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Dendrosicyos
Conservation status
	; Vulnerable (IUCN 3.1)
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Cucurbitales
Family:	Cucurbitaceae
Subfamily:	Cucurbitoideae
Tribe:	Coniandreae
Genus:	Dendrosicyos; Balf.f.
Species:	D. socotranus
Binomial name
	Dendrosicyos socotranus; Balf.f.

Dendrosicyos is a monotypic genus in the plant family Cucurbitaceae. The only species is Dendrosicyos socotranus, the cucumber tree. The species is endemic to the island of Socotra in Yemen, and is the only species in the Cucurbitaceae to grow in a tree form. The species name was originally spelled D. socotrana,^[2] but this is corrected to masculine grammatical gender according to the International Code of Nomenclature for algae, fungi, and plants.^[3]^[4]

Features

It has a bulbous trunk and a small crown.^[5] It was first described by Isaac Bayley Balfour in 1882.^[1] A recent molecular phylogenetic analysis of the family Cucurbitaceae found that the Dendrosicyos lineage is about twice as old as the island, and thus seems to be an island relic of a progenitor lineage that became extinct on the mainland.^[6]

The leaves are nearly round, covered with fine bristles, and slightly toothed. The yellow flowers (3 cm), males and females are present on the same plant for cross pollination. It reproduces only by seed. Fruits (3 x 5 cm) are green, turning brick-red when ripe.

Seedlings subjected to overgrazing and regeneration may be compromised over time, except for seedlings protected from goats by Cissus subaphylla. The species is considered endangered. In Soqotri, its name is qamhiyn.

Contrary to what its region of origin would suggest, Dendrosicyos socotranus responds well to being drenched and fertilized, if the temperature is greater than 20 °C. Individuals in their natural habitat can attain 3 meters (10 feet) in height. Trunks are succulent, but not bottle-shaped when young. The plant produces flowers when five years old. ^[7]

D. socotranus has a bottle-shaped base of fibrous wood (pachycaul). The trunk reaches a diameter of up to one meter. Numerous small twigs and branches grow from the stem, the bottle tree growth form making it distinctive.

The base consists largely of parenchyma. In it are embedded small xylem strands that are linked to each other (anastomosis). These xylem strands have a cambium, and a secondary phloem forms. There is not a consistently active cambium - it will always be sequentially formed on the periphery of a new cambium (cambium successively). Dendrosicyos is the first member of the Cucurbitaceae in which such meristem has been demonstrated.

The leaves are about 25 cm long and wide, its leaf edge being slightly thorny. On the underside of the leaves are trichomes of from two to seven cells, the cells often containing two cystoliths. The epidermal cells are lignified.

The flowers occur in the leaf axils. The species is monoecious - male and female flowers on one plant. The flowers are yellowish-orange with long petals and the fruits are ovate. The seeds are about 6 mm.

The plant contains dendrosycin, an iso-cucurbitacin with an unusual ring formation.^[8]

Proliferation and locations

Described in 1882 by Isaac Bayley Balfour, the species is generally described as endemic to the island of Socotra, although some sources (1887) state that it was present on the African continent in Djibouti. It is quite abundant on the dry parts of the island of Socotra, associated with Croton socotranus in the plains, and on calcareous soils to 500 m elevation. The species is well-adapted to dry sites. It is widely distributed in several vegetation types but has a rather fragmented distribution; over large areas there are only isolated trees or small relict populations, whilst in other areas it is relatively abundant. There are a few trees on the island of Samhah, but none on Darsah or Abd al Kuri.

Conservation status

In times of severe drought trees are cut down, pulped and fed to livestock. In some areas (Kilissan) this has resulted in its almost total eradication. On the plains, seedlings germinate and grow protected from livestock in the cover of dense vegetation such as provided by the spiny shrub Lycium sokotranum or by colonies of the succulent shrub Cissus subaphylla. Where there is no Cissus or Lycium, there is little if any regeneration. Thus, along the southern region of Socotra and on Samhah the recovery of populations of Dendrosicyos after drought is dependent on the presence of colonies of Cissus.

Nomenclature

The genus is in the pumpkin subfamily Cucurbitoideae and in the tribe Coniandreae. Dendrosicyos is the basal taxon within the tribe, all other genera of the tribe being a sister group.^[9]

The name Dendrosicyos socotranus means 'cucumber tree of Socotra'.

References

^ ^a ^b Miller, A. (2004). "Dendrosicyos socotrana". IUCN Red List of Threatened Species. 2004: e.T33691A9797777. doi:10.2305/IUCN.UK.2004.RLTS.T33691A9797777.en.
^ Isaac Bayley Balfour (1882), "Diagnoses Plantarum novarum et imperfecte descriptarum Phanerogamarum Socotrensium", Proceedings of the Royal Society of Edinburgh, 11: 498–514
^ The International Plant Names Index, retrieved 23 January 2016
^ McNeill, J.; Barrie, F.R.; Buck, W.R.; Demoulin, V.; Greuter, W.; Hawksworth, D.L.; Herendeen, P.S.; Knapp, S.; Marhold, K.; Prado, J.; Prud'homme Van Reine, W.F.; Smith, G.F.; Wiersema, J.H.; Turland, N.J. (2012), International Code of Nomenclature for algae, fungi, and plants (Melbourne Code) adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011, vol. Regnum Vegetabile 154, A.R.G. Gantner Verlag KG, ISBN 978-3-87429-425-6. Article 62, Ex 1, specifies that the noun Sicyos is treated as masculine.
^ Kingdon, Jonathan (1989). Island Africa: The Evolution of Africa's Rare Plants and Animals. Princeton, New Jersey: Princeton University Press. pp. 40–41. ISBN 0-691-08560-9.
^ Schaefer, H.; Heibl, C; Renner, S (2009). "Gourds afloat: a dated phylogeny reveals an Asian origin of the gourd family (Cucurbitaceae) and numerous oversea dispersal events". Proceedings of the Royal Society B. 276 (1658): 843–851. doi:10.1098/rspb.2008.1447. PMC 2664369. PMID 19033142.
^ Dendrosicyos socotrana: A tree squash extraordinaire
^ HA Hussein, O.B. Abdel-Halim, S.M. Marwan, A.A. El-Gamal, R. Mosana. Dendrocyin: an isocucurbitacin with novel cyclic side chain from Dendrosicyos socotrana Phytochemistry, Volume 65(18), 2004, p. 2551-2556. doi:10.1016/j.phytochem.2004.07.016
^ Kocyan Alexander, Li-Bing Zhang, Hanno Schaefer, Susanne S. Renner. A multi-locus chloroplast phylogeny for the Cucurbitaceae and its implications for character evolution and classification. Molecular Phylogenetics and Evolution, Volume 44(2), August 2007, pp. 553-577 doi:10.1016/j.ympev.2006.12.022

M. E. Olson: Stem and leaf anatomy of the arborescent Cucurbitaceae Dendrosicyos socotrana with comments on the evolution of pachycauls from lianas. Plant Systematics and Evolution, Band 239, 2003, S. 199–214 doi:10.1007/s00606-003-0006-1
The Ethnoflora of the Soqotra Archipelago by Anthony G Miller and Mirranda Morris, Royal Botanic Garden Edinburgh, 2004, ISBN 1-872291-59-7