Leucospermum

Leucospermum, commonly known as pincushions, is a genus of evergreen upright, sometimes creeping shrubs that is assigned to the Proteaceae, with currently 48 known species.

The shrubs mostly have a single stem at their base, but some species sprout from an underground rootstock, from which the plant can regrow after fire has killed the above ground biomass. In a larger group of species, specimens are killed by fire, and their survival depends on the seeds. In all species, seeds are collected by ants, which take them to their underground nests to feed on their ant breads, a seed dispersal strategy known as myrmecochory. This ensures that the seeds do not burn, so new plants can grow from them.

Leucospermum species mostly have seated, simple, mostly leathery, often softly hairy leaves, set in a spiral, with entire margins or more often, with 3–17 blunt teeth with thickened, bony tips, and without stipules at their foot. The flowers are organised with many together in heads with bracts on the under- or outside. The hermaphrodite flowers themselves are set on a common base that may be cylindrical, conical or flat, and have small bracts at their base. The flowers have a perianth that is hairy on the outside, particularly at the tip, and consists of four tepals that are merged into a tube. Usually the four anthers are merged individually with the tip the perianth lobes, and only in a few species, a very short filament is present that further down cannot be distinguished from the tepals anymore. While still in the bud, the pollen is transferred from the anthers to the pollen-presenter, a thickening at the tip of the style. At that stage, the style grows considerably and rips through the sutures between the two perianth lobes facing away from the centre of the flower head. The perianth lobes all four remain attached to each other, or with three, or the four free lobes all curl back on themselves (like the lid of a sardine can), rimming the top of the tube. The superior ovary consists of one carpel and contains a single ovary, and is subtended by four small scales. The fruit is an oval or almost globe-shaped nut.

Most species have very limited ecological ranges and distribution areas, and many are rare or endangered. The often attractive, large flower heads and evergreen foliage, the straight stems, combined with long flowering period makes that Leucospermum species and their hybrids are bred as garden ornamental and cut flower.

Description

upright=1.35|thumb|left|Morphology of the individual flower and leafupright=1.35|thumb|left|the positioning of flower buds on the [[Receptacle (botany)|receptacle as seen from above]]Most pincushions are upright shrubs or even small trees of high, that usually have a single main stem. Some species however only have trailing branches and can form low mats, in diameter. Yet another set of species grow several stems directly from a rootstock in the ground. This is an important character in distinguishing between some species. Dried specimens of L. pedunculatum and L. prostratum can be difficult to distinguish, but although both are prostrate species, the growth habits in the field differ considerably. In L. pedunculatum many horizontally spreading branches develop from an about main stem, in L. prostratum the branches rise from an underground woody rootstock. The leaves are alternately set along the stem, distanced and slightly pointing towards the tip of the branch or overlapping, mostly without, sometimes with a leaf stalk but always without stipules at their base, long and linear, elliptic, oblanceolate, oval, inverted egg-shaped or spade-shaped, the edge entire or with up to 17 teeth towards the tip, hairless or with a covering of soft cringy one-celled hairs, sometime interspersed with longer straight silky hairs.

thumb|right|Detail of an individual flower showing the white fruit that the genus is named after. Photo: Tony Rebelo.Leucospermum differs from genera such as Protea, Leucadendron, Mimetes, Diastella, Paranomus, Serruria, and Orothamnus by having the flower heads in the axils of the leaves (although often very near the tip of the branch), small and inconspicuous bracts subtending the head, brightly coloured styles that are straight or curve toward the center of the flower head and extend far from the perianth, giving the flower head the appearance of a pincushion, and large nut-like fruits covered by a pale and soft layer that attracts ants. The style breaks out of the bud at the side facing the rim of the head, and the perianth lobes may stick together with four or three forming a sheath, or roll back individually.

Cardinistyle

The six species that are assigned to the section Cardinistyle are sometimes called fireworks pincushions. They are all large upright shrubs, with only one main stem. The common base of the flowers is a narrow cone with a pointy tip. The flowers have styles of long that move downward when the flowers open, and are topped by a narrow pollen presenter ending in a sharp tip. L. reflexum has oval or narrowly oval greyish, felty leaves of long and wide. The perianth is yellow or scarlet long, and a style uniquely pointing downwards when the flower is open.

The species assigned to the section Leucospermum are sometimes called sandveld pincushions. Among it are both upright, spreading and creeping shrubs, and leaf-shapes vary from line- to egg- and wedge-shaped, but they all have felty hairy leaves, even when aged. The bud is usually straight, always with a sweet scent and colored brightly yellow. In the open flower, the three perianth lobes at the side of the center of the flower head remain attached, while the remaining lobe is free. The pollen presenter at the tip of the style is cylindrical or club-shaped. L. hamatum in 1983, Species within the genus are commonly known as pincushions.

Phylogeny

Comparison of homologous DNA has increased the insight in the phylogenetic relationships between the Proteaceae. Leucospermum belongs to a group that further only consists of genera endemic to the Cape Floristic Region, that together constitute the subtribe Leucadendrinae. Leucospermum is most related to Mimetes, which however is only monophyletic if both Diastella and Orothamnus would be included in it. A subgroup of Paranomus, Vexatorella, Sorocephalus and Spatalla is the sister group to the Leucospermum-Mimetes subgroup. The following tree represents those insights.

Subdivision

The genus Leucospermum is divided into nine groups called sections. These are Brevifilamentum, Cardinistyle, Conocarpodendron, Crassicaudex, Crinitae (synonym Diastella <small>Meisn. non (Salisb.) Endl.</small>), Diastelloidea, Hamatum, Leucospermum (synonym Hypophylloidea) and Timiditubus.

The following taxa are assigned to the respective sections.

L. alpinum subsp. alpinum = Vexatorella alpina
L. alpinum subsp. amoenum = Vexatorella amoena
L. glaberrimum = Leucadendron glaberrimum
L. involucratum = Leucadendron salignum
L. marginatum = Leucadendron spissifolium
L. obtusatum = Vexatorella obtusata
L. ovatum = Protea longiflora
L. rochetianum = Faurea rochetiana
L. zwartbergense = Leucadendron dregei

Names that cannot be assigned

For Leucadendron filiamentosum, L. polifolium and L. bellidifolium, no type specimens could be found, and their descriptions are too general to determine which Leucospermum species they are synonymous with. For L. obovatum, no description has been provided, so it is a nomen nudum.

Seed dispersal

upright=1.35|thumb|left|Seedhead of [[Leucospermum glabrum]]The fruits of Leucadendron have but one seed cavity, that does not open, and contains only one seed, a fruit type called nut. The fruits consist partly of a whitish, fleshy or gelatinous pericarp, a so-called elaiosome, that attracts ants because they contain chemicals that mimic pheromones. After the fruits fall from the plant, mostly Anoplolepis ants gather them, and carry them to their nest by sinking their jaws in the fleshy elaiosome. Once in the underground nests, the elaiosome is consumed. The smooth and hard seeds that remain do not fit the ants' small jaws, and are abandoned, protected from fire and seed eaters. The survival of the seeds is further enhanced by fungicidal and anti-bacterial substances that the ants excrete to keep their nests in a healthy condition. In the fynbos, this so-called myrmecochory is a strategy used by many plant species to survive the fire. Invasive ants species, like in South Africa Linepithema humile (Argentine ant), destroy the nests of the indigenous ants, and eat the elaiosomes where ever the seed has fallen, so that it is not protected against fire and can easily be found and eaten by mice and birds.

Fire

Periodic wildfires are an important factor in south and west South Africa. The occurrence of these fires among other things determines the extent of fynbos. All species that naturally occur in the fynbos have adaptations that ensure these species can survive the natural fire regime, but different species have different strategies. This is also true for the species of Leucospermum, even the few that occur outside the fynbos. A large majority of Leucospermum species is killed by fire because these have a single stem that only branches higher up, and are covered by a rather thin bark. One year after the fire however, many seedlings have occurred. All specimens within the area covered by the most recent fire, are therefore of the same age. After three to four years, these plants begin to flower and produce seeds, that do not yet germinate, but remain in the soil seed bank, until they get activated during the aftermath of a fire. Specimens belonging to these species are subject to biological aging (or senescence), and lose their vitality. The maximum life expectancy differs between twenty-five to thirty years in smaller species like L. truncatulum and L. oleifolia, to fifty to eighty years in L. praemorsum. For this group of species, fire is a prerequisite to rejuvenate and so maintain the population. If the fires occur as frequent as every two or three year however, the soil seed bank gets depleted because no new seeds are added, and the species may locally disappear. A number of large species (L. conocarpodendron, L. heterophyllum, L. patersonii, L. pedunculatum, L. profugum and L. royenifolium) have thick bark, which allows them to survive fires if these are not too intense, and so stretch their lifespan regularly beyond the interval between successive incidents. The fire survival rate in this group was estimated at 30–50 %. Since the fire destroys lower branches, regrowth only takes place from the higher branches, and the plants attain an umbrella-shape. A smaller group of Leucospermum species has a more effective method to survive fire. Above ground parts of these species die, but new shoots appear directly from the ground from woody tubers. This mechanism is best developed in the species of the section Crassicaudex (L. cuneiforme, L. gerrardii, L. innovans and L. saxosum) that mostly occur outside the fynbos, in areas with dominant summer rainfall where fires may be more frequent, but is also present in L. hypophyllocarpodendron, L. prostratum and L. tomentosum. The survival rate in this group is estimated at 95% or more. The young plants of these species can be distinguished because of the profuse development of side branches very low on the primary stem.

Cultivation

The breeding of pincushions provides an important export product in South Africa and a few other countries. L. conocarpodendron, L. cordifolia, L. lineare, L. patersonii and L. vestitum and a range of hybrids supply cut flowers.