''Didymeles'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial no ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...

s. 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 Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Afric ...

and the

Comoros The Comoros,, ' officially the Union of the Comoros,; ar, الاتحاد القمري ' is an independent country made up of three islands in southeastern Africa, located at the northern end of the Mozambique Channel in the Indian Ocean. ...

. 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 ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island country ...

, 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 conservatio ...

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

Description

Species are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

(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 surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''corolla''. Petals are usually ...

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 The term ''sepalum'' was coined ...

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

carpel Gynoecium (; ) 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) ''pistils ...

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). Stipules are considered part of the anatomy of the leaf of a typical flowering plant, although in many speci ...

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 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 panicle are ...

s. Each flower basically consists only of two

stamen The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filam ...

s. The carpellate ("female") flowers are arranged in thyrses (spike-like structures). At the base of the

inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed o ...

there are a number of empty

bract In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves. They may be smaller, larger, or ...

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 seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametoph ...

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, shell, germ or rind. Etymology The term is derived from ''integumentum'', which is Latin for "a covering". In a transferred, or ...

– 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 an indehiscent fruit in which an outer fleshy part ( exocarp, or skin, and mesocarp, or flesh) surrounds a single shell (the ''pit'', ''stone'', or ''pyrena'') of hardened endocarp with a seed (''kernel'' ...

, 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, Bournois – 12 May 1831, Paris) was an eminent French botanist known for his work collecting and describing orchids from the three islands of Madagascar, Mauritius and Réunion. Introd ...

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: Greece 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 ...

"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'' as the name of the

type species In 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 species that contains the biological type specimen( ...

of the genus. Placement in a

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

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 fu ...

of 2009 and the APG IV system 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 In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate ...

evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which ...

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