Tropicos | Name - Rachicallis DC.

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Rachicallis DC.

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Project Name Data (Last Modified On 8/23/2021)

Acceptance : Accepted

Note : Tribe Rondeletieae

Project Data (Last Modified On 8/23/2021)

Notes:

Rachicallis includes one unusual species of ericoid shrubs adapted for dry seaside habitats. It is distinctive in growth habit, with the small dry-adapted leaves clustered on short-shoots, apparently variable floral biology, small but showy yellow to orange corollas, and small loculicidal capsules. It has triangular interpetiolar stipules that often are overlapped along the reduced stems; subsessile narrow leaves with small thick-textured blades and and two tomentulose or villosulous groove on the lower surface; axillary flowers that do not extend much past the leaves, 4-merous flowers, and salverform corollas with the lobes imbricated in bud. Correll & Correll (1982) presented a good analytic figure. This species is found widely across the Antilles and is frequent in its particular habitat.

The leaves of Rachicallis are notably fleshy, stiff, and shiny on all surfaces. Lorence et al. (2012) noted that the leaves abaxially have two longitudinal grooves, on each side of the costa, but these grooves are usually very narrow and the costa is enlarged and swollen.The grooves are expanded to show a pilosulous inside surface on some older leaves, or perhaps on better hydrated leaves. The stipules are triangular and acute on the lateral stems and younger nodes of the main stems, but often with age they are stretched as the stem expands in diameter and apparently elongate a bit, to eventually become truncate. The fruits are rarely collected, possibly because of low reproductive rates but possibly because fruiting plants are easily overlooked.

The floral biology of Rachicallis was studied in the Bahamas by NIckerson & Tripp (1973). They documented these plants as apparently distylous but actually androdioecious here, with pollen produced in both floral forms but the stigmas and ovaries aborted in the apparently short-styled form (e.g., Dunn 406, MO). The biology of this species does not appear to have been studied elsewhere. Lorence et al. (2012) reported the plants as distylous or homostylous in the Yucatán Peninsuala, without further detail but with analyses noted on some herbarium specimens, including those studied by Nickerson & Tripp (e.g., Dunn 406, MO).

In their molecular analysis of Rondeletieae, Torres-Montúfar et al. 2020 found Rachicallis nested well within the tribe and related to several other Antillean genera, some of which are also xeromorphic plants found near seashores. Rachicallis is similar in general aspect to these and other ericoid Rubiaceae in dry vegetation in the Antilles. One commonly confused with Rachicallis is Strumpfia, differs in its white to pink corollas and fruits that are fleshy white drupes.

Some variant spellings of the genus name have been used, notably "Rhachicallis". (e.g., Correll & Correll, 1982) Rachicallis was circumscribed by some 19th-century authors to include plants now separated in Arcytophyllum, which are morphologically quite similar; Arcytophyllum differs, however, in its erose to lobed stipules, corollas with the lobes valvate in bud, and distribution at high elevations. Hooker (1873) apparently did not treat Rachicallis as a genus, either as an accepted genus or to synonymize it. He did include some of Candolle's individual Rachicalis species in Arcytophyllum (as Ereicoctis). Rachicallis was evaluated and narrowed to include just one seacoast species by Kuntze, who apparently was the first to treat this genus as monotypic and who also noted that Hedyotis americana Jacq. was an older name for these plants than Hedyotis rupestris Sw., the basionym of Rachicallis rupestris, which was the name that had been in use for these plants. Kuntze made the new combination Rachicallis americana (Jacq.) Kuntze here, but this was overlooked by some authors (e.g., Lorence, 1999), who instead cited the same combination in Rachicallis that was published independently but later by Hitchcock.

Author: C.M. Taylor.
The content of this web page was last revised on 19 August 2021.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

Distribution: Scrub vegetation on dunes, exposed limestone outcrops, and mangroves along sea coasts, 0-10 mm; northern and western Antilles and the Yucatán Peninsula.

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Erect shrubs, unarmed, terrestrial, without raphides in the tissues, generally low and densely branched, stems often with reduced internodes and stipules overlapping, bark waxy or corky. Leaves opposite but often borne on axillary short-shoots and often apparently fasciculate, subsessile, entire, with the higher-order venation not lineolate, apparently without domatia, succulent and coriaceous, abaxially with 2 longitudinal pilosulous grooves; stipules tubular and fused to the petioles, triangular sometimes becoming stretched and truncate, acute to bidenticulate, erect, valvate in bud, persistent, adaxially densely sericeous. Inflorescences axillary, with flowers solitary but often grouped at end of short-shoots and stems and appearing cymose, bracteate. Flowers subsessile, dimorphic and plants andromonoecious or perhaps bisexual and distylous or homostylous, subtended by 2 well developed bracts, protandrous, apparently fragrant, diurnal; hypanthium obconic; calyx limb developed, deeply 5-lobed, without calycophylls; corolla salverform, yellow to orange, medium-sized (5--8 mm long), internally glabrous, lobes 4, ligulate to elliptic, in bud imbricated (quincuncial), spreading at anthesis, without appendage but sometimes weakly crisped; stamens 4, inserted near middle of corolla tube, anthers narrowly oblong, dorsfiixed, dehiscent by linear slits, partially exserted, without appendage at top; ovary 2-locular, with ovules numerous in each locule, on axile placentas, stigmas 2, ovoid to linear, included or partially exserted. Fruit capsular, subglobose, septicidally dehiscent from apex, laterally flattened, weakly didymous, with disk portion enlarged, with valves perhaps eventually fully separating, rather small (2--3 mm long), chartaceous, smooth to weakly ribbed, with calyx limb persistent; seeds numerous per locule, angled, medium-sized (ca. 1 mm), entire, reticulated to striate.