apparently both be wind-dispersed and attach to animal fur. The dominant method of seed dispersal in Acanthaceae is explosive ejection when the capsule splits, aided by a hook-like appendage on the base of the seed stalk, the so-called retinaculum or jaculator. French botanist Yves Sél (1967, 1969) studied the mechanism of capsule opening in detail. The capsule opens due to unequal drying of the cell layers in the septum. The outer layers shrink strongly and contract, suddenly tearing the septum and the whole fruit in the middle along the central vein of the carpels. The capsule opens with a jerk; the pressed ends of the hooked appendages spring back strongly, acting like a lever, and the seeds are thrown to both sides like a thrown discus sometimes for tens of meters.

In some species of Blepharis, Barleria and other genera inhabiting arid climates, the dry mature capsule opens only in the rain, when the pectin-rich cells at the fruit tip swell, their adhesion weakens, and the fruit likewise opens with a jerk, scattering the seeds. Seeds of many desert species are covered or edged with silky hygroscopic hairs or scales that, when moistened, spread and swell and become sticky. The hairs orient the seed with the radicle toward the soil and glue it to soil particles so it is not washed away by water or blown off by the wind. Seeds of desert Acanthaceae germinate very quickly. In Blepharis grossa (Blepharis grossa), a root about 3 cm long grows overnight after wetting. In dry condition the seeds of such species can retain viability for a long time. P. G. Meier (1961) managed to germinate in the Munich Botanic Garden seeds of Petalidium setosum (Petalidium setosum) that had lain in the herbarium for 12 years.

Thanks to seed ejection, species of Acanthaceae often form dense stands almost without admixture of other plants and play a significant role in the herbaceous layer of moist tropical forests. Acanthaceae occur in all tropical and subtropical formations; they are common in open xerophilous forests, savannas and shrub thickets. In wet forests on the slopes of the Cordilleras they ascend to 1,600–1,800 m above sea level, and in the Western Himalayas even up to 2,000 m. At the same time Acanthaceae can be found on seashores: in the mangroves of the eastern hemisphere the spiny Acanthus ilicifolius (A. ilicifolius) forms thorny thickets. Spiny xerophilous species of Blepharis, Petalidium (Petalidium) and other genera occur in the Sahara, the deserts of southern Africa and Western Asia. Hygrophilous species (Hygrophila) grow along riverbanks, streams and in swamps, are partially flooded and live as aquatic plants. Some species of Ruellia, Justicia and other genera introduced beyond their native ranges have become weeds in many tropical and subtropical countries.

The family Acanthaceae is divided into five subfamilies: Nelsonioideae (Nelsonioideae), which, like Lamiaceae, have numerous ovules in the ovary locules; Thunbergioideae (Thunbergioideae), which have 2 ovules in each locule; Mendoncioideae (Mendoncioideae), in which one ovary locule does not develop and drupe-like fruits are formed; Acanthoideae (Acanthoideae) with several or two ovules per locule and imbricate corolla lobes in the bud; and Ruellioideae (Ruellioideae), characterized by twisted aestivation of the perianth segments. Acanthaceae do not have great economic importance. Some species are used in folk medicine and are cultivated as ornamental plants. The shape of the leaves of Mediterranean species of Acanthus underlies the architectural ornamentation of Corinthian and composite capitals, and the ornamentation of friezes, cornices and other elements of buildings.