Drosera

Plants annual or perennial [rarely subshrubs], deciduous, stems 1–2 cm (except also caulescent stems to 8(–20) cm in D. intermedia), usually forming over-wintering buds (hibernaculae). On both surfaces in strong sunlight, greener in shade (except D. tracyi, which lacks red pigment even in full sun), unlobed, suborbiculate, orbiculate, spatulate, or obovate, or cuneate to linear pink, or rose to pinkish lavender; stamens 5, usually connate basally; gynoecium 3-carpellate; styles 3, deeply bifid; stigma capitate. Capsules obovoid, splitting between placentae. Seeds 20–70, minute. x = 10.

Distribution

Nearly worldwide.

Discussion

Species ca. 170 (8 in the flora).

Species of Drosera are concentrated in Latin America, South Africa, Madagascar, Australia, and New Zealand.

Droseras, like all carnivorous plants, have leaves that attract, catch, digest, and absorb nutrients from small, mostly arthropod prey. They are characterized by gland-tipped multicelled hairs that move in response to stimuli and that catch and appress prey to the leaf blade, where sessile glands secrete enzymes that dissolve the soft tissues. The released nutrients enhance growth by supplementing those available from the poor soils where they grow.

All species of Drosera are capable of moving their trichomes in response to contact with digestible prey. According to C. (1875), this movement can be induced by the mere

touch of a part of a small insect with a single trichome. Besides having trichome movement, some species are able to curl their leaf blades to various degrees in order to maximize contact with prey.

Some species of Drosera may act as annuals, especially if the habitats dry out. The plants can be locally abundant. In most species, the flowers open only in the mornings on sunny days, or not at all on overcast days, and fruits may form from self-pollination. Some species, notably D. intermedia, may exhibit vegetative proliferation, portions of the flowers developing into leaves or plantlets. Some species form over-wintering buds called hibernaculae, requiring a cold period to break dormancy.

Some species of Drosera are reportedly utilized in herbal medicines to produce cough preparations and treat lung and skin ailments.

F. E. Wynne (1944) showed that seeds of North American Drosera are diagnostic for each species. The following key is adapted from various sources, and the species are presented in alphabetic order. Natural hybrids are rare in Drosera, and usually are sterile.

Selected References

Rivadavia, F. et al. 2003. Phylogeny of the sundews, Drosera (Droseraceae), based on chloroplast rbcL and nuclear 18S ribosomal DNA sequences. Amer. J. Bot. 90: 123–130.

Shinners, L. H. 1962d. Drosera (Droseraceae) in the southeastern United States: An interim report. Sida 1: 53–59.

Sorrie, B. A. 1998b. Distribution of Drosera filiformis and D. tracyi (Droseraceae): Phytogeographic implications. Rhodora 100: 239–260.

Wynne, F. E. 1944. Drosera in eastern North America. Bull. Torrey Bot. Club 71: 166–174.

Lower Taxa

Drosera anglica, Drosera brevifolia, Drosera capillaris, Drosera filiformis, Drosera intermedia, Drosera linearis, Drosera rotundifolia, Drosera tracyi

Key

1	Leaf blades filiform, not differentiated from petioles; stem base bulbose-cormose (from expanded petiole base); petals 7–17(–20) mm	> 2
1	Leaf blades linear or suborbiculate to obovate, elongate-spatulate, or cuneate, usually differentiated from petioles; stem base not bulbose-cormose; petals 3–7(–8) mm	> 3
2	Petals 7–10(–12) mm; leaf blades 8–25(–30) cm × 1 mm, glandular trichomes red to reddish purple, drying dark brown; scapes 6–26 cm.	Drosera filiformis
2	Petals 12–17(–20) mm; leaf blades 30–50 cm × 1–2 mm, glandular trichomes pale green, drying pale greenish brown; scapes 25–60 cm.	Drosera tracyi
3	Scapes stipitate-glandular; stipules absent or reduced to minute hairs; seeds black, crateriform.	Drosera brevifolia
3	Scapes glabrous; stipules present; seeds reddish brown, light brown, brown, or black, striate, areolate, papillose, crateriform, or ridged	> 4
4	Leaf blades suborbiculate, (broader than long); seeds finely longitudinally striate.	Drosera rotundifolia
4	Leaf blades linear, orbiculate, or spatulate to obovate or elongate-spatulate (longer than broad); seeds areolate, crateriform, or papillose	> 5
5	Stipules adnate to petioles	> 6
5	Stipules free from petioles or essentially so	> 7
6	Leaf blades obovate to elongate-spatulate; seeds fusiform, striate-areolate, 1–1.5 mm.	Drosera anglica
6	Leaf blades linear; seeds rhomboidal or oblong-obovoid, crateriform, 0.5–0.8 mm.	Drosera linearis
7	Plants always rosulate; petioles flat, sparsely glandular-pilose; petals usually pink, 6–7 mm; seeds coarsely papillose-corrugated, 0.4–0.5 mm.	Drosera capillaris
7	Plants rosulate when young, developing leafy stems 1–8(–20) cm; petioles filiform, glabrous; petals white, 3–6 mm; seeds uniformly papillose, 0.7–1 mm.	Drosera intermedia