25. Alocasia (Schott) G. Don in Sweet, Hort. Brit., ed. 3. 631. 1839.
海芋属 hai yu shu
Authors: Heng Li & Peter C. Boyce
Colocasia sect. Alocasia Schott in Schott & Endlicher, Melet. Bot. 18. 1832; Ensolenanthe Schott; Panzhuyuia Z. Y. Zhu; Schizocasia Schott ex Engler; Xenophya Schott.
Herbs, evergreen, rarely seasonally dormant, latex-bearing, medium sized to rarely arborescent and gigantic. Stem thick, often hypogeal, sometimes stoloniferous and bulbiferous, epigeal stem usually erect and later decumbent, rather less often elongated and creeping. Leaves few to several in terminal crown, less often scattered, sometimes each subtended by a cataphyll; petiole long [sometimes minutely asperous, minutely puberulent, or glandular], sheath relatively long; leaf blade sometimes pubescent abaxially, juvenile blade peltate, at maturity usually sagittate, less often ± hastate or cordate, but remaining peltate in some species, margin entire or sinuate [or slightly to deeply pinnatifid]; posterior divisions ovate or triangular; basal ribs well developed, wax glands present in axils of primary lateral veins and midrib; primary lateral veins pinnate, forming submarginal collective vein, 1 or 2 closely adjacent marginal veins also present, secondary and tertiary lateral veins arising from primaries at a wide angle, then arching strongly toward leaf margin, sometimes forming interprimary veins, higher order venation reticulate. Inflorescences 1 or 2 to many in each floral sympodium; peduncle usually shorter than petioles. Spathe persistent, erect, convolute, gaping only basally, strongly constricted between tube and blade, rarely not; tube with convolute margins, shorter than limb, ovoid or oblong, persistent and then splitting irregularly in fruit; limb oblong, usually boat-shaped, rarely arching, at anthesis at first erect, then reflexing and later usually deciduous. Spadix sessile, sometimes shortly stipitate, rarely obliquely adnate to spathe, shorter than spathe; female zone conic-cylindric, short, separated from male by a much narrower zone of sterile flowers; male zone usually cylindric; appendix conic to cylindric, with irregular, labyrinthine network of fissures. Flowers unisexual, naked. Pistil ovoid or oblong, 1-loculed or partially 3- or 4-loculed at apex; ovules 6-10, orthotropous, hemiorthotropous, hemianatropous, or anatropous; funicle short; placenta basal; stylar region short; stigma depressed capitate, ± distinctly 3- or 4-lobed. Male flowers 3-12(-36)-androus; stamens connate into obpyramidal, subhexagonal, truncate, rarely linear, synandria; thecae oblong to linear-oblong, lateral, dehiscing by apical pore; synandrodes shallow, obpyramidal, compressed, truncate. Fruit a reddish, ellipsoid or obconic-ellipsoid or subglobose odorless berry, 1-5-seeded; stigma remnants persistent. Seed subglobose to ellipsoid, rather large; testa thickish, smooth or scabrous; embryo broadly conic, broadly cylindric, or elongate; endosperm copious.
About 80 species: tropical Asia and Malaysia; eight species in China.
The genus Alocasia is vegetatively similar to Colocasia but may be readily differentiated by the presence of wax glands present in axils of the primary lateral veins and midrib on the abaxial surface of the leaf and by the mature fruit that are orange or red, odorless, and contain a few large seeds, this in marked contrast to Colocasia in which the fruits are inconspicuously colored, smelly, and contain many tiny seeds in slimy mucilage. In addition, the placentation is basal in Alocasia and parietal in Colocasia.
In respect to synflorescence architecture, Alocasia may be distinguished by the inflorescences that are generally paired, with each pair oriented ± tangential to the stem, whereas Colocasia has numerous inflorescences arising in a centrifugal sequence radial to the stem.
In China, Alocasia cucullata and A. macrorrhizos are never found away from human disturbance.