Roscoea


Roscoea is a genus of perennial plants of the family Zingiberaceae. Most members of the family are tropical, whereas Roscoea species are native to 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 become like petals. Some species are grown as ornamental plants in gardens.

Description

Roscoea is found from Kashmir through the Himalayas to Vietnam, extending northwards into China. There are up to 22 recognized species, of which 8 are endemic to China. Typically they grow in grassland, in screes or on the edges of deciduous woodland at heights of, coming into growth at the start of the monsoon season.
Species of Roscoea are small perennial herbaceous plants. They die back each year to a short vertical rhizome, to which the tuberous roots are attached. Like many members of the Zingiberales, Roscoea has "pseudostems": structures which resemble stems but are actually formed from the tightly wrapped bases of its leaves. The leaves are without a stalk. Lower leaves may consist solely of a sheath; upper leaves have a blade which is free from the pseudostem, and is oblong or lanceolate. The relative number of bladeless versus complete leaves is one distinguishing feature of the two clades into which the genus is divided.
The flowers are borne in a spike at the end of the pseudostems. The stalk bearing the flowers may be long, so that the flowers appear well above the leaves, or short, so that they appear between the upper leaf sheaths. Like other members of the ginger family, Roscoea flowers have a complex structure. Each flower has a tube-shaped outer calyx, which is split on one side and ends in two or three teeth. The petals are joined together at the base, and then divide into three lobes. The central lobe is upright and usually forms a hood; the two side lobes are narrower than the central one. The flower then has what appear to be three inner petals, which are actually formed from four sterile stamens. 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.
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.

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. 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 to bend over and contact the insect's back. However, in the only two species so far studied in detail, the actual pollinators were short-tongued pollen-collecting bees. 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. 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.

Taxonomy

Roscoea was named by the English botanist James Edward Smith in 1806. The type species is R. purpurea. The name honours Smith's friend William Roscoe, the founder of the Liverpool Botanic Garden. Roscoe is known to have been interested in "gingers" and to have grown a number of collections of this group of plants.

Evolution and phylogeny

A 2002 classification of the family Zingiberaceae, based on molecular phylogenetic analysis, placed Roscoea in the tribe Zingibereae, subfamily Zingiberoideae. It was most closely related to the genus Cautleya, and then to Rhynchanthus, Pommereschea and Hedychium. 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.
A molecular phylogenetic analysis of 15 species of Roscoea, based on nuclear ribosomal DNA, showed that the genus was monophyletic, and distinct from the closely related genus Cautleya. The 15 species fell into two clear groups, a Himalayan clade and a Chinese clade.
The two clades correspond to a geographical separation, the main distributions being divided by the section of the Brahmaputra River which flows south at the end of the Himalayan mountain chain. Historical biogeography study revealed that evolutionary split between Cautleya and Roscoea occurred during the middle and late Eocene to the early Oligocene, corresponding well to the proposed early uplift of the Himalayas and Qinghai-Tibetan Plateau. Roscoea species were then divided into two distinct clades, simultaneous with the rapid extrusion of Indochina and accompanied by the another Himalayan uplift around the Oligocene/Miocene boundary. It has been suggested that the genus may have originated in this area and then spread westwards along the Himalayas and eastwards into the mountains of China and its southern neighbours, supported by biogeographic reconstruction. Three species have an isolated distribution. R. brandisii, a member of the Himalayan clade, occurs to the south of the rest of the clade in the Khasia Hills in the modern Indian state of Meghalaya. R. australis, a member of the Chinese clade, occurs even further south, in the Chin Hills of Burma. R. ngainoi, discovered after the phylogenetic analysis discussed above, is found roughly between these two species, in the Indian state of Manipur.
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. 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.

Species

, the World Checklist of Selected Plant Families accepts the following species of Roscoea:
The hybrid R. auriculata × R. cautleyoides has occurred in cultivation, and has been given the name R. × beesiana Cowley & C.Whitehouse.

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 too much sun; 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, they escape frost damage in regions where subzero temperatures occur. Different species and cultivars then flower throughout late spring and summer into early autumn.
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 it is ripe, it will normally germinate in the following summer. Young plants should be potted on before the tuberous roots become too intertwined.

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. The Award of Garden Merit was given to 17 entries: