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.