ACANTHACEAE (Acanthus Family)

(Wasshausen, 1998)

Plants annual or more commonly perennial herbs (shrubs or small trees elsewhere). Stems often somewhat angled, usually bluntly 4-angled in Missouri species. Leaves opposite, lacking stipules, simple. Inflorescences axillary and sometimes also terminal, often reduced to clusters of flowers in the leaf axils, but in some genera in elongate spikes or small panicles. Flowers zygomorphic, perfect, hypogynous, subtended by 1 or more often conspicuous bracts. Calyces actinomorphic, (4)5-lobed, sometimes lobed nearly to the base. Corollas zygomorphic to nearly actinomorphic, 5-lobed or sometimes 2-lipped. Stamens 2 or 4, the filaments fused to the corolla tube, the anther sacs often appearing relatively distinct from each other and sometimes attached asymmetrically with one sac appearing terminal and the other sometimes appearing lateral. Staminodes sometimes also present, (1)2, particularly in some genera with only 2 functional stamens. Pistil 1 per flower, of 2 fused carpels. Ovary superior, with 2 locules, the placentation axile. Style 1 per flower, the stigmas 2. Ovules 1–8 per locule. Fruits capsules, 2-valved, explosively dehiscent longitudinally. Seeds variously shaped, often flattened, the outer coat usually becoming somewhat sticky or gelatinous when moistened, sometimes with appressed hairs that become erect when moistened. About 250 genera, about 2,600 species, cosmopolitan, but most diverse in tropical and subtropical regions.

The Acanthaceae can be difficult to separate from the Scrophulariaceae. In addition to their unusual, elastically dehiscent capsules, members of the Acanthaceae usually have capsules with fewer seeds and flowers subtended by often conspicuous bracts. At maturity, the fruits of most species dehisce suddenly by snapping open and discharging the seeds through the action of a springlike structure associated with each seed. Many species produce both chasmogamous (open-flowering) and cleistogamous (producing fruit without opening) flowers, the latter often produced toward the end of the growing season. Corolla measurements in the keys and descriptions below refer to chasmogamous flowers. Cleistogamous flowers have the corolla reduced in size, somewhat thickened, more or less club-shaped, and white to light yellow or pale green. Also, in a number of species, chasmogamous flowers are heterostylous; that is, some plants have flowers with longer styles and the stigmas positioned above the stamens, whereas others have flowers with shorter styles and the anthers positioned above the stigmas. This mixture of morphologies promotes cross-pollination by the insects that pollinate the flowers.