Drosera rotundifolia L., Round-leaved Sundew

Account Summary

Growth form and preferred habitats

A small, shallow-rooted, rosette-forming, bright red, short-lived, native perennial, D. rotundifolia has stalked leaves with orbicular laminae, 7 mm in diameter. Each leaf lamina bears around 200 stalked, sticky, red tentacles that attract and trap insects. The bright red colour of Drosera leaves and stems is due to cyanidin glycoside, an anthocyanin pigment.

Round-leaved Sundew is a small plant of wet, unshaded or lightly shaded, relatively open, strongly acid, wet peatland. It is frequent and occasionally abundant at the edges of bog pools on damp to wet moss carpets – often having to compete with Sphagnum moss; it also grows on damp, bare peat and on the seeping sides of drainage ditches cut in bogs dug for fuel turf. D. rotundifolia prefers full sun habitats, but it can tolerate a degree of shade from Calluna vulgaris (Heather) and other dwarf shrubs. It can survive overwinter completely shaded within Sphagnum cushions, after being overtopped by autumn growth of the moss. Wide fluctuations in the water table, including immersion for up to two weeks are tolerated, but due to its very shallow roots, dry periods must be only of short duration, if the plant is to survive. D. rotundifolia is replaced by D. anglica (Great Sundew) in the wettest areas of the bog, eg the bottoms of hollows and in intermittent or more permanent pools (Crowder et al. 1990).

Probably present on every patch of otherwise bare, acid peat bog and wet heath in the county, although sometimes it is rather scarce and needs to be actively searched out. With its tennis-racket shaped leaves this is the easiest sundew to identify and it is by far the most common of the three Drosera species we have in B & I, around twice as widespread as D. anglica (= D. longifolia) (New Atlas).

Variation

No varieties are recognised in Britain, although three have been described from continental Europe (Crowder et al. 1990).

Insect capture, plant nutrition and performance

Insect capture by the abundant glistening sticky leaf glands is effective in stimulating plant growth, reproductive success and survival through supplying additional nitrogen and phosphorus, the usual limiting nutrients on strongly acidic, oligotrophic bogland soils. However, plants without access to arthropods remain healthy and D. rotundifolia can take up inorganic nitrogen if it is available.

In Drosera, erect leaves have been found to be more efficient in capturing insects than horizontal leaves close to the soil surface. When an insect adheres to a leaf tentacle and struggles, this stimulates the bending of neighbouring tentacles and the in-rolling of the leaf lamina itself, both further entrapping the prey. These movements are believed to be responses to chemical rather than mechanical stimuli and they are mediated by hormones. One to two weeks after prey has been caught, the leaf reopens and the blackened remains, mostly chitinous pieces of legs and wings, fall off or are blown away (Crowder et al. 1990).

Observations of insect capture rate for D. rotundifolia in W Germany were higher than those published for another insectivorous genus (Pinguicula (Butterworts)) and were correlated with total leaf number and leaf area. It was calculated that the plant obtained around 24-30% of its nitrogen from insect capture (Karlsson et al. 1987; Schulze & Schulze 1990). Despite the advantage of large plants in capturing prey, the seasonal development of plant size was not simply related to insect capture but strongly influenced by leaf losses, flower-bud formation, flowering and seed set. All of these processes cost the plant photosynthetic and nutritive resources that take time to replenish before rosette regrowth and size recovery can take place (Schulze & Schulze 1990).

Individual plants are smaller in exposed and upland sites and there must be a lower size limit at which insect capture rates are inadequate and plants become doomed through starvation. Size of plant is generally related to the local abundance of prey and plants on most Fermanagh bogs are small, rarely over 6.0 cm tall, often with only 4-8 functional leaves, although exceptionally they can have around 20 on well grown individuals (Crowder et al. 1990).

Experimental studies in S Germany showed that leaf abscission and replacement is affected by the rate of insect capture and subsequent plant growth rate. Thus leaf number remains fairly constant, while the amount of stored nitrogen in the hypocotyl (the storage region between root and shoot), increases with insect supply. The experiments demonstrated that in Germany at least, insect capture is essential for growth of D. rotundifolia and that if small plants are continually deprived of insects, they will probably die.

Despite these relations between growth and prey, reallocation of resources from old leaves appears to be as essential as insect capture for plant development. It is likely the case that as for other insectivorous species, the captured prey mainly provides the nitrogen and phosphorus requirements of the plant, while other elements, especially calcium, magnesium and potassium have to be obtained from the soil (Schulze & Schulze 1990).

Reproduction

Rather tiny white flowers are produced in very small numbers on a leafless stem (ie a scape), from June to August, but they only open in really bright sunshine. Due to their closed nature, flowers are normally self-fertilised. In cloudy W Ireland, flowers rarely fully develop or open, but instead self-pollinate in bud (ie they are cleistogamous) (Garrard & Streeter 1983). Flowers that do manage to open, last only for a morning and are probably mainly selfed, although some degree of wind-pollination is possible (Crowder et al. 1990). The proportion of plants flowering is very variable, both from place to place and from year to year. For instance, in one study the percentage of plants flowering in Irish bogs varied from 35% near Ballymena, Co Antrim (H39) in 1965, to 73% at Rossbeg (given as Rosbeg), Co Donegal (H35) in 1963 (Crowder et al. 1990). Another study of Irish plants from ten sites found there was a mean of 6.7 capsules per plant and capsules contained a mean of 37 seeds (range 4-63) (Crowder et al. 1990).

Very little is known about seed dispersal, but the propagules are tiny and can float in water for months. Wind, water and animal feet (particularly those of birds), appear to be the most likely transporting agents.

In addition to seed, vegetative reproduction can take place by means of adventitious axillary buds that form new rosettes beside and under the main leaf rosette of the plant. Decay of the stem axis separates these rosettes from one another, allowing them to grow on and establish distinct plants. Growth buds can also develop on dying leaf blades, or on their petioles, forming penetrating roots as the parent leaf becomes moribund (Crowder et al. 1990).

In both D. rotundifolia and D. anglica new leaves grow in the centre of the rosette after flowering, which makes old inflorescences look as if they arise laterally. This can result in possible misidentification if other characters are not checked (A. Culham, in: Rich & Jermy 1998).

Hybrid

A rare hybrid occurs with D. anglica (Great Sundew), named D. × obovata Mert. & W.D.J. Koch (see account below).

Fermanagh occurrence

In Fermanagh, D. rotundifolia has been frequently recorded from 171 tetrads, 32.4% of those in the VC. It is very frequent and locally common on wet, acid bogland, including flushed areas of slightly higher pH, at all levels right up to Cuilcagh Gap near the summit of the highest mountain. Plants of D. rotundifolia on most Fermanagh bogs are small, often with only four to eight functional leaves, although exceptionally there can be around 20 leaves on well grown individuals.

Irish occurrence

Round-leaved Sundew is chiefly found in peaty uplands and on lowland raised bogs, so it is widely scattered across the whole of Ireland, but more frequent in the N, W and C of the country (New Atlas).

British occurrence

D. rotundifolia remains frequent in the N & W of Britain and formerly (pre-1970) it was also widespread in parts of C England (New Atlas). Losses in lowland S & E England continue due to habitat destruction (F.J. Rumsey, in: Preston et al. 2002).

European and world occurrence

Round-leaved Sundew is widespread throughout Europe, including S Greenland, Iceland and Scandinavia, but absent from most areas of the south continental mainland and all of the Mediterranean islands except Corsica (Jalas et al. 1999, Map 2962). Outlying stations occur in the Lebanon and in the Caucasus. Elsewhere, it is widespread and circumpolar in boreal and temperate zones worldwide, extending across N Asia to Korea, Japan and New Guinea. In N America, it ranges from the subarctic to Vancouver on the W Coast and Georgia in the east and thence to Alabama. It is absent from the Rocky Mountains (Hultén & Fries 1986, Map 999; Crowder et al. 1990).

Threats

While D. rotundifolia is tolerant of forms of disturbance such as grazing, occasional fires or trampling, undoubtedly there have been population losses locally in Fermanagh and elsewhere in B & I, mainly due to drainage and peat-cutting. At higher altitudes, losses are the result of forestry plantation, over-grazing, peat burning and consequent bog surface erosion.

• Find out more about the format of the species accounts
• View abbreviations
• View references for all species accounts
• View a gazetteer of all the sites