Geography and distribution
This species is restricted to a small part of central Tanzania. This region falls within the rain shadow of the Eastern Arc Mountain chain which, together with the sandy soils typical of the area, results in very dry growing conditions and a thornbush vegetation dominated by Acacia and Commiphora trees, and with sparse ground cover.
Flowering spike of Barleria aristata
The central Tanzanian bushlands are known for their high numbers of unique plant species. Indeed, Barleria aristata is just one of four Barleria species unique to the region, the others being B. granarii, B. pseudosomalia and B. subregularis, all of which have only been described as new to science in the last three years by Kew botanist Dr Iain Darbyshire.
Overview: Barleria aristata is an undershrub up to 2 m tall, the vegetative parts being covered in golden, star-shaped (stellate) hairs. The mature stems are woody with pale grey bark.
Leaves: The leaf stalks are absent or short, and the leaf blade is narrowly elliptic or lance-shaped, 3–8.5 × 1–2 cm, with 4 or 5 pairs of lateral veins impressed on the upper surface and prominent beneath.
Flowers: The flowers are held in dense conical or near-globose heads, 1.5–4 cm long, at the ends of the branches. The bracts are broad, with a long, slender, curved tip ending in a spine. The paired bracteoles are similar but narrower. The calyx is divided almost to the base into four highly unequal lobes. The front lobe is broad and elliptic or rhomboid in shape, 15–22 × 8–11 mm, and narrows into a pair of divergent spiny tips. The back lobe has a short, broadened base, 6–8 × 4–7 mm, before narrowing into a long, slender tip, 10–16 mm long, ending in a sharp spine. The two lateral lobes are 7–11 mm long and very slender. The flower is 35–42 mm long and blue or blue-purple with a paler throat. It has a narrowly cylindrical tube, 14–17 mm long, before dividing into 5 large lobes, the lowermost splitting from the tube slightly earlier than the remainder, each lobe being 15–22 mm long. The two white stamens are attached midway along the corolla tube, the anthers being held beyond the tube. There are also two (rarely three) minute staminodes less than 2 mm long and hidden within the tube. The ovary is short and hairless. The style is long and thread-like, the linear stigma being held beyond the flower tube.
Fruits: The fruit is a hairless capsule with two valves, elliptic in face-view, and flattened laterally with a short beak. The two seeds are disc-shaped and are covered in long, silky hairs. They are held on long hooks which help to project the seeds from the capsule when it opens explosively.
This species is typical of section Stellatohirta of the genus Barleria, in which the species all share the complex star-shaped hairs, the dense globose to spike-like flower-heads and the two-seeded capsules with silky-haired seeds. The genus as a whole is easily recognised by its unusual 4-lobed calyx, of which the front and back lobes are usually much broader than the lateral pair.
Threats and conservation
Stands of Barleria aristata in typical bushland habitat.
Barleria aristata has a very restricted range and is globally rare, with only two main population centres, one in the Kitonga Gorge-Lukose River area and one further north in the vicinity of Mpwapwa. However, it can be locally abundant or even dominant in the understorey and appears tolerant of moderate habitat disturbance, being able to withstand grazing pressure and apparently being at home along roadsides.
The busy Tanzania-Zambia highway has inevitably attracted local increases in human population in the Kitonga-Lukose area but human impact is stable at present and much of the bushland remains intact. While this species is therefore currently considered to be of Least Concern (LC), any future increase in the scale of habitat loss would quickly render it vulnerable.
The main populations are not covered by any formal conservation measures at present, although Barleria aristata has been recorded from the lowland portion of the Udzungwa Mountains National Park.
As with many species of Barleria, the striking flowers of B. aristata have high horticultural potential, yet only a few Barleria species are widely cultivated. They are said to be susceptible to disease.
Millennium Seed Bank: Seed storage
Barleria aristata is not currently represented in any seed bank. Attempts were made by the Millennium Seed Bank Tanzania team and Kew botanists to collect seeds in June 2008 but, as with many Acanthaceae species, high levels of seed predation and difficulties in finding mature seeds meant that insufficient quantities of viable ones could be collected.
Like many species in the genus, Barleria aristata has high horticultural potential, though it is not currently known to be in cultivation.
This species at Kew
Barleria aristata is well-represented in the collection of dried, pressed specimens housed in the Herbarium at Kew, which includes the type specimen collected by a Kew-led expedition to Tanzania in March 2006. The details of this specimen can be seen online in the Herbarium Catalogue. The oldest known collection of B. aristata dates back to 1925, and was collected by the German railway worker and prolific plant collector (Gustav) Albert Peter.