Acanthaceae

Acanthaceae is a large family of dicotyledonous flowering plants commonly known as the acanthus family. Comprising nearly 250 genera and around 2,500 species, it is predominantly tropical in distribution, though a limited number of species occur in temperate regions. Members of the family exhibit exceptional ecological diversity and occupy an extensive range of habitats including dense and open forests, scrublands, wetlands, valleys, coastal and marine zones, mangrove forests, and swamp environments. The family has four principal centres of distribution: Indonesia and Malaysia, tropical Africa, Brazil, and Central America.
Acanthaceae includes herbaceous plants, shrubs, woody climbers, and in some cases epiphytic species. Several genera are widely cultivated for ornamental, ecological, and medicinal purposes.

Morphological Description

Plants in the Acanthaceae family characteristically bear simple, opposite, decussate leaves that often lack stipules. Their leaf margins may be entire, toothed, lobed, or occasionally spiny. Many species contain cystoliths, calcium-rich crystalline deposits that appear as streaks or specks on the leaf surface.
The flowers are typically bisexual and range from nearly symmetrical to strongly zygomorphic. They are arranged in racemes, cymes, or grouped clusters, usually accompanied by a conspicuous bract, which in some species is large and colourful. Floral structure generally includes:

• Sepals divided into four or five lobes.
• Corolla tubular and either two-lipped or five-lobed.
• Stamens numbering two or four, commonly paired and inserted on the corolla tube.
• Ovary superior, bicarpellate, and with axile placentation.

The fruits are two-celled capsules that open explosively upon maturity. A distinctive feature of the subfamily Acanthoideae is the presence of a seed-dispersal mechanism involving a small, hook-like structure known as a retinaculum or jaculator. This organ attaches to the seed and forcefully ejects it from the capsule during dehiscence, a trait confirmed by high-speed imaging studies. Seed projection enhances dispersal distance and is a defining synapomorphy of the clade.
Gardeners in temperate climates often cultivate Acanthus mollis, known as bear’s breeches, while tropical horticulture frequently features genera such as Thunbergia and Justicia. Molecular phylogenetic studies have also placed the mangrove genus Avicennia within Acanthaceae, although it was historically treated in Verbenaceae or in its own family, Avicenniaceae.

Medicinal Uses

Traditional medicine in many cultures relies on species of Acanthaceae, particularly their leaves, which are applied externally to treat wounds and inflammatory conditions. Contemporary research indicates that members of the family may exhibit a wide range of biological activities, including:

• Antifungal
• Cytotoxic
• Anti-inflammatory
• Antipyretic
• Antioxidant
• Insecticidal
• Hepatoprotective
• Immunomodulatory
• Anti-platelet aggregation
• Antiviral properties

Notable medicinal species include:

• Acanthus ilicifolius, used extensively in South and Southeast Asian ethnomedicine for ailments such as asthma, diabetes, leprosy, hepatitis, snakebite, and rheumatoid arthritis.
• Acanthus ebracteatus, rich in polyphenols and valued for its antioxidant properties; its leaves are used to prepare herbal tea in Thailand and Indonesia.

Phytochemistry

Phytochemical analyses of the family have identified a variety of compounds, including:

• Glycosides
• Flavonoids
• Benzenoids
• Phenolic compounds
• Naphthoquinones
• Triterpenoids

These compounds account for many of the pharmacological actions observed in traditional and experimental studies.

Systematics and Classification

The taxonomic treatment of Acanthaceae has undergone revision since the nineteenth century. The family was first comprehensively classified in 1847 by Christian Gottfried Daniel Nees von Esenbeck. Subsequent revisions include:

• Gustav Lindau (1895), who divided the family into Mendoncioideae, Thunbergioideae, Nelsonioideae, and Acanthoideae. The first three groups lack retinaculate fruits, a distinction that continues to underpin modern taxonomic debates.
• Cornelis Eliza Bertus Bremekamp (1965), who restricted Acanthaceae to those genera with retinaculate fruits, transferring others to families such as Scrophulariaceae, Thunbergiaceae, and Mendonciaceae. Within his circumscription, Acanthaceae was divided into Acanthoideae and Ruelloideae, based on characteristics such as cystolith presence and pollen type.

A 2000 systematic study by Scotland and Vollesen recognised 221 genera, grouping most genera with retinaculate fruits into seven well-defined infrafamilial taxa. Contemporary usage distinguishes:

• Acanthaceae sensu stricto (ss): including only clades with retinaculate fruits—Acantheae, Barlerieae, Andrographideae, Whitfieldeae, Ruellieae, and Justiceae.
• Acanthaceae sensu lato (sl): including the above, plus Thunbergioideae, Nelsonioideae, and Avicennia.

Evolution and Fossil Record

Recent molecular dating studies place the origin of the Lamiales stem lineage at around 106 million years ago (MY), with Acanthaceae diverging approximately 67 MY ago. The crown group—comprising surviving lineages—dates to roughly 54 MY before present. These molecular estimates predate the earliest fossil evidence confidently attributed to Lamiales, which appears in the middle Eocene (48–37 MY).
Fossilised pollen attributed to Acanthaceae is known from the upper Miocene, with the oldest dated to around 22 MY.