Drosera capensis (), the Cape sundew, is a perennial rosette-forming carnivorous herb in the flowering plant family Droseraceae. It is native to the Cape region of South Africa, where it grows in permanently wet, nutrient-poor habitats. Its elongated, roughly oblong leaves are held semi-erect and have a distinct petiole. It is quite a variable plant with several recognised growth forms, some of which develop a short stem. As in all sundews, the leaves are covered in stalked glands that secrete sticky mucilage. These attract, trap, and digest arthropod prey, obtaining nutrients that supplement intake from the substrate in which the plant grows. D. capensis has dramatically mobile leaves that curl around captured prey, preventing its escape and facilitating digestion.
First recorded in the late 17th century, D. capensis was one of the five Drosera species included in the first edition of Carl Linnaeus' Species plantarum. A relatively large, 'showy' species that flowers readily and is considered very easy to grow, it was cultivated in Europe as a curiosity from the mid-18th century and is now one of the most widely grown sundews. It has also been extensively studied, including as a potential source of bioactive compounds of pharmacological interest, and was the first sundew to undergo whole-genome sequencing. Although often uncommon and localised in its native range, it has become naturalised in several countries following deliberate introductions and is listed as an invasive species in New Zealand.
Characteristics Habit D. capensis is a perennial herb forming a rosette typically in diameter, although occasionally up to . Its narrow, oblong leaves are held semi-erect. As in all sundew species, the laminae are covered with stalked glands ("tentacles") that secrete a sticky mucilage. These attract, trap, and digest insects and other arthropods as prey. New leaves grow from the centre of the rosette and the remains of old leaves gradually form a short stem, typically around in length but sometimes as long as . The roots are sparse, thickened, and attain a length around , rarely branching. In summer, the plant produces one or two relatively tall inflorescences, each with between 15 and 30 flowers. The petals are typically pale purple.
Growth forms D. capensis is quite variable across its range, and several distinct forms have been informally identified. In his 1986 book Insect-eating Plants and How to Grow Them, Adrian Slack referred to two forms: "typical" form with laminae up to wide and long, flowers up to wide, which develops a stem and a semi-trailing habit "narrow-leaf" form, smaller overall, with narrower leaves, minimal stem formation, and smaller flowers that are produced earlier
Peter D'Amato described four forms in his 1998 book The Savage Garden: Cultivating Carnivorous Plants: D. capensis - "typical" with red-tentacled leaves long, and forming a scrambling stem D. capensis - "narrow" with narrow leaves and less tendency to form a stem D. capensis - "alba" similar to the "narrow" form but with white flowers and pale tentacles D. capensis - "red" similar to the "narrow" form but entirely reddish-maroon in colour when growing in bright light, with deep pink flowers
Robert Gibson noted the difficulty of assigning unlabelled plants to any of the informally described forms, due to short descriptions and the plants' variation in response to growing conditions. He detailed nine different forms:
+ informally described by Gibson (2020)FormHabitLeavesPetioleLaminaFlowersStigmasStemLocalitiesWide-leavedRobustGreen, underside sparsely covered with eglandular hairs>3 mm wide, about 60% width of lamina>3 mm, up to 8mm wideLarge, ~3 cm, usually dark purpleIrregularly obovate to elliptic, widening conspicuously from terete style segmentsConspicuous, to 15 cmWestern part of Western Cape: Silvermine Nature Reserve; Vogelgat Private Nature Reserve; former Bergvliet FarmNarrow-leavedTypicalOlive green to reddish, to about 12 cm long, underside sparsely covered with eglandular hairs<=3 mm<=3 mm, similar length to petiole and up to twice the width~2 cm, pale purpleTerete to narrowly ovateNone, or up to 5 cm in older plantsTypical form in the wildRed-leavedTypicalVibrant red under bright illumination, underside with few eglandular hairsAs narrow-leavedAs narrow-leavedUp to 2 cm, style segments dark purpleIrregularly ovateAs narrow-leavedPrimarily Gifberg, but also Travellers Rest, Koue Bokkeveld plateau and Table MountainWhite-floweredTypicalGolden green; tentacles with colourless stalks and green to pale red glandsAs narrow-leavedAs narrow-leaved10–14 mm, white petalsModerately expanded(not described)Appears to have emerged spontaneously in cultivationMiniCompactOlive-green, congested1.5–2 mm3–4 mmUp to 2 cm, dark purple, on disproportionately tall scape(not described)At most a few cm longHigh mountain areas near Ceres, Bainskloof (Matroosberg), upper slopes of Baviaanskloof between 1000 and 1800 mTraveller's RestGiantGreen or fully red, very large (rosettes to ~30 cm)Longer than lamina, about 2 mm wideAbout 3 mm wideAbout 1.5 cm, pale purple, style segments terete(not described)Banks of Doring River at Traveller's Rest, east of ClanwilliamPaddle-shape leaved (1)Sparse, reminiscent of D. anglicaGreen~2 mm~6 mm(not described)(not described)(not described)Very wet site at Gydo Pass near Ceres, ~1000mPaddle-shape leaved (2)Open with distinct internodesGreen(as previous?)(as previous?)(not described)(not described)ConspicuousBainskloof Pass, also possibly Waaihoekskloof near Ceres, and Montague Pass.Paddle-shape leaved (3)CompactTo 6 cm long, spirally arranged, underside with abundant eglandular hairsTo 1 mm wideTo 3 mm wideUp to 1.6 cm wide, petals dark purpleDark purple, irregularly obovateFormed only very slowlyIn cultivation, origin not known
Gibson also noted occasional plants of the narrow-leaved form with enlarged bracteoles with insect-trapping hairs, and some red-leaved plants developing a row of insect-trapping hairs on the petals.
Leaves and carnivory Morphology D. capensis has elongated, roughly rectangular leaves with a distinct petiole typically similar in length to the lamina, although in some growth forms it may be significantly longer. Vernation is geniculate-involute, the lamina first unfolding from the petiole, and the lamina margin and tentacles unfurling outward from the leaf axis.
In section, the petiole is a narrow trapezoid with a thickened midrib on the lower (abaxial) side. It has a covering of fine, translucent white hairs, sparse on the upper (adaxial) surface and sparse to moderate on the lower surface.
The lamina is usually slightly wider than the petiole. It has long, glandular hairs (tentacles) around the margins of the upper surface, surrounding shorter glandular hairs in the middle of the lamina. The lower surface of the lamina lacks tentacles but has a sparse to moderate covering of shorter, translucent white, spreading hairs.
D. capensis has conspicuous stipules at the leaf bases; these are brown and roughly triangular, membranous, with the apex intact or slightly divided into setae, long and wide.
Drosera species exhibit numerous different types of glandular trichomes (hairs). In addition to the large "tentacles" involved in prey capture, leaves of D. capensis have two types of microscopic glandular trichomes: Type 3 trichomes (short, unbranched, biseriate, stalk two- or three-celled, gland vertically divided) and Type 10 trichomes (unbranched short glandular hairs, stalk biseriate, gland multicellular).
Movement and prey capture
Like all sundews, D. capensis attracts, captures, and digests prey by means of stalked glandular trichomes (hairs or tentacles) that secrete droplets of sticky mucilage containing digestive enzymes. When a potential prey item contacts the marginal tentacles, they bend toward the centre of the leaf, bringing the prey item into contact with more tentacles. Adjacent marginal tentacles also move toward the prey, making prey escape more difficult and facilitating digestion.
D. capensis is among the sundew species in which the lamina itself also exhibits significant movement. When prey is detected, the leaf blade slowly bends inwards around the location of the prey, enfolding it. In D. capensis, the leaf can bend through more than 180°, "rolling" around the prey. The extent to which the leaf bends depends on the location of the prey; the closer the prey to the tip of the leaf, the more pronounced the bending response.
Experiments involving treatment of leaves of D. capensis with an auxin (indole-3-acetic acid) and an auxin transport inhibitor (2,3,5-triiodobenzoic acid) demonstrated that bending of the leaf is caused by a hormonal growth stimulus transported from the tip of the leaf to the bending point and induced by a signal originating at the location of the prey.
Mucilage The mucilage produced by the glandular tentacles of D. capensis is an aqueous solution of about 4% of an acidic polysaccharide containing xylose, mannose, galactose, glucuronic acid, and ester sulphate in the ratio 1:6:6:6:1, with a pH of 5. It is viscous—about six times more viscous than water at —and so sticky that when fresh, it can form threads up to long.
Flowers and seeds
The plant produces one, two, or occasionally three inflorescences, typically tall, but sometimes only or up to . The scape, pedicels, and abaxial surface of the sepals have a moderate covering of coarse, translucent white, spreading hairs.
Each inflorescence bears numerous flowers, usually 15–30+ but occasionally as few as 6. The five-petalled flowers are arranged along one side of the peduncle and open sequentially, starting at the base. The pedicels are semi-erect and long. Sepals are obovate, long and wide, and usually green. Petals are also obovate, long and wide and typically pale purple, sometimes with a darker purple base. Forms with dark purple or white petals also exist.
The five stamens are long with a pink filament and yellow anthers and pollen. The apex of the filament is expanded between the anthers. The ovary is green, obovoid, long, and in diameter. The three purple styles are long, forked at the base and lie subhorizontal, expanding into stigmata that are cylindrical to irregularly obovate.
The flowers are self-fertile and autogamous (self-pollinating). Copious seeds are produced in dehiscent capsules, ripening 4–5 weeks after pollination. The seeds are dark brown, shiny, and cylindrical with tapered ends, approximately long and in diameter. The testa is deeply ridged longitudinally, with slightly shallower transverse ridges forming a net-patterned (reticulated) surface.
Biochemistry Sundews produce a range of bioactive compounds, including naphthoquinones, flavonoids and their glycosides, and other secondary metabolites. D. capensis contains the naphthoquinone ramentaceone (7-methyljuglone) and trace amounts of plumbagin (its isomer). These compounds have antifeedant, allelopathic, and antimicrobial properties and are believed to play a defensive role in plants. They are also of interest for their potential medical uses, including antibacterial and anticancer applications.
D. capensis contains the flavonoids myricetin and quercetin, which are also of pharmacological interest. The species has been the subject of studies aiming to increase the biosynthesis of its naphthoquinones and flavonoids by applying elicitors including jasmonates and Agrobacterium rhizogenes, and in cell culture by inducing callus formation.
Genetics D. capensis is a tetraploid species with a chromosome number of 2n=40.
The red-leaved form of the species was the first sundew—and the first carnivorous plant from order Caryophyllales, which also includes Nepenthes, Aldrovanda, Drosophyllum and Dionaea—to undergo whole-genome sequencing. The genome spans a total of 264 Mbp.
Distribution and habitat