Tropicos | Name - !Dentella J.R. Forst. & G. Forst.

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!Dentella J.R. Forst. & G. Forst.

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Project Name Data (Last Modified On 12/19/2023)

Acceptance : Accepted

Note : Tribe Spermacoceae

Project Data (Last Modified On 12/19/2023)

Notes:

Dentella includes about a dozen weedy, mat-forming, rather delicate tropical herbs. Most of the species are found in Australia, and several more have been described from Asia but not all of them are now recognized. One species, Dentella repens, is widely distributed and apparently adventive across the Pacific and in scattered localities in the Neotropics (Lorence et al, 2012) and Indian Ocean (Airy Shaw, 1932; Verdcourt, 1989; Razafimandimbison & Manjato, 2019), and this web page addresses mainly that species.

Dentella is characterized by its low, often herbaceous habit, small and sometimes rather succulent leaves, membranaceous interpetiolar stipules, solitary 5-merous flowers that are subsessile and terminal or pseudoaxillary, and small (2-3 mm), rounded, dry fruit with the well developed calyx limb persistent on it. The flowers are rather small, with the funnelform corolla 4--6 mm long but showy. The corollas vary in color from white to pink or flushed with violet, and sometimes have a distinct "eye" in the center formed by the yellowish green anthers and stigmas in the throat, and perhaps also by pigment variation. The corollas are often distinctive in having toothed or dentate lobes, though the teeth may not be developed. The fruits are capsular in their dry mature form with several seeds free inside the walls, but they are apparently not dehiscent. These fruits perhaps disperse in water, given how often Dentella is associated with wet ground and waterways. Dentella has bisexual, homostylous flowers with the anthers and stigmas borne near the middle of the corolla tube, except one Australian species, Dentella dioeca, apparently is dioecious. This genus has not been reviewed as whole; the most comprehensive study of it was the regional commentaries of Airy Shaw (1932, 1934). Modern information about most of its species is compiled in the Australian Plant Census. Plants from outside Australia have been named as native species in various Asian regions, but whether they are indeed native vs. adventive is unclear.

Dentella repens is the most widespread and commonly collected species of the genus, and also one of the morphologically variable species of Rubiaceae. The plants are prostrate and root at the nodes, and are reported to form mats more than half a meter in diameter. Dentella repens can be recognized by its generally small leaves, 2-12 mm long, subsessile solitary flowers less than 1 cm long, and its ovaries and fruits that are covered with odd, distinctive, papillose to bulbiform or club-shaped trichomes, which usually become enlarged in fruit. The rounded to broadly triangular, membranaceous stipules are persistent and also distinctive, and only a few other non-adventive Australian species also have these distinctive ovary and fruit trichomes.See Kajita et al. (2023: fig. 1) for useful photos of these characters in this species.

Beyond this, though, plants of Dentella repens vary widely in pubescence, from glabrous to pilose or hirsute; in leaf shape, from narrowly elliptic to broadly ovate; and in development of the teeth on the corolla lobe. There seems no pattern to the variation at least among adventive plants, and glabrous and pubescent plants as well as broad-leaved and narrow-leaved may be found in the same area. The manner of dispersal of this plant across the various oceans is unknown, and it seems likely it is more widespread but has been overlooked as distinct from locally known Campanulaceae (see discussion of their similarities below).

Airy Shaw (1934: 284) considered the "Indo-Malayan region" of southern Asia to be the main range of Dentella repens (whether he meant its native range or its center of morphological diversity is not clear), and noted that it shows similar broad mophological variation or plasticity all across its range. Unsurprisingly a number of morphological variants and regional populations have been described as distinct species, including by Airy Shaw himself. He separated Dentella serpyllifolia (Airy Shaw, 1932) based on its glabrous fruits and asymmetric calyx, and later noted (Airy Shaw, 1934: 300-301) that specimens sent to him from Java to demonstrate the occurrence of intermediates (the view of the Javanese botanists) were not considered to be intermediate by him, but that separation of these two variants as forms rather than species could be preferable. Verdcourt (1989) subsequently continued to recognize the Dentella serpyllifolia form as different, and treated it as a variety not based on his study of the group, but with a statement that classification was intended to present a taxonomic compromise between Airy Shaw's original (1932) view and that of Bakhuizen in the Java flora.

The Rubiaceae phylogenetic analysis of Bremer & Eriksson (2009) found Dentella as the sister group of Pentodon, and these both placed in the Spermacoceae.Dentella is similar to and has been confused with Campanulaceae, and in particular with Wahlenbergia, which also has opposite leaves, inferior ovaries, dry capsular fruits, and a sympetalous corolla with toothed lobes; Wahlenbergia however lacks interpetiolar stipules and has 2-5-locular ovaries and dehiscent fruits. The similarity is marked enough that one species, Wahlenbergia erecta, was originally described in Dentella.

Author: C.M. Taylor
The content of this web page was last revised on 12 August 2023.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

Distribution: Swampy areas, cultivation ponds, muddy roadsides, creek edges, and other wet disturbed areas, generally at low elevations and often near or on sea coasts; most species in Australia, one species adventive also across the Pacific and sporadically in the Indian Ocean and Caribbean.

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