Neotropical Plocospermataceae

Nicky Biggs

Royal Botanic Gardens, Kew, UK. 

Description

Shrubs or small treelets to 5 m; branches pale grey-brown, often with thin, finely fissured bark, not lenticellate, glabrous or hairy, the hairs normal and glandular. Stipules absent.  Leaves evergreen, whorled or (sub)opposite, simple, entire, coriaceous, glossy and scabrous above, matt and pubescent or nearly glabrous below, with some minute glandular hairs on both sides, ovate, elliptic to oblong, the apex mostly emarginate, but sometimes obtuse, with a slightly revolute margin, midrib sunken above, prominent below; petiole short, pubescentInflorescence axillary, racemose, 1-7 flowered; peduncles and pedicels short, sparsely pubescent; plants dioecious, some with functionally female flowers (anthers present without viable pollen), others with functionally male flowers (with non-functional ovary lacking style and stigma, but their pollen viable).  Flowers slightly zygomorphic; calyx fused at base, lobes 5(-6), subequal, narrowly triangular, apex acuminate, sparsely pubescent or glabrous outside, ciliate, glabrous inside; corolla tube blue violet to purple, broadly funnel-shaped with 5 lines of hairs outside below the lobes or glabrous, glabrous inside, lobes 5(-6), imbricate in bud, subequal, spreading, (sub)orbicular, sparsely pubescent outside, glabrous inside, ciliate; stamens 5(-6), inserted on lower part of corolla tube, equal, slightly exserted, filaments glabrous, anthers oblong, basifixed, apex rounded, base deeply cordate, dehiscent by a longitudinal lateral split; ovary superior, narrowly ovoid or oblong, nectariferous tissue at base, carpels (2-)4, locule 1, style short, glabrous; stigma slender, twice dichotomously divided, minutely pubescent with glandular hairs above, slightly exerted.  Fruit a 2-valved capsule, linear fusiform, glabrous, longitudinally ribbed; seeds 1-4, narrow, smooth, the apices with a pappus of unbranched hairs.

Notes on delimitation

  • Previously placed in Loganiaceae and left there by Leeuwenberg (1967) owing to the shape of the stigma and anthers and because of the absence of latex cells, even though it was the only member of Loganiaceae to have a tuft of hairs on the seed.
  • Recent molecular and phytochemical analyses of Plocosperma supports that status of a seperate family which is resolved at the base of the Lamiales phylogeny, and that there are no close links with the Gentianales nor the Solanales as previously thought (Struwe et al. 1994, Endress et al. 1996, Oxelman et al. 1999).

Distribution in the Neotropics

  • Mexico.
  • Guatemala.
  • Nicaragua.

Distinguishing characters (always present)

Key differences from similar families

  • The insertion of the hairs on the seed of Plocosperma is like that of some Apocynaceae genera (Echites P.Browne and Macrosiphonia Müll.Arg.) but it does not have the apocarpous fruit and partially sterile anthers.
  • The fruit of Plocosperma determines the placement of the genus. It is an elongated capsule, subterete, multicostate, 1-locular, 2-valvate, 1-4 seeded.
  • The seeds are linear fusiform, with an ochraceous pappus-like brush of unbranched hairs at the apex.
  • These characters distinguish Plocosperma from Solanaceae, Verbenaceae and Hydrophyllaceae.

Number of genera

  • One monotypic genus, Plocosperma Benth., is recognized.

Status

  • Native, cultivated.

General notes

  • Three species (P. buxifolium Benth., P. microphyllum Baill. ex Solered. and P. anomalum S.F. Blake) have been described in Plocosperma, but they cannot be clearly separated. The variation among the species was based on differences in indumentum, leaf size, flowers and fruit.
  • Guatemalan specimens have larger leaves than Mexican specimens.
  • The size of flowers and fruits is more or less correlated with leaf size.
  • Leeuwenberg (1967) recognized P. microphyllum and P. anomalum as synonyms of P. buxifolium.

Important literature

Chiang, F. and Frame, D. 1987. The identity of Lithophytum (Loganiaceae, Plocospermae). Brittonia 39(2): 260-262.

Endress, M.E., Sennblad, B., Nilsson, S., Civeyrel, L., Chase, M.W., Huysmanns, S., Grafstrom, E. and Bremer, B. 1996. A phylogenetic analysis of Apocynaceae s.s. and some related taxa in Gentinales: a multidisciplinary approach. Opera Bot. Belg. 7: 59-102.

Leeuwenberg, A.J.M. 1967. Notes on the American Loganiaceae 1. Revision of Plocosperma Benth. Acta Bot. Neerl. 16(2): 56-61.

Oxelman, Backlund, M. and Bremer, B. 1999. Relationships of the Buddlejaceae s.l. investigated using parsimony jack-knife and branch support analysis of chloroplast ndhF and rbcL sequences data. Syst. bot. 24: 164-182.

Struwe, L. 2004. Plocospermataceae. pp. 299-300. In: Smith, N., Mori, S.A., Henderson, A., Stevenson, D.W. and Heald, S.V. (eds.). Flowering Plants of the Neotropics. The New York Botanical Garden, Princeton University Press, Princeton.

Struwe, L., Albert, V.A. and Bremer, B. 1994(1995). Cladistics and the family level classification of the Gentianales. Cladistics 10: 175-206.

How to cite

Biggs, N. (2009). Neotropical Plocospermataceae. In: Milliken, W., Klitgård, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://www.kew.org/science/tropamerica/neotropikey/families/Plocospermataceae.htm.