Didymeles

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 IV system.

Didymeles
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
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Order: Buxales
Family: Buxaceae
Genus: Didymeles
Thouars
Species

Didymeles integrifolia J.St.-Hil.
Didymeles madagascarensis Willd.
Didymeles perrieri Leandri

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.[1] Fossils assignable to this genus have been found in New Zealand,[2] suggesting a much wider distribution in the past.

The conservation status of D. integrifolia has been assessed as of "least concern".[1]

Description

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.

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.[3] 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.[4] 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.[3]

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.[4]

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.[4]

Taxonomy

Genus

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.[5] In 1805, Jean Henri Jaume Saint-Hilaire established Didymeles integrifolia as the name of the type species of the genus.[6]

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.[4] More recently, molecular phylogenetic studies have repeatedly shown that the genus is sister to the Buxaceae.[4][7][8] As of 2017, two treatments are in use. The APG III system of 2009 and the APG IV system of 2016 put Didymeles in the Buxaceae.[9][10] Other sources keep Didymelaceae as a separate family in the order Buxales.[4][11]

Species

The genus consists of three species of evergreen trees.[12][13]

  • Didymeles integrifolia J.St.-Hil. (syns Didymeles excelsa Baill., Didymeles madagascarensis Willd.) — Madagascar, Comoros[13]
  • Didymeles madagascarensis Willd. — Madagascar, Comoros[12]
  • Didymeles perrieri Leandri — Madagascar[13]

There may also be other species, undescribed as of 2013.[1]

Phylogeny

One possible evolutionary relationship among the genera of the Buxales is shown below:[8]

Buxales
 

Didymeles

 
 
 
 

Buxus

 
 

Haptanthus

 
 
 
 

Sarcococca

 
 
 

Styloceras

 
 

Pachysandra

 
 
 
 
 

References
  1. "Didymeles Thouars". Catalogue of the Vascular Plants of Madagascar, Tropicos. Missouri Botanical Garden. Retrieved 2014-09-30.
  2. Stevens, P.F. "Didymelaceae". Angiosperm Phylogeny Website. Retrieved 2014-09-28.
  3. Watson, L.; Dallwitz, M.J. (1992–2014). "Didymelaceae". The families of flowering plants. Retrieved 2014-09-30.
  4. 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.
  5. 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.
  6. "Didymeles integrifolia J. St.-Hil". Tropicos. Missouri Botanical Garden. Retrieved 2014-09-30.
  7. 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
  8. Shipunov, A.B.; Shipunova, E. (2011), "Haptanthus story: Rediscovery of enigmatic flowering plant from Honduras", American Journal of Botany, 98 (4): 761–763, doi:10.3732/ajb.1000307, PMID 21613172
  9. 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, archived from the original on 2017-05-25, retrieved 2010-12-10
  10. Angiosperm Phylogeny Group (2016). "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV". Botanical Journal of the Linnean Society. 181 (1): 1–20. doi:10.1111/boj.12385. ISSN 0024-4074.CS1 maint: uses authors parameter (link)
  11. Köhler, Egon & Kubitzki, Klaus (2006). "Didymelaceae". The Families and Genera of Vascular Plants, Volume IX. Berlin: Springer-Verlag. pp. 129–131.
  12. The Plant List.org: Didymeles species . accessed 9.9.2015.
  13. "Didymeles Thouars". Tropicos. Missouri Botanical Garden. Retrieved 2014-09-30.
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