Roscoea

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Roscoea
R. auriculata
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Monocots
Clade: Commelinids
Order: Zingiberales
Family: Zingiberaceae
Genus: Roscoea
Sm.[1]
Species

See text.

Roscoea is a genus of

perennial plants of the family Zingiberaceae (the ginger family). While most members of the ginger family are tropical, Roscoea species are native to cooler mountainous regions of the Himalayas, China and its southern neighbours. Roscoea flowers superficially resemble orchids, although they are not related. The flowers of Roscoea have a complicated structure, in which some of the showy coloured parts are not formed by petals, but by staminodes, sterile stamens
which have evolved to resemble petals. Some species are grown as ornamental plants in gardens.

Description

Roscoea is found from

Himalayas to Vietnam, extending northwards into China. There are up to 22 recognized species, of which 8 are endemic to China.[1][2] Typically they grow in grassland, in screes or on the edges of deciduous woodland at heights of 1,200–5,000 metres (3,900–16,400 ft), coming into growth at the start of the monsoon season.[3]

Species of Roscoea are small perennial

pseudostems; structures which resemble stems but are actually formed from the tightly wrapped bases of its leaves ("sheaths"). The leaves are without a stalk (petiole). Lower leaves may consist solely of a sheath; upper leaves have a blade which is free from the pseudostem, and is oblong or lanceolate (i.e. considerably longer than it is wide).[2] The relative number of bladeless versus complete leaves is one distinguishing feature of the two clades into which the genus is divided.[4]

The flowers are borne in a spike at the end of the pseudostems. The stalk (

staminodes). Two lateral staminodes form what look like upright petals, often also hooded in shape; two other staminodes are fused together to form a prominent central "lip" or labellum.[2][Note 1]

Flower of Roscoea cautleyoides showing the petals and staminodes

The single fertile stamen has a short filament bearing a cylindrical anther. The connective tissue between the anther's two pollen sacs extends outwards at its base to form spurs. The ovary has three "cells" or locules, eventually producing many small arillate seeds. The single functional style extends upwards through a grove in the stamen to appear above its top.[2][7]

Flower of Roscoea auriculata showing the "spurs" on the stamen

Flower structure and pollination

The orchid-like flowers with a long floral tube appear to be an adaptation for pollination by long-tongued insects specializing in this type of flower.[8] The design of the flower suggests that the lip acts as a landing platform and that if a pollinator puts its head into the flower in order to obtain nectar, it will push down on the spurs on the stamen, causing the anther (and the stigma which is held in front of it) to bend over and contact the insect's back.[9] However, in the only two species so far studied in detail (R. cautleyoides and R. humeana), the actual pollinators were short-tongued pollen-collecting bees.[10] In at least one species, R. schneideriana, it has been shown that if cross-pollination does not occur, the stigma bends over towards the anthers, thus effecting self-pollination.[8] One suggestion is that although the original pollinators may have been long-tongued insects, these are now absent from at least some of the areas where Roscoea occurs, so that the genus has been able to survive in its alpine habitats through the presence of generalist pollinators and self-compatibility.[10]

Taxonomy

Roscoea was named by the English botanist James Edward Smith in 1806. The type species is R. purpurea.[1] The name honours Smith's friend William Roscoe, the founder of the Liverpool Botanic Garden[11] (remnants of which can now be found at Croxteth Hall).[12] Roscoe is known to have been interested in "gingers" (Zingiberales) and to have grown a number of collections of this group of plants.[13]

Evolution and phylogeny

A 2002 classification of the family

Zingibereae, subfamily Zingiberoideae. It was most closely related to the genus Cautleya, and then to Rhynchanthus, Pommereschea and Hedychium.[14] The family is mainly tropical in distribution. The unusual mountainous distribution of Roscoea and the closely related Cautleya may have evolved relatively recently as a response to the uplift taking place in the region in the last 50 million years or so due to the collision of the Indian and Asian tectonic plates.[4]
A molecular phylogenetic analysis of 15 species of Roscoea, based on nuclear
Burma, R. australis).[4]
Himalayan clade
Chinese clade
Distribution of Roscoea according to Ngamriabsakul, Newman & Cronk (2000); Himalayan clade shown in blue, Chinese clade shown in red. Black circle marks the distribution of R. ngainoi, discovered later.[15]

The two clades correspond to a geographical separation, the main distributions being divided by the section of the

Burma.[17] R. ngainoi, discovered after the phylogenetic analysis discussed above, is found roughly in an area in-between these two species, in the Indian state of Manipur.[15]

The material of R. tibetica included in the analysis came from Chinese sources. Later research showed that plants from Bhutan and southern Tibet which had previously been considered to be R. tibetica were actually a new species, R. bhutanica, which belongs to the Himalayan clade.[18] There are few clear-cut morphological differences between the two clades; one is that compared to the Himalayan group, members of the Chinese clade have a larger number of bladeless leaves (i.e. leaves which have only a sheath forming part of the pseudostem).[4]

Species

As of October 2011, the World Checklist of Selected Plant Families accepts the following species of Roscoea:[1]

The hybrid R. auriculata × R. cautleyoides has occurred in cultivation, and has been given the name R. × beesiana Cowley & C.Whitehouse.[1]

  • Variation among the species
  • Roscoea auriculata K.Schum.
    Roscoea auriculata K.Schum.
  • Roscoea capitata Sm.
    Roscoea capitata Sm.
  • Roscoea cautleyoides Gagnep.
    Roscoea cautleyoides Gagnep.
  • Roscoea humeana Balf.f. & W.W.Sm.
    Roscoea humeana Balf.f. & W.W.Sm.
  • Roscoea kunmingensis S.Q.Tong
    Roscoea kunmingensis S.Q.Tong
  • Roscoea nepalensis Cowley
    Roscoea nepalensis Cowley
  • Roscoea purpurea Sm.
    Roscoea purpurea Sm.
  • Roscoea scillifolia (Gagnep.) Cowley
    Roscoea scillifolia (Gagnep.) Cowley
  • Roscoea tumjensis Cowley
    Roscoea tumjensis Cowley
  • Roscoea wardii Cowley
    Roscoea wardii Cowley

Cultivation

Roscoeas in cultivation

Some Roscoea species and cultivars are grown in gardens, particularly rock gardens, as ornamental plants. Coming from monsoon regions, they require moisture in the summer but relatively dry conditions in winter. A moisture-retaining but well-drained soil is recommended, with a mulch of a material such as bark. They vary in their tolerance of sun exposure, most requiring shade for at least part of the day. In a Royal Horticultural Society trial, R. ganeshensis and R. purpurea f. rubra were the most harmed by excessive sunlight; R. cautleyoides, R. × beesiana and R. scillifolia the most resistant. In cultivation they do not appear above ground until late spring or even early summer; thus if planted deeply enough, up to 15 cm (6 in), they escape frost damage in regions where subzero temperatures occur. Different species and cultivars then flower throughout late spring and summer into early autumn.[3][5]

They can be propagated by careful division in the autumn of the individual growth points produced by a large plant, or by seed. Seed should not be allowed to dry out. If sown immediately upon ripening, it will normally germinate in the following summer. Young plants should be potted on before the tuberous roots become too intertwined.[3][5]

RHS trial

From 2009 to 2011, the Royal Horticultural Society held a trial involving around 100 entries submitted by British and other European sources. All proved hardy (rating H4, i.e. hardy anywhere in the British Isles). The Award of Garden Merit was given to 17 entries:[5]

  • R. auriculata and its cultivar 'Summer Deep Purple'
  • R. × beesiana
  • Cultivars of R. cautleyoides – 'Abigail Bloom', 'Himalaya', 'Jeffrey Thomas' and, subject to availability, 'Purple Queen'
  • R. forrestii
  • Forms and cultivars of R. humeana – f. humeana, f. lutea and f. tyria and, subject to availability, the cultivar 'Stephanie Bloom'
  • A form and cultivars of R. purpurea – f. rubra, cultivars 'Dalai Lama', 'Helen Lamb' and 'Red Neck'
  • Subject to availability, R. wardii
  • Three cultivars whose origins are unclear – R. 'Hartington Raw Silk', R. 'Kew Beauty' (possibly a hybrid between R. cautleyoides and R. humeana) and, subject to availability, R. 'McBeath's Pink'.

Notes

  1. ^ Some sources use the term "staminode" only for the lateral staminodes. See e.g. Wilford (2012, p. 79).

References

  1. ^ a b c d e WCSP (2011), World Checklist of Selected Plant Families, Royal Botanic Gardens, Kew, retrieved 2011-10-01, search for "Roscoea"
  2. ^ a b c d Wu, Delin; Larsen, Kai, Roscoea, retrieved 2011-10-01, in Wu, Zhengyi; Raven, Peter H. & Hong, Deyuan, eds. (1994), Flora of China, Beijing; St. Louis: Science Press; Missouri Botanical Garden, retrieved 2011-10-01
  3. ^ a b c Wilford, Richard (1999), "Roscoeas for the rock garden", Quarterly Bulletin of the Alpine Garden Society, 67 (1): 93–101
  4. ^
    doi:10.1017/s0960428600000032
    , retrieved 2011-10-03
  5. ^ a b c d Wilford, Richard (2012), "Roscoea on trial", The Plantsman, New Series, 11 (2): 78–85
  6. ISBN 978-3-8331-1253-9
    , p. 801
  7. ^ Wu, Delin; Larsen, Kai, Zingiberaceae, retrieved 2011-10-01, in Wu, Raven & Hong 1994
  8. ^
    PMID 18682439
  9. ^ Ngamriabsakul, C. (2005), "Morphological study of the versatile anther group in the tribe Zingibereae (Zingiberaceae)" (PDF), Walailak Journal of Science and Technology, 2: 11–12, archived from the original (PDF) on 2012-06-12, retrieved 2011-10-22, p. 18
  10. ^
    PMID 21668599
  11. ISBN 978-0-19-866189-4
    , p. 435
  12. OCLC 953223962.{{cite book}}: CS1 maint: location missing publisher (link
    )
  13. ^ Cowley 1982, p. 748
  14. PMID 21665595
  15. ^
    doi:10.1111/j.0107-055X.2008.00186.x
  16. ^
    doi:10.1016/j.gr.2015.02.004
    .
  17. ^ Cowley 1982
  18. doi:10.1017/s0960428600000202, archived from the original
    (PDF) on 2012-06-12, retrieved 2011-10-04

Bibliography

External links

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