Developmental morphology of Saxicolella amicorum and S-submersa (Podostemaceae: Podostemoideae) from Ghana

Saxicolella (six spp.) is a podostemoid genus occurring in tropical west Africa (Cameroon, Ghana, Nigeria). Taxonomically used characters such as root (with holdfasts), pollen (dyads in many Podostemoideae), capsules (with ribs) and seeds are demonstrated and discussed. This paper deals with the structure and development of two species, which are endemic to rivers in southern Ghana: Saxicolella amicorum J.B.Hall and Saxicolella submersa (J.B.Hall) C.D.K.Cook & Rutish. (syn. Polypleurum submersum J.B.Hall). Saxicolella amicorum has simple, one-flowered stems up to 3cm long, whereas S. submersa has branched, many-flowered stems up to 25cm long. Vegetative shoots can reach 12cm (S. amicorum) and even 50cm (S. submersa) in length. The latter species was previously placed in the Asian genus Polypleurum because the long floating axis was misinterpreted as a root which would be typical for Polypleurum. The long floating axis of S. submersa develops exogenous leaves and is actually a stem. Both S. amicorum and S. submersa have various features in common: vegetative parts (roots, stems, leaves) are elongate and very thin (diameter less than 1 mm); prostrate roots are narrow ribbons (twice as wide as thick); endogenous shoots in opposite pairs along the root; leaves usually simple and filiform; leaf bases with two attached ear-like stipules; spathella club-shaped to ellipsoidal; erect flowers with a solitary stamen; ovary ellipsoidal to fusiform, bilocular; capsules nearly isolobous, with three prominent ribs per valve (i.e. eight ribs per capsule including sutural ribs). Evolutionary dynamics of the root structures in African Podostemoideae such as Saxicolella include: formation of green prostrate ribbons as a result of dorsoventral root flattening; reduction of root caps; occurrence of adhesive hairs and exogenous holdfasts which are disk- or finger-like. Structural diversity and developmental patterns in the Ghanaian Saxicolella species are compared with other African Podostemoideae. (C) 2002 The Linnean Society of London.