Roussea
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''Roussea simplex'' is a woody climber of 4–6 m high, that is endemic to the mountain forest of
Mauritius Mauritius, officially the Republic of Mauritius, is an island country in the Indian Ocean, about off the southeastern coast of East Africa, east of Madagascar. It includes the main island (also called Mauritius), as well as Rodrigues, Ag ...
. It is the only species of the genus ''Roussea'', which is assigned to the family
Rousseaceae Rousseaceae is a plant family in the order Asterales containing trees and shrubs. The fruit is a berry or capsule. Leaves are simple, with toothed margins. Leaf stipules are not seen in this group. The family contains four genera and twelve or t ...
. It has opposing, entire, obovate, green leaves, with modest teeth towards the tip and mostly pentamerous, drooping flowers with yellowish recurved tepals, and a purse-shaped orange corolla with strongly recurved narrowly triangular lobes.


Description


Stems and leaves

''Roussea simplex'' is a
liana A liana is a long-Plant stem, stemmed Woody plant, woody vine that is rooted in the soil at ground level and uses trees, as well as other means of vertical support, to climb up to the Canopy (biology), canopy in search of direct sunlight. T ...
of 4–6 m high. The wood vessels have very oblique oval openings which are subdivided by about 20 (maximally 50) bars (this is called scalariform), while the side walls have pits in rows and lack spiral-shaped thickenings. Its young stems are firm and have thick nodes. Leaves are set opposite to each other, but several pairs can be close to each other creating a whorl-like cluster.
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 at the base of the leaf stems are absent, while the leaf stems themselves are about 1 cm long and are covered with felty hairs. The leaf blades are robust, large, and have a long inverted egg-shape (7–12 × 3–5½ cm). The base is rounded to slightly wedged, the margin is slightly serrated, particularly towards the tip and the tip is pointed or blunt. The top of the leaf blade is darker green and without hair, the underside is lighter and has some hair. The light green main vein splits pinnately and the secondary veins end at the leaf margin. The leaves contain long resin channels. Glandular peltate hairs are said to be present.


Flowers

In the axis of the leaf mostly single (but occasionally up to four) pendulous flowers arise on a flowerstem of about 2 cm long that is covered in felty hairs. These flowers are mostly pentamerous (but sometimes 4-merous), large and robust, measuring approximately 2½ cm. The flowers have a slightly sweet scent reminiscent of yeast, a possible adaptation to the preference of the gecko pollinator. The
anther 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 are ripe before the stigmas, meaning that individual flowers are first male and subsequently female (or protandrous). The
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 are leathery, persistent, initially spreading and whitish during the male phase, to become more yellowish and deflexed during the female phase, and eventually more or less spreading again and pale green during development of the fruit. The corolla is bell- or purse-shaped, strongly deflexed in the upper half, with individual petals thick, long egg-shaped to narrowly triangular, yolk yellow near the base and gradually becoming a warm orange near the tip, with felty hairs on the outside, more dense toward the tip. The stamens alternate with the petals, are yolk yellow, large, completely covering the ovary and pistil, with filaments triangular and leaning towards the middle, with large anthers facing outwards, away from the stigma. The pollen grains are circular, 25–30 μm in diameter, secreted in a wet, sticky, yellowish fluid, which easily attaches to smooth surfaces. After three to four days the stamens drop from the flower and so reveal the pale female parts, which have a large central style and a bud-shaped stigma, that is greenish in the male phase and turns yellow in the female phase. The fruit is a pale green berry shaped like an angular flat onion. It contains many minute, flat and ovoid seeds. Nectar is released in generous quantities during both male and female phases.


Taxonomy


Taxonomic history

''Roussea simplex'' was described in 1789 by James Edward Smith, who placed it in the
Campanulaceae The family Campanulaceae (also bellflower family), of the order Asterales, contains nearly 2400 species in 84 genera of herbaceous plants, shrubs, and rarely small trees, often with milky sap. Among them are several familiar garden plants bel ...
. In 1830,
Alphonse Pyramus de Candolle Alphonse Louis Pierre Pyramus (or Pyrame) de Candolle (27 October 18064 April 1893) was a French-Swiss botanist, the son of the Swiss botanist Augustin Pyramus de Candolle. Biography De Candolle, son of Augustin Pyramus de Candolle, first devot ...
however rejected this assignment and considered ties with the
Escalloniaceae Escalloniaceae is a family of flowering plants consisting of about 130 species in eight genera. In the APG II system it is one of eight families in the euasterids II clade (campanulids) that are unplaced as to order. More recent research has pro ...
, Loganaceae or Goodeniaceae. His father,
Augustin Pyramus de Candolle Augustin Pyramus (or Pyrame) de Candolle (, , ; 4 February 17789 September 1841) was a Swiss people, Swiss botany, botanist. René Louiche Desfontaines launched de Candolle's botanical career by recommending him at a herbarium. Within a couple ...
, eventually erected a then
monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...
family Rousseaceae in 1839.
John Lindley John Lindley Fellow of the Royal Society, FRS (5 February 1799 – 1 November 1865) was an English botanist, gardener and orchidology, orchidologist. Early years Born in Old Catton, Catton, near Norwich, England, John Lindley was one of four c ...
saw close relationships with ''
Argophyllum ''Argophyllum'' is a genus in the Argophyllaceae family comprising eighteen species of shrubs and small trees. They feature alternate, simple leaves, often silvery white underneath. They appear in Australia and New Caledonia, where several specie ...
'', '' Ixerba'' and '' Brexia'' and included ''Roussea'' in the Brexiaceae in 1853. Most later authors agreed with Lindley, but differed about the
rank A rank is a position in a hierarchy. It can be formally recognized—for example, cardinal, chief executive officer, general, professor—or unofficial. People Formal ranks * Academic rank * Corporate title * Diplomatic rank * Hierarchy ...
of this grouping, considering an order, family, subfamily (within the
Saxifragaceae Saxifragaceae is a family of Herbaceous plant, herbaceous Perennial plant, perennial flowering plants, within the core eudicot Order (biology), order Saxifragales. The Taxonomy (biology), taxonomy of the family has been greatly revised and the ...
or the Escalloniaceae) or tribe (in the Escallonioideae in the Saxifragaceae). Other authors remained doubtful about the connection with ''Brexia'', as suggested by differences in pollen, anatomy and chemistry. Since the development of methods to compare
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
to determine relationships between organisms (
phylogenetics In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
), the affinities of ''Roussea'' to the
Asterales Asterales ( ) is an Order (biology), order of dicotyledonous flowering plants that includes the large Family (biology), family Asteraceae (or Compositae) known for composite flowers made of Floret#floret, florets, and ten families related to th ...
have been affirmed repeatedly.


Modern classification

Phylogenetic analysis puts a clade consisting of ''
Abrophyllum ''Abrophyllum'' ( syn.: ''Brachynema'' F.Muell.) is a monotypic genus of flowering plants in the family Saxifragaceae ''sensu lato'' according to Engler, A. in Engler & Prantl and Schulze-Menz, G. K. in Melchior, 1964; placed in Subfamily Esca ...
'', ''
Cuttsia ''Cuttsia viburnea'' is a shrub or bushy tree which has toothed leaves and panicles of white flowers, and that is Endemism, endemic to eastern Australia. It is sometimes called silver-leaved cuttsia, and confusingly also native elderberry, honey ...
'' and ''
Carpodetus ''Carpodetus'' is a genus of flowering plants in the Rousseaceae family. It was formerly considered to lie within the Escalloniaceae. Its species occur in New Guinea, New Zealand, the Solomon Islands, and Vanuatu. The genus is characterised by sm ...
'' in the subfamily Carpodetoideae as
sister A sister is a woman or a girl who shares parents or a parent with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to ref ...
to ''Roussea'', on its own representing the Rousseoideae. Together these two make up the family Rousseaceae.


Phylogeny

The following tree represents the current insights in the relationships of ''Roussea'' with other taxa.


Reassigned species

One species that was described as ''Roussea'' has been reassigned to another family later on. * ''R. salicifolia'' = ''
Pouteria ''Pouteria'' is a genus of flowering plant, flowering trees in the gutta-percha family (biology), family, Sapotaceae. The genus is widespread throughout the tropical Americas, with outlier species in Cameroon and Malesia. It includes the canistel ...
salicifolia'' (
Sapotaceae 240px, '' Madhuca longifolia'' var. ''latifolia'' in Narsapur, Medak district, India The Sapotaceae are a family of flowering plants belonging to the order (biology)">order Ericales">family (biology)">family of flowering plants belonging to th ...
).


Etymology

''Roussea'' was named after
Jean-Jacques Rousseau Jean-Jacques Rousseau (, ; ; 28 June 1712 – 2 July 1778) was a Republic of Geneva, Genevan philosopher (''philosophes, philosophe''), writer, and composer. His political philosophy influenced the progress of the Age of Enlightenment through ...
. The species
epithet An epithet (, ), also a byname, is a descriptive term (word or phrase) commonly accompanying or occurring in place of the name of a real or fictitious person, place, or thing. It is usually literally descriptive, as in Alfred the Great, Suleima ...
''simplex'' is derived from Latin and means simple or characterized by one element.


Distribution and habitat

''Roussea simplex'' is an endemic climbing shrub of Mauritius, currently known to be confined in nine locations around the south-east, centre and south-west regions of the island. This climbing shrub mainly grows in wet, high elevation cloud forests, such as at Le Pouce, Mauritius' third highest mountain .


Ecology

The flowers of ''Roussea'' produce copious amounts of nectar and are pollinated by the blue-tailed day gecko ''Phelsuma cepediana''. The fruit secretes a gelatinous substance that contains the minute seeds. The blue-tailed gecko licks up this secretion and disperses the seeds in its droppings. ''and'' ''cited on'' More recently, the Mauritius bulbul, ''Hypsipetes olivaceus'' was revealed as a more efficient pollinator of ''Roussea'' given its long beak and ability to fly longer distances.


Pests and diseases

'' Technomyrmex albipes'', a small ant introduced to Mauritius that came originally from the Indo-Pacific area, seals the flowers of ''Roussea'' with clay to protect mealy bugs. These drink the sap and excrete a sugary urine that is collected by the ants. The ants attack animals that visit the plant, and so prevent the blue-tailed day gecko from pollinating the flowers and from eating the fruit, and in this way seriously hampering the reproduction of ''Roussea''. Two new species were reported to visit flowers for nectar, the native '' Plagiolepis madecassa'' and the invasive yellow crazy ant, '' Anoplolepis gracilipes''. However, it has also been reported that the infestation by alien ants varies seasonally and occurs mostly on plants that have collapsed closer to the ground, due to competition by surrounding alien invasive plants, such as strawberry guava.


Conservation

In 1937 ''Roussea'' was common on Mauritius, but it is now considered
critically endangered An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...
. Some 250 plants in nine localities are reported to survive, but the population is in rapid decline having lost 50% of individuals within a decade. The species is relatively little affected by deforestation because it grows mainly along mountain ridges or crater rims or in other rocky areas which have been largely spared by the extensive deforestation that occurred in Mauritius and the most important driver of its decline is competitive displacement by invasive alien plants, mainly the Strawberry guava and to a lesser extent introduced invasive rats that eat flowers and flower buds and introduced invasive macaques that break its branches, and destroy its flower buds, flowers and unripe fruits as well as predate on its main endemic pollinator, the Mauritius Bulbul. Seeds germinate well within three weeks when sowed on tree fern stems in its native habitat but fail to germinate on the ground or on other plants, and both germination rate and early seedling survival were higher if invasive alien plants are removed from their vicinity.Trotzer, Serina (2013). The impact of invasive alien plants on recruitment of the Mauritian endemic ''Roussea simplex'' in the species’ stronghold on Le Pouce Mountain in Mauritius. Bachelor thesis, University of Applied Sciences Bremen Bremen, Germany


References

{{Taxonbar, from1=Q12049874, from2=Q17516080, from3=Q87133004 Rousseaceae Endemic flora of Mauritius Monotypic Asterales genera