Megistostegium Hochr.

Koopman, Margaret M., 2011, A synopsis of the Malagasy endemic genus Megistostegium Hochr. (Hibisceae, Malvaceae), Adansonia (3) 33 (1), pp. 101-113 : 106-108

Genus Megistostegium Hochr.

Annuaire du Conservatoire et du Jardin botanique de

Genève 18-19: 221 (1915). — Type: Madagascar. Tulear .

Massif de la Table. F. Geay 5295, 5301 (P).

Synonym:

Macrocalyx Costantin & Poisson, Comptes Rendus des Séances de l’Académie des Sciences Paris 147: 637 (1908).

Homonyms:

Macrocalyx Miers ex Lindl., The Vegetable Kingdom: 764 (1847).

Macrocalyx Trew , Nova Acta Physico-medica Academiae Caesareae Leopoldino-Carolinae Naturae Curiosorum Exhibentia Ephemerides sive Observationes Historias et Experimenta 2: 332 (1761).

Macrocalyx Tiegh. , Bulletin de la Société botanique de France 42: 357 (1895).

MORPHOLOGY

Prostrate or erect shrubs or trees to 8 m tall.Branches cylindrical with white/grey cork-like bark, a cracked appearance and knobby protrusions extending from the limbs. Roots few-branched, covered with a dark, papery bark and often run close to the soil surface. Leaves, petioles, stipules, peduncles, epicalyx, calyx and adaxial base of corolla densely,stellate-pubescent. Leaves borne on short shoots or not, clustered or prominently absent at branch tips. Stipules persistent or caducous, linear to lanceolate. Flowers large, solitary, axillary. Peduncles articulate. Epicalyx 4-lobed, lobes fused about halfway, campanulate, petaloid, accrescent in fruit. Calyx lobes 5, white to light pink, hidden by the conspicuous epicalyx. Petals 5, white, pink or red, obovate to narrowly elliptic, overlapping or not. Nectar copious, sweet, produced in a large trichome-filled cavity created by a flap of basal petal tissue to the adaxial side of corolla. Staminal column (5-9 cm) gives rise to 13-30 stamens with long delicate filaments in no discernable whorls, and crowned with 5 white apical teeth. Anthers monothecate and reniform. Pollen large (> 100 µm), periporate, spherical, intectate with a microscabrate surface; apertures (140-150) are at the level of the exine, delimitation distinct, no annulus, pore membrane structureless. Grains echinate, echinae (80-110) smooth arranged uniformly. Style red, 5-9 cm in length, branched with 5 capitate stigmas. Buds longer than wide; the fourfused epicalyx lobes being distinctively pleated and coming to a sharp point at the bud apex. Young buds (2-7 mm) covered with ferruginous hairs but become green in appearance as the bud grows in size and gains pigmentation in the last days of maturation before flowering. Fruits dry, globose, 5-locular, septicidal capsules subtended by a mature accrescent epicalyx which easily catches the wind and probably acts to disperse the reniform, tomentose seeds. Seeds gray (2[-3] per locule, 3-5 mm long, 2-4 mm wide, 2.5-4 mm tall).

CONSERVATION STATUS OF MEGISTOSTEGIUM Megistostegium is restricted to the xeric, deciduous scrublands of southwestern Madagascar and associated coastline ( Fig. 2). These forests are home to some of the most distinctive plant communities on the island with extremely high levels of endemism at many taxonomic levels ( Nicoll & Langrand 1989) and remain severely and continuously threatened by anthropogenic habitat disturbance. Megistostegium , in particular, is threatened by intense herbivory by goats and other livestock within and outside of protected areas, by invasive plants that no doubt compete and often better acquire the small amounts of available water (e.g., Opuntia Mill. ) in these dry forests, and by the potential (if not actual) extinction of pollinators (Koopman 2008).

KEY TO THE SPECIES OF MEGISTOSTEGIUM HOCHR.

1. Tree; leaves thin/flexible; flowers zygomorphic; staminal column curved, well exserted past corolla (3-5 cm long) ...................................................................... M. nodulosum

— Prostrate to tall shrub; leaves fleshy; leaf base rounded; flowers actinomorphic, staminal column straight, exserted equal to or barely past corolla (0-3 cm long) ....................... 2

2. Prostrate subshrub restricted to calcareous rocky plateaus in extreme southern Madagascar; stipules foliaceous, persistant; leaves large (1.8-5.4 cm × 1.3-3.4 cm), clustered at branch tips; flowers large (epicalyx 3.9-5.0 cm long), often pendant; epicalyx dark maroon; corolla forming a wide cup (apical diameter more than 1.2 cm) .............................. M. perrieri

— Multi-stemmed shrub (0.5-4 m tall) on sand or rocky sand throughout southwest Madagascar; stipules minute, early caducous; leaves small (0.5-1.8 cm × 0.4-1.3 cm), borne on short shoots, often absent from branch terminals; flowers small (epicalyx 2.9-3.9 cm long), erect; epicalyx orange-red; corolla tight around staminal column (apical diameter 0.6-1.1 cm) ........................................................................................ M. microphyllum