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The Podostemaceae (eudicots, Malpighiales) are adapted to rivers that exhibit distinct high-low water seasonality, mainly in the tropics. They attach to submerged rocks with ribbonlike or crustose green roots that cover the substrate like a carpet. Pronounced root dorsiventrality resulted in disklike crusts lacking root caps. African Podostemoideae show a bewildering array of forms not known from other flowering plants, such as (i) foliage leaves having a basis with two sheaths (e.g., Ledermanniella linearifolia), (ii) modular shoot construction with repeated stem cups (Ledermanniella ledermannii), (iii) endogenous origin of flowers along stems (Dicraeanthus africanus), and (iv) epiphyllous flowers (Ledermanniella letouzeyi). Important morphological transformations specific to African podostemoids include a shift from erect to inverted flowers in the spathella and unilocular ovaries arising via septum loss. New matK sequence data and new morphological data for eight African Podostemaceae species of the genera Dicraeanthus, Djinga, and Ledermanniella are combined with previously published sequences representing all major groups to test the placement of the African taxa in the family. All podostemoids studied from continental Africa form a clade that is sister to the Madagascan genera Endocaulos and Thelethylax. The sister of this African-Madagascan lineage is the clade comprising all Asian podostemoids and the American genus Podostemum, whereas all other New World podostemoids and the subfamily Tristichoideae are more basal.

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 3 cm long, whereas S. submersa has branched, many-flowered stems up to 25 cm long. Vegetative shoots can reach 12 cm (S. amicorum) and even 50 cm (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.

This paper discusses problems with labelling plant structures in the context of attempts to create a unified Plant Structure Ontology. Special attention is given to structures with mixed, or doubtful identities that are difficult or even impossible to label with a single term. In various vascular plants (and some groups of animals) the structural categories for the description of forms are less distinct than is often supposed. Thus, there are morphological misfits that do not fit exactly into one or the other category and to which it is difficult, or even impossible, to apply a categorical name. After presenting three case studies of intermediate organs and organs whose identity is in doubt, we review five approaches to categorizing plant organs, and evaluate the potential of each to serve as a general reference system for gene annotations. The five approaches are (1) standardized vocabularies, (2) labels based on developmental genetics, (3) continuum morphology, (4) process morphology, (5) character cladograms. While all of these approaches have important domains of applicability, we conclude that process morphology is the one most suited to gene annotation.

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