Roussea

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Roussea
Drawing from Iconographia generum plantarum, F. Beck, 1838
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Asterales
Family: Rousseaceae
Subfamily: Rousseoideae
Horan.
Genus: Roussea
Sm.
Species:
R. simplex
Binomial name
Roussea simplex

Roussea simplex is a woody climber of 4–6 m high, that is endemic to the mountain forest of

pentamerous, drooping flowers with yellowish recurved tepals, and a purse-shaped orange corolla with strongly recurved narrowly triangular lobes.[1]

Description

Stems and leaves

Roussea simplex is a liana 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. Stipules 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.[1][2]

Flowers

In the axis of the leaf mostly single (but occasionally up to four) pendulous flowers arise on a

protandrous). The sepals 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.[1][2][3]

Taxonomy

Taxonomic history

Roussea simplex was described in 1789 by

Brexiaceae in 1853. Most later authors agreed with Lindley, but differed about the rank of this grouping, considering an order, family, subfamily (within the Saxifragaceae 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 to determine relationships between organisms (phylogenetics), the affinities of Roussea to the Asterales have been affirmed repeatedly.[4]

Modern classification

Phylogenetic analysis puts a clade consisting of Abrophyllum, Cuttsia and Carpodetus in the subfamily Carpodetoideae as sister to Roussea, on its own representing the Rousseoideae. Together these two make up the family Rousseaceae.[4]

Phylogeny

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

family Campanulaceae

subfamily Rousseoideae

Roussea

subfamily Carpodetoideae

Reassigned species

One species that was described as Roussea has been reassigned to another family later on.[5]

Etymology

Roussea was named after Jean-Jacques Rousseau.[4] The species epithet simplex is derived from Latin and means simple or characterized by one element.[6]

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.[7] This climbing shrub mainly grows in wet, high elevation cloud forests, such as at Le Pouce, Mauritius' highest mountain .[8]

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.[9] 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.[10]

Pests and diseases

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.[12]

Conservation

In 1937 Roussea was common on Mauritius, but it is now considered

traveller's palm) may outcompete Roussea. Kew Gardens is now trying to grow the species from seed, which turns out to be very difficult, because without passing through the gecko's gut, the seeds succumb to a fungal infection. Seeds are now grown in a sterile environment.[9]

References

  1. ^
    ISBN 978-3-540-31050-1
  2. ^ a b c James Edward Smith (1789). Plantarum icones hactenus. pp. VI. Retrieved 2016-03-18. Roussea simplex.
  3. ^ Hansen, Dennis M. (2005). "Pollination of the enigmatic Mauritian endemic Roussea simplex (Rousseaceae): birds or geckos?" (PDF). Ecotropica. 11. Society for Tropical Ecology: 69–72.
  4. ^
    doi:10.1111/j.1095-8339.2001.tb01122.x
    . Retrieved 2016-03-17.
  5. ^ "Roussea". The Plantlist. Retrieved 2016-03-27.
  6. ^ "simplex". Dictionary.com. Retrieved 2016-03-17.
  7. doi:10.1080/17550874.2017.1402382
    .
  8. ^ "Roussea simplex". Encyclopedia of Life. Retrieved 2016-03-18.
  9. ^
    JSTOR 20492551. cited on "Roussea simplex"
    . Kew Royal Botanic Gardens. Retrieved 2016-03-17.
  10. doi:10.1016/j.jnc.2019.04.002
    .
  11. ^ "Plagiolepis madecassa". AntWiki: The Ants --- Online.
  12. doi:10.1007/s10530-020-02272-y
    .
  13. ^ Kerry Scott Walter; Harriet J. Gillett (1998). 1997 IUCN Red List of Threatened Plants. IUCN.
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