Acalypha

The genus Acalypha is one of the most diverse in the spurge family (Euphorbiaceae). It is distinguished from other genera (such as Euphorbia or Croton) by a unique combination of highly distinct morphological traits, many of which are linked to its adaptation to wind pollination (anemophily).

Acalypha stands out by its simple, often toothed leaves, spike-like inflorescences with separate male and female flowers. Its flowers are highly simplified lacking both petals and the specialised floral structures seen in relatives such as Euphorbia. Furthermore, they lack nectar-secreting structures (disc absent), as they do not need to attract insect pollinators.

Many species are easily recognised by the striking, leaf-like bracts that surround the female flowers. After pollination this bract grows significantly (accrescent) and develops a serrated, lobed, or laciniate margin, completely or partially enveloping and protecting the capsule (the fruit). Most species present highly laciniate or pinnatipartite styles unlike the simple or bifid styles common in the family. This likely increases the surface area exposed to capture floating pollen. In the inconspicuous male flowers, the anthers have hanging pollen sacs that elongate and curl like threads or tiny worms upon maturing (vermiform). This morphological characteristic is highly unusual among Euphorbiaceae.

Flowers and inflorescences are typically arranged in a clear and consistent pattern, a key taxonomic feature for distinguishing species.

FORMAL MORPHOLOGICAL DESCRIPTION

Herbs, annual or perennial, undershrubs, shrubs, or small trees. Monoecious or rarely dioecious. Indumentum of simple and/or glandular trichomes, rarely of stellate trichomes.

Leaves alternate, undivided, generally petiolate, stipulate. Stipules paired at the leaf base, sometimes caducous. Stipels rarely present at apex of petiole or leaf base. Leaf blades entire or, more commonly, dentate or crenate-serrate; pinnately, palmately or pinni-palmately veined.

Inflorescences terminal or axillary, frequently both, unisexual or androgynous, generally solitary. Male inflorescences spicate, densely flowered, with several flowers at each node subtended by a minute bract. Female inflorescences generally spicate, sometimes racemose or paniculate, with 1-3(-5) flowers at each node, usually subtended by a large, foliaceous bract, which becomes accrescent in fruit (subgenus Acalypha) and is generally dentate or lobed; sometimes subtended by a small, non-accrescent bract, entire or lobed. Androgynous inflorescences almost always with female flowers at proximal nodes and male flowers at distal nodes, rarely with female flowers distal and male proximal (gynecandrous inflorescences).

Flowers unisexual, apetalous, disc absent. Male flowers very small, shortly pedicellate, globose in bud; calyx divided into 4 small, valvate sepals; stamens 4–8(–16), on a slightly raised receptacle; filaments free or basally connate; anthers with divaricate or pendulous thecae, unilocular, elongate, later vermiform; pollen grains oblate-spheroidal, with 3–5 pseudopores, tectate, psilate; pistillode absent. Female flowers generally sessile or subsessile, pedicellate in a few species; calyx of 3–(4–5) small, imbricate or valvate sepals, free or connate at base; ovary [1–2]3-carpellate, surface often papillose or muricate, usually pubescent; ovules solitary in each locule, anatropous; styles reddish, free or basally connate, several times divided into filiform segments, rarely bifid or entire; staminodes absent.

Allomorphic female flowers present in some species, generally terminal (sometimes median or basal) in the inflorescences; ebracteate, long-pedicellate or subsessile; calyx as in normal female flowers; ovary and fruits 1–2-locular.

Fruits capsular, small, usually 3-lobed, dehiscing septicidally into 3 bivalved cocci; generally surrounded by the accrescent female bract. Seeds small, ovoid or ellipsoid, usually carunculate, smooth or foveolate, rarely tuberculare; endosperm present, whitish; embryo straight; cotyledons broad and flat.