Роридула: Насекомоядное растение Южной Африки

Insect trapping shrublet from South Africa

Roridula (/r//ɒ/'/r//ɪ//dj//ʊ//l//ə/; from Latin roridus "dewy") is a genus of evergreen, insect trapping shrubs, with two species, of about 1⅓–2 m (4–6⅔ ft). It is the only genus in the family Roridulaceae. It has thin, woody, shyly branching, upright, initially brown, later grey stems, with lance to awl shaped leaves crowded at their tips. The star symmetrical flowers consist from the outside in of five, green or reddish, free sepals, alternating with five white, pink or purple, free petals. Further to the middle and opposite the sepals are five stamens 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 a superior ovary. The leaves and sepals carry many sticky tentacles of different sizes, that trap insects. Roridula does not break down the insect proteins, but bugs of the genus Pameridea prey on the trapped insects. These later deposit their feces on the leaves, which take up nutrients from the droppings. The species can be found in the Western Cape province of South Africa. They are commonly known as dewstick or fly bush in English and vlieëbos or vlieëbossie in Afrikaans.

The two species in the genus Roridula are slender evergreen shrubs up to 1⅓ m (4 ft) or 2 m (6⅔ ft) high. They grow from a taproot with few side roots. The perennial, upright, shyly branching stems are leafless, except near the top. The leaves are arranged alternately, crowded at the tip of the branches, almost as if in a rosette. The leaves are awl shaped, lack both stipules and a leaf stalk, either with entire margins or with distanced line shaped lobes. The leaves and calyx are set with different sizes stalked glands or tentacles that secrete a resin.

Carl Linnaeus described Roridula in 1764.

The genus Roridula is endemic to parts of the mountains in the west and southwest of the Western Cape province of South Africa. R. dentata can be found in the mountains of Tulbagh, Clanwilliam and Ceres. R. gorgonias occurs between Somerset West and Swellendam. Both species grow on acidic and nutrient poor, sandy soils, in locations that are relatively moist, at altitudes of 900–1200 m (2950–3950 ft) for R. dentata and 150–925 m (500–3000 ft) for R. gorgonias. Unlike Drosera, Roridula gorgonias secretes a very sticky resinous substance, mainly containing acylglycerides and triterpenoids that are insoluble in water. This implies that insects can even be trapped during rainy weather. Roridula does not respond by bending tentacles to struggling insects, unlike Drosera, 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. Carnivorous plants like Drosera secrete enzymes that break down proteins (so called proteases) 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 the bug Pameridea roridulae, which quickly close in on the trapped insects and feed on them. The bugs consequently defecate on the leaves. In a nitrogen 15 tracing experiment, where prey insects enriched with the rare heavy nitrogen isotope were eaten by Pameridea bugs, the share of N15 increased, showing that the plant had taken up nitrogen nutrients from the captured insects. Pameridea was shown to have a thick greasy layer that prevents direct contact between the resin on Roridula tentacles and the insect's cuticle, this allows them to roam freely across the plants. 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 genus Synema can be found on the plant
and these may both prey on the captive insects as well as on the resident bugs. The unrelated Australian genus Byblis resembles Roridula in having sticky tentacles, which do not secrete digestive enzymes and also lives together with hemipteran bugs in much the same way.

Fragments of fossil leaves, morphologically very close to extant Roridula, have been found in two pieces of amber of Eocene age (35–47 million years old), from the Yantarny mine near Kaliningrad. 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. The surface of the leaf consists of small six 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 an apical pore, are all characters shared with extant Roridula species. These fossils differ from today's dewsticks in having smaller tentacles, and in lacking a prominent midrib on the underside of the leaf. The pieces of amber with the roridulid remains were found in a deposit 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 the northern hemisphere during the Eocene questions the assumption that the family originated in Gondwana, about 90 million years ago.

Early settlers in parts of the Cape used to hang Roridula branches from the ceiling to catch flies.