Hemithrinax Hooker 1883
Creators: name: Henderson, Andrew
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Plantae
Tracheophyta
Liliopsida
Arecales
Arecaceae
Hemithrinax

description: HEMITHRINAX
Hemithrinax was proposed by Hooker in Bentham & Hooker (1883) to accommodate a Cuban species that had been described by Grisebach (1866) as Trithrinax compacta. Subsequently two other species were described, both from Cuba. Muñiz & Borhidi (1982) divided the genus into two sections, Macrocarpae and Hemithrinax based on León’s (1941) earlier work. Borhidi & Muñiz (1985) classified Hemithrinax as a subgenus of Thrinax. However, molecular data has shown that Hemithrinax is probably more closely related to Leucothrinax than to Thrinax and should be recognized at the generic level (Asmussen et al. 2006, Lewis & Zona 2008, Roncal et al. 2008, Cano et al. 2018). Moya (2019) has given a detailed account of the nomenclature of Hemithrinax species.
Morphology
In the following discussion, morphology is treated in detail and several attributes of Hemithrinax not used in delimiting species are discussed. A detailed generic description can be found in Dransfield et al. (2008). In complete contrast to Coccothrinax, Hemithrinax is remarkable for the distinctiveness of the species and the great amount of morphological diversity.
There are very few records for stem length and diameter. Craft (2017) reported that the stems of H. compacta can reach 15 m tall and 30 cm diameter while those of H. rivularis are sometimes short and subterranean. All species have solitary stems.
Leaf sheaths of Hemithrinax are split at the base. Leaf sheath fibers are scored as three states. Fibers are stout, woo dy, and separate into long, single, curled fibers (H. ekmaniana); or they are wiry and separate into curled, multi-fiber strands (H. rivularis); or leaf sheaths are not fibrous or have a few, single fibers along the margins (H. compacta). Leaf sheath apices are seldom present on specimens and thus this variable is seldom scored.
Petioles of H. ekmaniana are relatively short so that the leaves appear crowded together in a compact crown (Fig. 19A), giving it a quite different appearance from the other species. At the apex of the petiole, and base of the blade, there is a well-developed adaxial hastula and a much less developed abaxial hastula. Pulvini in Hemithrinax are unusual; they have elongate, interfold filaments attached adaxially (Fig. 19B). These may always be present but soon fall or are broken off, and possibly are somewhat shorter in H. rivularis. Leaf blade segments of H. ekmaniana are considerably shorter than the other two species, and the apices of the middle leaf segment apices are blunt and rounded and only briefly splitting (Fig. 19C). The leaves of the other two species are considerably larger and the segments are attenuate at the apex and more deeply split. Abaxially, leaf segments are without indumentum but have close rows of small, whitish dots between the veins together with fewer, larger, scattered brownish or greenish scales. This is the same as that seen in Leucothrinax. Transverse veinlets are inconsistent. Hemithrinax ekmaniana appears to lack transverse veinlets and H. compacta and H. rivularis either have or do not have them. Zona (2021) has commented on the glossy sheen on the adaxial surface of the leaves of H. rivularis.
Inflorescences are remarkably diverse. They are scored as erect, at least initially, above the leaves, with few to numerous partial inflorescences (H. rivularis); or erect above the leaves, with few partial inflorescences at apex of the inflorescence (H. ekmaniana. Fig. 19A); or compact amongst leaf sheath bases (H. compacta). This contrast in inflorescences in closely related taxa is quite common in palms (Henderson 2002). There are 3–9 partial inflorescences. Hemithrinax compacta has swollen, woody rachis bracts. In the other two species they are tubular, closely sheathing, membranous, and brown tomentose. Rachises of the partial inflorescences are flattened in cross-section, markedly so in H. compacta, and rachillae are unevenly spaced, with some in groups. Rachillae are glabrous at or near anthesis. The pedicels are so short that the bracteoles appear to directly subtend the flowers. Flowers are spirally arranged along the rachillae. The perianth consists of a six-lobed cupule. The 6–8 stamens are short and form a compact ring around the gynoecium at anthesis, and the anthers are extrorse. This is quite a different arrangement from the other four genera of the Thrinax unit; these have elongate stamens that spread irregularly at anthesis, and the anthers are latrorse.
Fruits of Hemithrinax are unusually variable in size. Fruits of H. compacta and H. ekmaniana are 5.0–6.0 mm long, while those of H. rivularis are almost three times the size, at 14.3 mm long. Based on fruit size, Muñiz & Borhidi (1982) divided the genus into two sections— Hemithrinax (H. compacta, H. ekmaniana) and Macrocarpae (H. rivularis). Fruits are recorded as yellowish (although fruit color is seldom recorded) and the surfaces are smooth or sometimes with projecting fibers.
Seed surfaces are smooth or shallowly lobed. In longitudinal section seeds are intruded by the hilum from base to about half way (Fig. 19D) (as in Leucothrinax). Embryos are subapical to almost lateral.
description: Published as part of Henderson, Andrew, 2023, A revision of Coccothrinax, Hemithrinax, Leucothrinax, Thrinax, and Zombia (Arecaceae), pp. 1-115 in Phytotaxa 614 (1) on pages 89-91, DOI: 10.11646/phytotaxa.614.1.1, http://zenodo.org/record/8389307