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BIODIVERSITY REFERENCE
 
   Drosera L.   
 
Sundew


Kingdom: Plantae
Phylum: Tracheophyta – vascular plants
Subphylum: Magnoliophytina – flowering plants
Class: Magnoliopsida – dicotyledons
Order: Nepenthales
Family: Droseraceae


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.
World distribution: About 100 species, throughout tropical and temperate parts of the world, especially Australia and South Africa.


Drosera anglica leaves

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.
Even so, obligately cleistogamous flowers, with the petals adhering together, are sometimes recorded and very rarely, at least in D. rotundifolia, entire populations may be of this type. Apomixis (i.e. agamospermy) has been reported as occurring, though rarely, in all three species.
Wind pollination also happens and the hybrid of D. anglica with D. rotundifolia (= D. × obovata) sometimes occurs.

Like other carnivorous plants, Drosera species are much grown by enthusiasts and are vulnerable to over-collection from the wild.



Drosera rotundifolia L.
Drosera rotundifolia
Drosera rotundifolia

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.


 

Drosera intermedia Hayne
Drosera intermedia

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.


 

Drosera anglica Hudson
Drosera anglica

 
 
 
 
 
Mature plants of D. anglica are recognisable by their proportionally narrower leaves and by their flowering stems being up to twice as long as the leaves and arising centrally (though sometimes bent at the base and arising more at one side). Although widespread it demands wetter, more nutrient-rich sites, including base-rich fen, and is more vulnerable to drainage. Consequently it is extinct in much of England, though still very locally common in western Scotland.



 

Drosera anglica × rotundifolia (= D. × obovata Mert. & Koch)
Drosera anglica x rotundifolia

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.


Photographs
D. anglica: Kirkudbrightshire, 1977
D. intermedia: Cornwall, 2000
D. × obovata: Wester Ross, 1968
D. rotundifolia: South Uist, Outer Hebrides, June 2006


Sources
Valuable compilation, extensive bibiography and principle information source used here:
•   Crowder, A.A., Pearson, M.C., Grubb, P.J., & Langlois, P.H., (1990). Biological flora of the British Isles. Drosera L. Journal of Ecology 78: 233-267.
Additional sources:
•   Chandler, G.E., & Anderson, J.W., (1976). Studies on the nutrition and growth of Drosera species with reference to the carnivorous habit. New Phytologist 76: 129-141.
•   Clancy, F.G., & Coffey, M.D., (1977). Acid phosphatase and protease release by the insectivorous plant Drosera rotundifolia. Canadian Journal of Botany 55: 480-488.
•   Ragetli, H.W.J., Weintraub, M., & Lo, E., (1972). Characteristics of Drosera tentacles. I. Anatomical and cytological detail. Canadian Journal of Botany 50: 159-168.


Useful links (carnivorous plants)
•   The Carnivorous Plant FAQ (Barry Meyers-Rice, for the International Carnivorous Plant Society)
    http://www.sarracenia.com/faq.html
    A wealth of information on carnivorous plants in general (especially their cultivation), good overviews of the individual genera (including Drosera of course) and with valuable sections on ecology and conservation.
 
•   Galleria Carnivora
    http://www.sarracenia.com/galleria/galleria.html
    An excellent and comprehensive if slightly quirky archive of photographs of carnivorous plants.
 
•   Carnivorous Plants (Michael Zenner)
    http://www.rdrop.com/users/mvz/plants.htm
    Nice, clear, illustrated summary of the major genera.



© A.J. Silverside
Page first hosted at www-biol.paisley.ac.uk/bioref/, December 2000; transferred to lastdragon.org with minor edits and replacement photographs of D. rotundifolia, October 2009
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