Martiodendron excelsum (Benth.) Gleason

Shrubs to small trees 1 – 15 m tall, rarely up to 25 m, up to 30 (– 55) cm in diameter, usually without buttresses. Leaf rachis (1.5 –) 5.5 – 14.5 (– 17) cm long; petiole 1.5 – 3.2 cm long; petiolule 3 – 5 mm long; leaflets (3 –) 5 – 6 (– 7), rarely unifoliolate leaves at the base of the inflorescences, the blades chartaceous, margins commonly revolute, the terminal ones (7 –) 8 – 12 (– 19) × (3.2 –) 5 – 9 (– 11) cm, 1.4 – 2 (– 2.6) times longer than wide, elliptical to ovate, more rarely oblong, apex cuspidate to acuminate to rounded, base usually rounded to truncate or cordate, less commonly obtuse; axillary buds elliptic to oblong in outline, (4 –) 5 – 6.7 × 1.9 – 2.7 mm, apex acuminate to cuspidate. Inflorescences thyrsoid, distichous, terminal, with elongated primary and secondary axes from which cymes are formed, (7 –) 15 – 25 (– 40) × 6 – 20 (– 27) cm. Flower buds 1.5 – 3 cm long, straight, apex almost always straight, acute to acuminate; sepals 1.5 – 3 × 0.1 – 0.4 cm, one of them generally narrower than in other species, sometimes the abaxial sepal patent and the others reflexed; petals 0.9 – 2 (– 3) × 0.7 – 1.5 cm; stamens 4 or 4 + 1 abaxial staminode, rarely 5 stamens, the abaxial one less-developed than the others, anthers (0.8 –) 1.2 – 2 × 0.1 – 0.25 cm, densely pubescent, the pubescence generally concentrated in the middle portion of the anther; carpel 5 – 7 × 2 – 3 mm, laterally glabrous, pubescent along the abaxial suture; style 5 – 12 mm long. Fruits almost always elliptical, sometimes slightly asymmetrical, 5.5 – 9 (– 10) × 2.8 – 5 × 0.3 – 0.8 cm, 1.7 – 2.2 times longer than wide, glabrous, greenish or yellow to orange, sometimes with pink or red spots along the sutures when mature, wings each 3 – 7 mm wide in the middle portion of the fruit, seminiferous nucleus occupying most of the fruit, ca. 6 – 10 times wider than the widest of the two wings in the middle portion of the fruit.

Distribution, Habitat and Ecology: — The species occurs in Guyana and in the Brazilian state of Roraima (Figure 15), in the basins of the Branco and Essequibo Rivers. It may also occur in Venezuela, near to the Brazilian and Guayanan borders. The species is almost always found in campinaranas, varzeas and riverbank forests, and rarely in upland savannas but always close to rivers, on sandy or sandy-clay soils. Ducke (1948) considered it one of the most frequent and characteristic species of the marginal forest of the Rio Branco in Roraima. In Guyana, the species is dominant along the edges of Essequibo River in the Iwokrama Reserve (P. Mutchnick 834). Borges et al. (2014) reported M. excelsum occurring in Mato Grosso, where only M. elatum occurs, but the authors did not cite any specimen. The species is shorter and occurs in considerably drier areas than the other Amazonian species (Figure 16). It is often found as shrubs to small trees 1 to 6 m tall. Median trees with 6 – 15 m tall are common, rarely reaching a maximum of 25 m tall. Although Silva et al. (2005) and Koeppen & Iltis (1962) gave heights of 30 – 50 m and 28 – 33 m, respectively, and the specific epithet invokes a large tree (see etymology); we found no specimens in situ or in herbaria with label data indicating a stature greater than 25 m. However, we examined all of the specimens cited by Silva et al. (2005) and all but three of those cited by Koeppen & Iltis (1962); indeed, the total number of specimens of the species examined for the present study was almost triple the number mentioned by Koeppen and Iltis. Perhaps this discrepancy is due to misinterpretation of the tree height given in feet rather than meters on many older specimens. Because this was the first species of the genus to be described, the idea of height expressed by its name should not be taken as compared to other species of the genus. Specimen label data (Lewis, G. P. 1607) and personal observation indicate that fruits of Martiodendron excelsum are often found on the banks of rivers and that they float in water due to an airspace in the inner chamber of the samara. These observations, combined with the relatively reduced samara wings of the species relative to other members of the genus, indicate an evolutionary shift away from anemochoric dispersal, which presumably is the dominant form of seed dispersal in the other species of the genus, toward hydrochoric dispersal. Transitions to hydrochoric dispersal via floating fruits are postulated for several other taxa of Dialioideae found in seasonally inundated riverine forests and / or close to rivers, including some species of Dialium, Dicorynia, and Apuleia (Falc ã o et al., 2016; 2020 b; 2022; in prep). Other examples of mainly anemochoric groups with shifts towards hydrochory are found in other subfamilies of Leguminosae like in the genera Machaerium Pers. and Pterocarpus Jacq., of subfamily Papilionoideae (Lozano & Klitgaard 2006; Schley et al., 2021).