General Info – summary

This 3m high, Tree is often a multi-stemmed shrub. Simple aromatic Leaves lack stipules, are elliptic to lanceolate & occur in whorls of up to 4. Margins may be narrowly scalloped. Bisexual, zygomorphic, bracteate, oil producing Flowers have bilabiate, joined yellow petals, attached didynamous stamens and 1 staminode. Single style & pistil + superior ovary. Fruit a small 2-lobed woody capsule with many seeds.

Description

Bowkeria citrina

Previous Names: Nil.

SA Tree No. 672.1

Common names: (Afr) Geelskulpblombos. (Eng) Yellow Shell-flower.

Family: Stilbaceae: (Family with 6 genera and about 14 species). Local genera on this website include Anastrabe, Bowkeria, Halleria, and Nuxia. Members of this family have simple and entire Leaves that lack stipules and have recurved margins. Non-stellate hairs are present. Flowers are bisexual and zygomorphic or regular and the corolla is funnel-shaped and usually 2-lipped. The usually 4 Stamens are epipetalous (born on or arising from the petals), and the usually superior Ovary initially has 2 locules each with 1 ovule. Fruit is usually a dehiscent capsule but may be a berry.

Name derivation: Bowkeria – named after two naturalists Col. H. Bowker (1826-1900) – and his sister M. S. Barber who was a botanist. He was better known as an Entomologist and Botanist. He was also the co-author with Roland Trimen of a book on ‘South African Butterflies’. The entire genus Bowkeria has 3 species) is endemic in Southern Africa. The other 2 species are B. cymosa (the tallest Bowkeria) and B. verticilata (both are mountainous species). citrina – lemon yellow – referring to the flowers.

Conservation: National Status: Rare but not threatened. Assessed: 2005/09/15. (K.E. Steiner and K. Naidoo). This plant has a very restricted natural range.

Tree

The Tree may reach 3m high, but it is usually a multi-stemmed aromatic shrub with sessile glands on many parts. The Bark is often dark grey (photos 629 & 590 under Leaves) to greyish white (photo 544). In addition, there are small light coloured and distinctly raised lenticels (usually raised corky oval or elongated areas on the plant that allow the uncontrolled interchange of gases with the environment – photos 629 & 544). Fine soft golden Hairs are present on young branches.

: 176M1 2014.12.02 Walter Sisulu NBG. Photo: David Becking.

: 629 2015.04.14 Walter Sisulu NBG. Photo: David Becking.

: 544 2014.10.28 Walter Sisulu NBG. Photo: David Becking.

Leaves

The soft, slightly leathery Leaves occur in whorls of 3 or 4 (photo 590) or they are occasionally opposite (photo 590). They are narrowly elliptic to lanceolate and up to 8 x 1cm (photo 540). All leaves are simple (have a single blade which may have incisions that are not deep enough to divide the leaf into leaflets). The Margin is hairy (photo 118) and narrowly scalloped (photo 540) – especially in young plants. It may also be entire (with a continuous margin, not in any way indented). The Blade may have fine hairs of different intensity. The Upper surface is a dull olive green with a sunken midrib. The Lower surface is much lighter and has a clearly visible yellow protruding midrib (photo 540). The midrib and lateral veins are yellowish and tend to arch along the margin prior to reaching it. All veins are best viewed when the leaf is held against the sun (photo 118). The Apex tapers to a point. The Base also tapers – slightly less so. The Petiole (leaf stalk) is greenish yellow, hairy and up to 7mm long (photo 540). Stipules (basal appendages of the petiole) are absent. Leaves have a pleasantly lemony aromatic flavour when crushed. This is the only species in the genus Bowkeria with this diagnostic characteristic.

: 590 2015.10.20 Walter Sisulu NBG. Photo: David Becking.alter Sisulu

: 118 2016.05.31 Walter Sisulu NBG. Photo: David Becking. Leaf against the sun.

: 540 2014.10.28 Walter Sisulu NBG. Photo: David Becking.

Flowers

The attractive bisexual axillary and solitary Flowers are bracteate (bearing Bracts – modified specialised leaves usually found with inflorescences and flowers). There are 2 darker green lanceolate subtending bracts below each flower (photo 822). The flowers are zygomorphic (irregular flower with the corolla divisible into 2 equal halves in one plane only) and are up to 2 x 2cm. Usually, 1 but up to 3 flowers develop in leaf axils. The 5-lobed, hairy (photo 293) Calyx has longish different sized sepals with overlapping edges (photo 822). Three sepals point forwards (largest in the centre) and two backwards. The Petals are joined to form a bell-shaped tube with many attractive small red marks near the interior base and are more clearly visible if the tube is opened (photo 807). The Corolla is removable as a single piece. The attractive, bright yellow flowers are bilabiate (2-lipped) and scuttle-shaped (photo 168). The yellow colour is unique for this genus. The other species all have white flowers. The Upper lip is 2-lobed and the Lower lip is 3 lobed. The 4 Stamens are in 2 pairs and are attached to the base of the petals. Each pair has stamens of equal length but a different length from the other pair. They are said to be didynamous (having 4 stamens disposed in 2 pairs of 2 different lengths – photo 686). Stamens are not exserted (do not stick out) beyond the mouth of the petals. The anterior pair is basally twisted. The Anthers have 2 divergent pollen sacs (photo 807). A single Staminode is also present. There is a single Pistil (a unit of the Gynoecium, the female element of the flower, composed of the Ovary, Style and Stigma). The superior bilocular Ovary contains many ovules and has a long single Style (photo 293) which extends after the anthers have developed (this can be seen in photos 686 & 293). This plant is pollinated by oil-collecting bees (Rediviva sp.), which feed the collected oil to their larvae (Dec-May). In 2014, this plant was also flowering in October at Walter Sisulu NBG (photo 590 – under Leaves).

: 822 2016.11.29 Walter Sisulu NBG. Photo: David Becking.

: 293 2018.01.11 Walter Sisulu NBG. Photo: David Becking. Dissected.

: 807 2017.02.07 Walter Sisulu NBG. Photo: David Becking. Dissected.

: 453 2014.12.09 Walter Sisulu NBG. Photo: David Becking.

: 168 2014.12.02 Walter Sisulu NBG. Photo: David Becking.

: 686 2014.12.23 Walter Sisulu NBG. Photo: David Becking. Dissected.

Fruit

The eventually brown, ovoid, slightly woody Fruit is a small – about 10 x 5 mm Capsule (a dry fruit resulting from the maturing of a compound ovary, which usually opens at maturity by one or more lines of dehiscence). Here it splits into 2 valves (separable part of a pod) and the line of dehiscence lies between the carpels (carpel – female flower part containing a stigma, style (usually), ovary & ovule/s). The carpel is an organ (generally believed to be a modified foliar unit – related to leaves) at the centre of a flower, bearing one or more ovules and having its margins fused together or fused with other carpels to enclose the ovules in an ovary. The now open Calyx (photo 301) remains attached to the base of the capsule (photo 301) which splits into 2 valves. Fruit and flowers often develop at the same time. Many very small ellipsoid Seeds are produced. (Feb-Jul).

: 301 2016.09.15 Walter Sisulu NBG. Photo: David Becking.

Distribution & Ecology

This range restricted plant is Endemic (Endemism is the ecological state of a species being unique to a defined geographic location) in South Africa. Here there are 5 natural populations. Bowkeria citrina is naturally located in North-West KwaZulu-Natal, and southern Mpumalanga in moist highland grasslands. Plants occur on forest margins, stream banks, below rocky hillsides-including cliff edges, margins of evergreen forests and has an altitude range of 1 400 – 1 800m. This plant secrets non-volatile oil that attracts female oil collecting bees in the genus Rediviva. The oil is collected deep within the flowers by the extra-long front legs of the female bee and fed to bee-larvae. In the process the female bee aids with pollination. “Ordinary” bees are also involved in the nectar collection and in pollination.

Ethnobotany

These slow growing plants do best if planted in the open – in moist areas. Although the plant is not frost sensitive, it will not survive a prolonged dry period. Cuttings are the best means of propagation.