Didymeles

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Didymeles
Didymeles - Hist vég îles France Tab III labelled.jpg
Labels: ae = pistillate (carpellate, "female") flower, natural size; f = fruit, natural size; k = seed, natural size; ad = staminate ("male") flower, natural size; D = stamens, enlarged
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
Order: Buxales
Family: Didymelaceae (or Buxaceae)
Leandri
Genus: Didymeles
Thouars
Species

See text.

Didymeles is a genus of flowering plants. It is variously treated as the only genus of the family Didymelaceae or in the family Buxaceae, as in the APG III system. The genus consists of two species of evergreen trees, although more may yet to be described. It is native to Madagascar, where it is found in humid montane habitats, and the Comoro Islands, although it formerly had a wider distribution. Species are dioecious (i.e. "male" and "female" flowers occur on separate plants). Individual flowers have a very simple structure, without obvious petals or sepals. Female flowers have been interpreted in different ways, either as having two carpels or as occurring close together in pairs, each with a single carpel.

Description[edit]

Didymeles species are evergreen trees. The simple leaves are leathery in texture, with untoothed (entire) margins. There are no stipules. The flowers are unisexual, with the two kinds borne on separate plants (i.e. the species are dioecious). The staminate ("male") flowers are arranged in short panicles. Each flower basically consists only of two stamens.[1] The carpellate ("female") flowers are arranged in thyrses (spike-like structures). At the base of the inflorescence there are a number of empty bracts followed by up to 15 bracts arranged in a spiral, each of which is at the base of a side branch of the inflorescence. At the apex of each side branch there is a structure made up of two larger "scales", each with a smaller "scale" at its base, and two carpels, somewhat separated from one another, often with a small "hump" in between.[2] The pollen of Didymeles is distinctive. The grain has three furrows (colpi), as is commonly the case for eudicots, but each colpus then contains two circular openings or pores.[1]

The carpellate flowers have been interpreted differently. When the genus was first described, the two carpels were considered to belong to a single flower, each "female" flower having two carpels just as the "male" flowers have two stamens. This interpretation has been accepted by some later researchers. The "scales" then correspond to tepals of the single flower. The alternative interpretation, preferred by von Balthazar et al., is that there are two flowers very close together, each with a single carpel. Only the smaller "scale" might then correspond to a tepal.[2]

Each carpel has a broad two-crested stigma on a short style, and usually contains a single ovule. The integument – the covering on the outside of the ovule – is in the form of a long coiled tube, an unusual feature in flowering plants. The fruit which forms after fertilization is a fleshy drupe, to which the stigma remains attached.[2]

Taxonomy[edit]

The genus was first described by Louis-Marie Aubert du Petit-Thouars in 1804. He described the staminate ("male") flowers as composed only of two stamens and the pistillate (carpellate, "female") flowers as composed only of two pistils. For this reason he named the genus from the Greek διδύμος "twin" and μελὲς "members". He described the genus as containing a tree from Madagascar, but did not name the species.[3] In 1805, Jean Henri Jaume Saint-Hilaire established Didymeles integrifolia as the name of the type species of the genus.[4]

Placement in a family was uncertain for a long time: von Balthazar et al. list 9 families or orders it was placed in, or allied with, between 1873 and 1974. Placed in a family of its own, Didymelaceae, it was associated with the Buxaceae from the 1980s onwards, based on chemical, leaf, pollen and wood characters.[2] More recently, molecular phylogenetic studies have repeatedly shown that the genus is sister to the Buxaceae.[2][5][6] As of 2014, two treatments are in use. The APG III system of 2009 puts Didymeles in the Buxaceae.[7] Other sources, including Peter F. Stevens at the Angiosperm Phylogeny Website, keep Didymelaceae as a separate family in the order Buxales.[8][2][9]

Phylogeny[edit]

One possible evolutionary relationship among the genera of the Buxales is shown below:[6]

Buxales

Didymeles





Buxus



Haptanthus





Sarcococca




 Styloceras



 Pachysandra






Species[edit]

As of September 2014, Tropicos accepts two species:[10]

There may also be other species, undescribed as of 2013.[11]

Distribution, habitat and conservation[edit]

The genus Didymeles is restricted to Madagascar and the Comoros. In Madagascar, the two species are found in humid montane habitats, up to 1,500 m, in two disconnected areas in the north-east and south-east of the island.[11] Fossils assignable to this genus have been found in New Zealand,[8] suggesting a much wider distribution in the past.

The conservation status of D. integrifolia has been assessed as of "least concern".[11]

References[edit]

  1. ^ a b Watson, L.; Dallwitz, M.J. (1992 onwards). "Didymelaceae". The families of flowering plants. Retrieved 2014-09-30.  Check date values in: |date= (help)
  2. ^ a b c d e f von Balthazar, M.; Schatz, G.E. & Endress, P.K. (2003). "Female flowers and inflorescences of Didymelaceae". Plant Systematics and Evolution 237 (3–4): 199–208. doi:10.1007/s00606-002-0262-5. 
  3. ^ du Petit-Thouars, L.-M. Aubert (1804). "Didymeles". Histoire des végétaux recueillis dans les îles de France, de Bourbon et de Madagascar (in French). Paris: Madame Huzard. OCLC 65363354. Retrieved 2014-09-28. 
  4. ^ "Didymeles integrifolia J. St.-Hil.". Tropicos. Missouri Botanical Garden. Retrieved 2014-09-30. 
  5. ^ Worberg, Andreas; Quandt, Dietmar; Barniske, Anna-Magdalena; Löhne, Cornelia; Hilu, Khidir W. & Borsch, Thomas (2007), Phylogeny of basal eudicots: Insights from non-coding and rapidly evolving DNA, Organisms Diversity & Evolution 7 (1): 55–77, doi:10.1016/j.ode.2006.08.001 
  6. ^ a b Shipunov, A.B.; Shipunova, E. (2011), Haptanthus story: Rediscovery of enigmatic flowering plant from Honduras, American Journal of Botany 98: 761–763, doi:10.3732/ajb.1000307 
  7. ^ Angiosperm Phylogeny Group (2009), An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III, Botanical Journal of the Linnean Society 161 (2): 105–121, doi:10.1111/j.1095-8339.2009.00996.x, retrieved 2010-12-10 
  8. ^ a b Stevens, P.F. (2001 onwards). "Didymelaceae". Angiosperm Phylogeny Website. Retrieved 2014-09-28.  Check date values in: |date= (help)
  9. ^ Köhler, Egon & Kubitzki, Klaus (2006). "Didymelaceae". The Families and Genera of Vascular Plants, Volume IX. Berlin: Springer-Verlag. pp. 129–131. 
  10. ^ "Didymeles Thouars". Tropicos. Missouri Botanical Garden. Retrieved 2014-09-30. 
  11. ^ a b c "Didymeles Thouars". Catalogue of the Vascular Plants of Madagascar, Tropicos. Missouri Botanical Garden. Retrieved 2014-09-30.