| Roridula | |
|---|---|
 | |
| Roridula dentata | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Asterids |
| Order: | Ericales |
| Family: | Roridulaceae Engl. &Gilg (1924)nom.cons. |
| Genus: | Roridula L. (1764) |
| Species | |
 | |
| Roridula distribution | |
Roridula (/rɒˈrɪdjʊlə/; from Latinroridus "dewy") is a genus ofevergreen, insect-trapping shrubs, with two species, of about 1.3–2 m (4.3–6.6 ft). It is the only genus in the familyRoridulaceae. It has thin, woody, shyly branching, upright, initially brown, later grey stems, with lance- to awl-shaped leaves crowded at their tips. Thestar-symmetrical flowers consist from the outside in of five, green or reddish, freesepals, alternating with five white, pink or purple, freepetals. Further to the middle and opposite the sepals are fivestamens with the anthers initially kinked down. These suddenly flip up if the nectar-containing swelling at its base is being touched. The center of the flower is occupied by asuperior ovary. The leaves and sepals carry many sticky tentacles of different sizes, that trap insects.Roridula does not break down the insectproteins, but bugs of the genusPameridea prey on the trapped insects. These later deposit theirfeces on the leaves, which take up nutrients from the droppings. The species can be found in theWestern Cape province of South Africa. They are commonly known asdewstick orfly bush in English andvlieëbos orvlieëbossie inAfrikaans.[1][2][3][4][5][6]
The two species in the genusRoridula are slenderevergreenshrubs up to 1.3–2 m (4.3–6.6 ft) high. They grow from ataproot with fewside roots. Theperennial, upright, shyly branching stems are leafless, except near the top. The leaves are arrangedalternately, crowded at the tip of the branches, almost as if in a rosette. The leaves areawl-shaped, lack bothstipules and aleaf stalk, either withentire margins or withdistanced line-shaped lobes. The leaves andcalyx are set with different sizes stalked glands or tentacles that secrete a resin.[1]
The5-merousbisexual flowers are set with several inracemes amidst the crowded leaves at the tip of the branches. The freesepals andpetals are well-developed andstar-symmetrical. The broadly inverted egg-shaped petals are white, reddish pink or bright purple and are overlapping in a circle in the bud. Inside the petals are fivestamens that sit opposite the sepals and below theovary. Thefilaments are free, line-shaped, topped byanthers that are connected with the filament near their base. These anthers have two pollen-containing cavities that open inwards through four short slits near the top. The anthers are initially flipped downward on the outside. They are subtended by a swelling that contains a cavity that containsnectar. When the swelling is touched, the anther explosively swings up, releasing the pollen through the slits. Thesuperior ovary in the center of the flower consists of three merged carpels, that together protect three cavities within which are one to fouranatropous ovules each of which is covered by asingle layer. The uprightstyle tapers towards the top and carries a small globe-shapedstigma or expands towards the top into an inverted cone-shaped stigma, covered in small grains. The smooth, cartilaginous,dehiscentfruit opens with three valves. The small, dark reddish brown seeds areellipsoid in shape either with a smoothed netted structure or angular with three sutures and with prominent warts or a honeycomb-like structure.[1]
Both species are diploids having six sets of homologue chromosomes (2n=12).[1]
R. dentata can be distinguished by its larger habit of up to 2 m, the line-shaped lobes along its leaves and the umbel-like inflorescences, whereasR. gorgonias is smaller, up to 1.3 m (4.3 ft), has entire leaves and spike-like inflorescences.[7][8]
Carl Linnaeus describedRoridula in 1764.[1] The nameRoridula derives fromroridus, aLatin word meaning "dewy", which refers to the fine drops of liquid on the tentacles that give the leaves a dewy appearance.[4]
Over time, different botanists have held different views on the affinities of the genusRoridula.Jules Émile Planchon thought it should be assigned to theOchnaceae in 1848.George Bentham andJoseph Dalton Hooker, two of the nineteenth century's most influential British botanists however, included it in 1867 in theDroseraceae. In 1912,Johannes Gottfried Hallier regardedRoridula as a specialized member of theClethraceae.Hutchinson in 1959, andArthur Cronquist in 1981 included it in theByblidaceae. Current insights in the relationships of the Roridulaceae, based on a 2015 DNA-analysis, are summarized in the following tree.[9]
| core Ericales |
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The genusRoridula isendemic to parts of the mountains in the west and southwest of theWestern Cape province of South Africa.R. dentata can be found in the mountains ofTulbagh,Clanwilliam andCeres.R. gorgonias occurs betweenSomerset West andSwellendam. Both species grow on acidic and nutrient-poor, sandy soils, in locations that are relatively moist, at altitudes of 900–1,200 m (3,000–3,900 ft) forR. dentata and 150–925 m (500–3,000 ft) forR. gorgonias.[3][4]
The seeds ofRoridula dentata germinate shortly after a fire has destroyed the vegetation. Flowering mostly occurs in September and December.[3]
Like many other carnivorous plants,Roridula has a rather poorly developed root system, and grows on acidic and leached soils in humid locations.Roridula shares its habitat with severalDrosera species. LikeDrosera,R. gorgonias strongly absorbs UV and this is assumed to attract flying insects.[1] BothDrosera andRoridula trap large numbers of various flying insects.[10] UnlikeDrosera,Roridula gorgonias secretes a very sticky resinous substance, mainly containingacylglycerides andtriterpenoids that are insoluble in water. This implies that insects can even be trapped during rainy weather.[11]
Roridula does not respond by bending tentacles to struggling insects, unlikeDrosera, that secrete a sticky mixture of saccharides or proteins. Instead, it carries three types of glandular hairs that differ in size: long, medium, and short. The long tentacles are less sticky, and by struggling, the insect comes in contact with the much more sticky medium and short glands, which completely immobilize it.[12]
Carnivorous plants likeDrosera secrete enzymes that break down proteins (so-calledproteases) from the captured insects, and so make available nitrates to these plants, that grow in soils with low ammonium and nitrate content.R. gorgonias however lacks proteases and is thus unable to extract these nutrients from trapped prey directly. Instead, each plant houses individuals of thebugPameridea roridulae, which quickly close in on the trapped insects and feed on them. The bugs consequently defecate on the leaves. In anitrogen-15 tracing experiment, where prey insects enriched with the rare heavy nitrogen isotope were eaten byPameridea bugs, the share of N15 increased, showing that the plant had taken up nitrogen nutrients from the captured insects.[13]
Pameridea was shown to have a thick greasy layer that prevents direct contact between the resin onRoridula tentacles and the insect'scuticle, this allows them to roam freely across the plants.[14]
R. dentata also has a hemipteran resident,Pameridea marlothi, and may receive nitrogen nutrients in much the same way. In addition, several crab spider species of the genusSynema can be found on the plantand these may both prey on the captive insects as well as on the resident bugs. The unrelated Australian genusByblis resemblesRoridula in having sticky tentacles, and lives together with hemipteran bugs in much the same way.[13] UnlikeRoridula,Byblis also produces its own digestive enzymes, but this was not proven until 2005.[15]
Pameridea is assigned to theMiridae, a family of bugs that further live from sucking plant juices. AlthoughPameridea depends on insects with their high protein content that have been captured byRoridula for completing its life cycle, it can survive on plant juices. In case of a fire, the bugs probably evacuate their home plant and fly off. Even if they do not find anotherRoridula specimen, the bugs can sit out the period untilRoridula plants have germinated and sufficiently grown, by sucking juices of other plant species.[16]
Fragments of fossil leaves, morphologically very close toextantRoridula, have been found in two pieces ofamber ofEocene age (35–47 million years old), from theYantarny mine nearKaliningrad. These leaf tips are set with both non-glandular hairs and stalked glands (or tentacles), and have a linear-triangular shape that narrows to a terminal stalked gland. The hairs are on both surfaces and the margins, consist of a single cell, taper to a point, and are 10 to 80 μm long. The stalked glands are restricted to the margins and lower surface. The glands consist of many cells, forming a tapering stalk and a thick, hoof-shaped or egg-shaped head, with a small pore at its very tip. The stalks are 20–350 μm long (one stalk reaching 1.4 mm), the heads 20–120 μm long and 10–40 μm wide. Organic remains and hairs of other plant species are positioned on the heads, suggesting these were sticking to a secretion from the heads. Thesurface of the leaf consists of smallsix-sided cells at the base of the leaf and long, larger cells from the center to the tip of the leaf. These epidermal cells are 3–54 μm long and 6–18 μm wide.Stomata of 20–38 μm long and 15–25 μm wide are restricted to the underside of the leaf. Leaf shape, size and shape of the stomata and epidermal cells, the presence of non-glandular hairs, and of stalked glands that strongly differ in size on the lower leaf surface and on the margins including a terminal tentacle, as well as the head of the tentacle having anapical pore, are all characters shared with extantRoridula species. These fossils differ from today's dewsticks in having smaller tentacles, and in lacking a prominent midrib on the underside of the leaf.[17]
The pieces of amber with the roridulid remains were found in adeposit formed in a forested swamp on a nutrient-poor and carbonate-free soil in a coastal area, with both angiosperm and conifer trees in a warm-temperate or subtropical climate. The presence of roridulids in theNorthern Hemisphere during the Eocene questions the assumption that the family originated inGondwana, about 90 million years ago.[17]
Early settlers in parts of the Cape used to hangRoridula branches from the ceiling to catch flies.[16]
