Isoglossa variegata

A newly described rare species of forest floor herb, discovered growing in the glasshouses at Kew!

Isoglossa variegata
Isoglossa variegata

Species information

  • Scientific name: Isoglossa variegata I.Darbysh
  • Conservation status: Isoglossa variegata is considered to be globally threatened, with a provisional IUCNrating of Endangered (EN B1ab(iii), B2ab(iii)).
  • Habitat: This species is recorded from rainforest at 800 to 1,100 m above sea level, being noted from both primary and disturbed forest and in the understorey of plantations. In a collection made in 1950 it was recorded as locally dominant in the herb layer on Mount Bomole in the Usambara Mountains.
  • Known hazards: None known.

Taxonomy

  • Class: Equisetopsida
  • Subclass: Magnoliidae
  • Superorder: Asteranae
  • Order: Lamiales
  • Family: Acanthaceae
  • Genus: Isoglossa

About this species

Isoglossa variegata was first collected in the East Usambara Mountains by the prolific German plant collector, Albert Peter, in December 1914. Several subsequent herbarium collections were made from the same region over the course of the twentieth century. One gathering from 1950, by the eminent East African botanist and long-term Kew staff member Dr Bernard Verdcourt, bears the inscription 'NAME URGENTLY DESIRED'. However, due in part to the very limited taxonomic research into East African Acanthaceae until recent times and in part to the confusion over the generic placement of this species (see discussion below) the species remained undescribed.

Whilst working on the Acanthaceae for the Flora of Tropical East Africa, Dr Iain Darbyshire of the Herbarium at Kew stumbled across living material of this species, flowering in the Princess of Wales Conservatory, during a lunchtime stroll in the gardens in November 2006. Further investigation revealed this collection to have been donated to Kew by Mariette Manktelow, then a researcher at Uppsala University in Sweden with an interest in the Acanthaceae family. She had collected the plant in the Nguru Mountains, a new site for this species. With this excellent live material complementing the existing herbarium specimens, sufficient material was available for this species to at last be formally described and Isoglossa variegata was published in late 2009, almost 60 years after Dr Verdcourt’s urgent plea.

Geography & Distribution

Isoglossa variegata is restricted to the East Usambara and Nguru mountain ranges within the ancient Eastern Arc Mountain chain of Tanzania, perhaps the most biologically rich region of tropical Africa.

Description

More Information
Dr. Verdcourt's Isoglossa variegata herbarium specimen

Dr Verdcourt's herbarium specimen of Isoglossa variegata

Isoglossa variegata is a slender perennial forest floor herb with striking variegated leaves and a slender spike of small white and purple-marked, two-lipped flowers.

The stems are initially creeping but become ascending or erect in the leafy and flowering portion, being 10 to 30 cm tall. The leaves are stalked for up to 45 mm and have a heart-shaped blade of up to 7.5 by 5.5 cm. The upper surface of the leaves is variegated, paler around the midrib and along the lower portion of the principal veins. The upper leaf surface has a few conspicuous pale hairs between the veins. There are also finer hairs along the midvein above, lateral veins beneath, and along the margin.

The flowering spike is 5 to 14 cm long, rarely with a single branch developing in the lower half. The flowers are well spaced and solitary, or paired along the axis. The bracts are obovate (egg-shaped in outline, with the broadest portion near the apex). The bracts are up to 6 by 3.5 mm and glandular-hairy. Each flower is held above a pair of lance-shaped bracteoles 0.8 to 2.5 mm long. The five sepals are narrowly lance-shaped, 2 to 3 mm long and are glandular-hairy. The flower is white and 9 to 11 mm long. The petals are initially united into a tube (5.5 to 6.5 mm long), and then they split into two lips. The upper lip is shortly two-lobed; the lower lip has three distinct lobes and pale purple markings towards the throat.

The two stamens are attached to the corolla tube. The two anther cells on each stamen are offset, overlapping for half their length, and are hairy. The ovary is largely hairless and narrows into a slender style, which is hairy towards the base. The fruits and seeds of this species are currently unknown.

The pollen of this species differs from the 'typical' pollen type in Isoglossa, which has led to past disagreement over its generic placement; several of the herbarium specimens have been provisionally identified as a Justicia. However, other morphological characters more closely match Isoglossa than Justicia, particularly with regard to the anthers. Furthermore, several species of Isoglossa in tropical Asia share the same pollen type with Isoglossa variegata.

Threats & Conservation

This species is currently known from only ten collections from the East Usambara and Nguru mountain ranges. It is apparently absent from many forests within these ranges, the reasons for which are unknown. Although it can be locally abundant, the widespread clearance of lowland and sub-montane forest in the vicinity of Amani (this species’ stronghold) is likely to have significantly reduced or destroyed several of the populations. Few of its sites fall within formally protected forest and it is therefore likely that further population declines will continue to occur. It is therefore considered to be globally threatened, with a provisional IUCN rating of Endangered.

This species at Kew

This species is currently known in cultivation only from the Royal Botanic Gardens, Kew, where it can currently be found on display in the Princess of Wales Conservatory.

Several dried and pressed specimens are held in the Herbarium at Kew, including the type (or reference) collection. Herbarium specimens are made available to bona fide researchers by appointment.

Useful Links

Further information on a selection of Kew’s collections of Isoglossa variegata can be found on the Kew Herbarium Catalogue

Find out more about the Flora of Tropical East Africa project

A wealth of further information on the biodiversity of the Eastern Arc Mountains can be found at the Eastern Arc homepage, which is run by the Eastern Arc Mountains Conservation Endowment Fund.

Search Kew's science databases for more information on this species

References & Credits

Darbyshire, I. (2009). Taxonomic notes and novelties in the genus Isoglossa (Acanthaceae) from east Africa. Kew Bulletin 64: 401–427.

Kew Science Editor: Dr. Iain Darbyshire
Copy editing: Emma Tredwell

While every effort has been taken to ensure that the information contained in these pages is reliable and complete, the notes on hazards, edibility and suchlike included here are recorded information and do not constitute recommendations. No responsibility will be taken for readers’ own actions. Full website terms and conditions.

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