British distribution: Three species, throughout Britain but more northern and western and following the distribution of wet heathland, absent or extinct over large areas, particularly of central and eastern England.
A genus of 'carnivorous' plants, trapping and digesting small insects and other invertebrates by means of the glandular hairs ('tentacles') on their leaves. Most species of sundew occur in nutrient-poor environments, with all three British species occuring in wet, peaty situations deficient in nitrogen and phosphate.
The 'tentacles' respond to touch, bending inwards to trap and hold the prey by means of the sticky mucilage at their tips. Initial reaction begins within ten seconds and may sometimes be completed within three minutes, especially in the case of the longer tentacles on the leaf margins. The whole leaf lamina may also close slowly around the prey.
This tentacle reaction varies according to the nature of the stimulus; Clancy & Coffey (1977) (working with D. rotundifolia) recorded no reaction to water droplets or glass and only slight movement in response to plastic. They found nitrogenous objects and compounds elicited greatest response.
The tentacles secrete a protease and a broad-spectrum acid phosphatase for the enzymic breakdown of the captured prey. Again, Clancey and Coffey found the enzymes were secreted only after specific stimuli. Uptake of nitrogen and phosphorus has been proven, and no doubt other nutrients are absorbed as well, giving Drosera species a valuable supplement to the limited minerals they can obtain from the peaty soils.
Nutrient absorbtion and transport is evidently also a function of the tentacles. Anatomical details are given by Ragetli et al. (1972), though their studies were apparently based on only one species, which is not named anywhere in their paper!
The three British species (discussed individually below) are all small, short-lived perennials, only a few centimetres high, forming rosettes and overwintering as a bud of tightly rolled leaves. Elsewhere in the world, notably in Australia, species can be much larger, bushy or scrambling, have larger, brightly coloured flowers and grow in a greater diversity of habitats. Our three species are the only ones in Europe, with all three occurring in suitable habitats in much of western and northern Europe and with D. rotundifolia occurring more widely, as is the case within Britain. The closely related species, Drosophyllum lusitanicum, which has long, strap-shaped leaves covered with similar tentacles, grows in dry places in Spain and Portugal.
Our species all have spikes of small, white flowers that are autogamous (self-fertilise). On dull days, the flowers may not open at all, and it has been claimed that this is the usual state, the flowers automatically self-fertilising without opening (cleistogamous). However, the present author's experience is that the flowers normally do open.
Like other carnivorous plants, Drosera species are much grown by enthusiasts and are vulnerable to over-collection from the wild.
D. rotundifolia is the commonest and most widespread species in Britain and is usually easily recognisable by its rounded leaf-blades in mature plants. It grows readily in a range of different acid mire types, including blanket bogs and raised bogs, amongst Sphagnum moss or on wet peat. Typical microhabitats are Sphagnum 'lawns', margins of bog pools and on the sides of Sphagnum hummocks. It seems to be a ready colonist and has occurred at least twice on wet, inner-city wasteground in central Glasgow.
D. intermedia is widespread but generally a much scarcer species, though very characteristic of damp heaths in southern England. In Scotland it is mostly near the west coast. Immature and aberrant plants of the other species are not uncommonly mistaken for D. intermedia and serious doubt must attach to some, if not most or all, inland Scottish records.
The neat, small leaves are narrowed into the stalks. Importantly, the flowering stem arises laterally from below the terminal rosette, so coming up from the side of the plant, whereas in D. anglica the flowering stem appears central. Unfortunately, D. anglica shows a degree of variation in this much-used identification feature and plants must be examined carefully.
D. intermedia sometimes grows with the other species but more typically prefers dryer conditions, often on flushed peaty banks or on bare, exposed, peaty mud on loch margins. Sites are often those that are flooded in winter but dry out in the summer months. Nevertheless, a growth variant (ecotype?) sometimes occupies shallow, permanent bog pools.
D. × obovata is a sterile hybrid, but, as shown here in this rather duff photograph (from having to perch on the quaking edge of a bog pool), plants can show 'hybrid vigour'. It is sporadic with the parents, with the majoriy of British records being from north-west Scotland.
The hybrid of D. rotundifolia with D. intermedia has also been recorded in Britain, but only on very few occasions.