''Didymeles'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s. It is variously treated as the only genus of the family Didymelaceae — or in the family

Buxaceae The Buxaceae are a small family of six genera and about 123 known species of flowering plants. They are shrubs and small trees, with a cosmopolitan distribution. A seventh genus, sometimes accepted in the past (''Notobuxus''), has been shown by ...

, as in the

APG IV system The APG IV system of flowering plant classification is the fourth version of a modern, mostly molecular-based, system of plant taxonomy for flowering plants (angiosperms) being developed by the Angiosperm Phylogeny Group (APG). It was publish ...

. The genus ''Didymeles'' is restricted to

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

and the

Comoros The Comoros, officially the Union of the Comoros, is an archipelagic country made up of three islands in Southeastern Africa, located at the northern end of the Mozambique Channel in the Indian Ocean. Its capital and largest city is Moroni, ...

. 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. Fossils assignable to this genus have been found in

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, suggesting a much wider distribution in the past. The

conservation status The conservation status of a group of organisms (for instance, a species) indicates whether the group still exists and how likely the group is to become extinct in the near future. Many factors are taken into account when assessing conservation ...

of ''D. integrifolia'' has been assessed as of "least concern".

Description

Species are

dioecious Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

(i.e. "male" and "female" flowers occur on separate plants). Individual flowers have a very simple structure, without obvious

petal Petals are modified leaves that form an inner whorl surrounding the reproductive parts of flowers. They are often brightly coloured or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''corol ...

s or

sepal A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 Etymology The term ''sepalum'' ...

s. Female flowers have been interpreted in different ways, either as having two

carpel Gynoecium (; ; : gynoecia) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more ...

s 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

stipule In botany, a stipule is an outgrowth typically borne on both sides (sometimes on just one side) of the base of a leafstalk (the petiole (botany), petiole). They are primarily found among dicots and rare among monocots. Stipules are considered part ...

s. The flowers are unisexual, with the two kinds borne on separate plants (i.e. the species are

). The staminate ("male") flowers are arranged in short

panicle In botany, a panicle is a much-branched inflorescence. (softcover ). Some authors distinguish it from a compound spike inflorescence, by requiring that the flowers (and fruit) be pedicellate (having a single stem per flower). The branches of a p ...

s. Each flower basically consists only of two

stamen The stamen (: stamina or stamens) is a part consisting of the male reproductive organs of a flower. Collectively, the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filament ...

s. The carpellate ("female") flowers are arranged in

thyrse A thyrse is a type of inflorescence in which the main axis grows indeterminately, and the subaxes (branches) have determinate growth. Gallery File:Syringa Leipzig 2011.jpg, ''Syringa ''Syringa'' is a genus of 12 currently recognized spec ...

s (spike-like structures). At the base of the

inflorescence In botany, an inflorescence is a group or cluster of flowers arranged on a plant's Plant stem, stem that is composed of a main branch or a system of branches. An inflorescence is categorized on the basis of the arrangement of flowers on a mai ...

there are a number of empty

bract In botany, a bract is a modified or specialized leaf, associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves in size, color, shape or texture. They also lo ...

s 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

s, somewhat separated from one another, often with a small "hump" in between. The

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

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. 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 had two stamens. This interpretation has been accepted by some later researchers. The "scales" then correspond to

tepal A tepal is one of the outer parts of a flower (collectively the perianth). The term is used when these parts cannot easily be classified as either sepals or petals. This may be because the parts of the perianth are undifferentiated (i.e. of very ...

s 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. Each carpel has a broad two-crested stigma on a short style, and usually contains a single ovule. The

integument In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, Exoskeleton, shell, germ or Peel (fruit), rind. Etymology The term is derived from ''integumentum'', which is Latin for "a coverin ...

– 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 In botany, a drupe (or stone fruit) is a type of fruit in which an outer fleshy part (exocarp, or skin, and mesocarp, or flesh) surrounds a single shell (the ''pip'' (UK), ''pit'' (US), ''stone'', or ''pyrena'') of hardened endocarp with a seed ...

, to which the stigma remains attached.

Taxonomy

Genus

The genus was first described by

Louis-Marie Aubert du Petit-Thouars Louis-Marie Aubert du Petit-Thouars (5 November 1758, Saint-Martin-de-la-Place, Bournois – 12 May 1831, Paris) was a French botanist known for his work collecting and describing orchids from the three islands of Madagascar, Mauritius and R� ...

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 Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

"twin" and "members". He described the genus as containing a tree from Madagascar, but did not name the species. In 1805, Jean Henri Jaume Saint-Hilaire established ''

Didymeles integrifolia ''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. The genus ''Didymeles'' is restricted to Madagascar and the Comoro Islands ...

'' as the name of the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

of the genus. Placement in a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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. More recently, molecular phylogenetic studies have repeatedly shown that the genus is sister to the Buxaceae. , two treatments are in use. The

APG III system The APG III system of flowering plant classification is the third version of a modern, mostly molecular-based, system of plant taxonomy being developed by the Angiosperm Phylogeny Group (APG). Published in 2009, it was superseded in 2016 by a f ...

of 2009 and the

of 2016 put ''Didymeles'' in the Buxaceae. Other sources keep Didymelaceae as a separate family in the order Buxales.

Species

The genus consists of three

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

evergreen In botany, an evergreen is a plant which has Leaf, foliage that remains green and functional throughout the year. This contrasts with deciduous plants, which lose their foliage completely during the winter or dry season. Consisting of many diffe ...

trees.The Plant List.org: ''Didymeles'' species
. accessed 9.9.2015. * ''

'' ( syns ''Didymeles excelsa'' , ''Didymeles madagascarensis'' ) — Madagascar, Comoros * '' Didymeles madagascarensis'' — Madagascar, Comoros * '' Didymeles perrieri'' — Madagascar There may also be other species, undescribed .

Phylogeny

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

References

{{Taxonbar, from=Q5274259 Buxaceae Eudicot genera Flora of Madagascar Flora of the Comoros Trees of Africa Trees of Madagascar Dioecious plants