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Biol 50.6 25/11/03

Christmas curiosity or
medical marvel?
A seasonal review of mistletoe
Jonathan Briggs
British Waterways
A fascinating feature of Christmas, mistletoe is a rather mysterious parasite with a complicated
folklore and, perhaps, significant potential in medicine. This review concentrates on Viscum
album, the ‘original’ mistletoe. Its distribution and conservation in Britain, traditions and
potential in medicine are considered within the wider context of the species worldwide.
Viscum album, (Figure 1) is just one of some 1300 mistletoe them from the fruits. A few genera (Nuytsia and species worldwide. All are plant parasites, and in their Misodendron) have developed wind-dispersed seeds.
native lands many have similar folklore and superstitions Unusual pollination through animal interaction is charac- to our own British species. Mistletoes are the dominant members of the Santalales, a plant order containing mostly Despite their similarities, it seems that the Loranthaceae parasitic members. Others in the order include the and Viscacae arose separately within the Santalales and Santalaceae (sandalwoods) and the Balanophoraceae.
their adaptation as aerial parasites is convergent. They Mistletoes were once lumped into the Loranthaceae, but are have very different floral appearance, with the rather now split into several separate families. The largest and dowdy Viscaceae flowers (Figure 1) eclipsed by the elongate most important of these are the Loranthaceae sensu stricto colourful flowers of the Loranths (Figure 2). The two families (over 900 species) and the Viscacaeae (over 400 species).
have similar worldwide distributions, occurring throughout Smaller families include the Eremolepidaceae and the tropics and subtropics, including sub-Saharan Africa, Central and South America and Australasia. The Viscaceae Mistletoes are all hemi-parasites of trees and shrubs, also venture north into Europe and North America with 100 joining to the host via an intimate xylem connection called a Viscum species in the Old World and 200 Phoradendron haustorium (see Box 1). Most take only water and solutes from the host, manufacturing their own metabolites Viscum has 45 species in Africa and 30 in Madagascar, with through photosynthesis. Some, notably the forestry pest just a few of the V. album group spreading into temperate Arcuethobium species of North America, have reduced European and Asian areas. Though fairly uniform in its floral scale-like leaves and approach holoparasitism. The majority characteristics, the plant form varies from the familiar are aerial parasites, though a few, including the appropriately V. album through to V. minimum, an African species with named Australian Christmas Tree Nuytsia floribunda, are much reduced leaves and stems, parasitic on fleshy root parasites. Most also have distinctive methods of seed Euphorbia species (Figure 3). V. album is the only mistletoe dispersal, usually with distinctively sticky fruits that found in Britain, though others are found in continental adhere to new host branches following dispersal by birds.
Arceuthobium species disperse their seeds by squirting Title image: Viscum album, Tenbury Wells mistletoe market. Biologist (2003) 50 (6)
A s e a s o n a l r e v i e w o f m i s t l e t o e Figure 1. Viscum album is a including oak, in southern and central Eastern Europe.
and host preferences of the 1970s study, but did not confirm European mistletoe taxa are further increased by subdivi- a decline. Although many recorders’ anecdotes suggested a sion of V. album into three subspecies: local decline with orchard removal, the mistletoe distribu- •V. a. platyspermum (syn V. a. album) on deciduous hosts tion increased, particularly across the Midlands and south- ern England (Figure 7), possibly reflecting increased •V. a. abietis on Firs in central and southern Europe recording effort in the 1990s (Briggs, 1995 and 1999).
V. a. laxum (syn V. a. austriacum) on Pine and Spruce in Although non-quantitative, the 1990s survey highlights differences across the country in habitat and host preference.
The Asiatic V. album populations belong to a fourth sub- Gardens topped the list of favourites (39%), followed by orchards (30%), then parks (12%) and woodland (5%). Thisemphasises the preference for open and man-made habitats.
Mistletoe in Britain – hosts and distribution
Within habitats, host preferences varied. Gardens had thelongest host list with apple Only V. a. platyspermum occurs in Britain. It is also wide- spread in continental Europe, though rare in the north and extreme south-west. The British distribution is centred in the south and west midlands, with particularly good populations in Herefordshire. This was first described by Bull, in 1864, who listed 30 host species in Herefordshire with a further 23 in other counties, but he emphasised that cultivated apple (Malus domestica; Figure 5) was by far the most popular (Bull, 1864). Today the host list stands at over 200 tree species.
The first national mistletoe survey in the early 1970s (Perring, 1973) mapped the species on a two by two km square (tetrad) basis and assessed host frequency. The results suggested that mistletoe distribution is influenced cruciatum showing its red berries. indirectly by man, as most records were of plants on non- native or planted trees. Cultivated apple was confirmed as Herefordshire, and parkland records increased to 25% in the most frequent host, followed by hybrid limes (Tilia spp), the east. Host preference hierarchy altered accordingly, hawthorn (Crateagus spp), hybrid poplars (Populus spp) and with proportion of lime rising to 25% in the east, reflecting false acacia (Robinia pseudacacia; Figure 6). Mistletoe the parkland records, and apple at >70% in the north. The seems to prefer its hosts in an open situation rather than longest host lists are in the stronghold areas, with mistletoe woodland, and so orchards, parkland and gardens are ideal, survival elsewhere closely linked to favoured hosts.
if somewhat artificial, habitats. Its distribution (Figure 7) wasconcentrated in Herefordshire, Gloucestershire, Worcester- Animal associations
shire and Somerset, with scattered records elsewhere in the Nearly all mistletoe species rely on animals for pollination and seed dispersal, and most also provide food for animals, including several obligate mistletoe feeders. The seed dis- Figure 5. Viscum album growing Biologist (2003) 50 (6)
A s e a s o n a l r e v i e w o f m i s t l e t o e terflies including the great purple (Atlides halesus), Thicket (Callophrys spinetorum) and Johnson’s (Mitoura johnsoni), and the British tortrix moth (Celypha woodiana) (Figure 8) whose larvae are leaf miners in V. album leaves.
There are, presumably, many more mistletoe inver- tebrates that get less attention than butterflies and moths, or are difficult to sample from mistletoe high up in trees, leading to under-recording of, even very com- mon, species. This is neatly demonstrated in Britain, where we have ‘at least’ three mistletoe bugs and one mistletoe weevil. All are probably under-recorded. Ofthe bugs, one is the plant-sucking homopteran, Psylla Figure 6. Viscum album host (left) and habitat (right) preference across viscid. The others are heteropterans, Anthocoris visci Britain. Data from the 1990s survey. and Orthops viscicola. Both seem to be associated withthe psyllid, with Anthocoris probably feeding on it.
In Europe the main vectors are the mistle thrush (Turdus Orthops was recorded as a new species in France in 1888, and viscivorous), a defecating vector, and blackcap (Sylvia atri- within six months enthusiastic Herefordshire naturalists capilla), a beak-wiping vector. Usually, only mistle thrush had undertaken surveys and recorded it as common – overwinters in Britain and is our main vector. Some studies though they had obviously overlooked it before. Over a century suggest that other birds fail to recognise the white (as later, the mistletoe weevil Ixapion variegatum was recorded opposed to orange, red or black) berries as food. Curiously, in an orchard in Herefordshire, as a new British species, by the etymology of Turdus viscivorous suggests it was named National Trust surveyors. This species was also previously after its fondness for the red V. cruciatum berries rather known in France – we must wait to see if it too becomes Mistle thrush’s common occurrence outside the ranges of both European Viscum species suggests that it is not a Viscum album outside its main range
dedicated mistletoe specialist. It is certainly an inefficientvector, tending to defecate long strings of seeds held on There are many small V. album populations outside their threads of viscin, with only one or two attaching to the host main range, most arising from deliberate introductions. Two branch. In contrast, the Australian mistletoe bird (Dicaeum are in North America, far from their native area. Others are hirundinaceum) carefully ‘wipes its bottom’ on a branch much more ‘ local’ since most, if not all the populations in the each time it passes a seed. In continental Europe, Blackcaps east, north and extreme west of Britain, must be considered are also much more efficient, wiping each seed onto a introductions. Populations in America include a flourishing branch from their beaks. Recent changes in Blackcap win- colony in Sonoma County, California. Established by the tering patterns have led to some wintering in Britain, par- eminent botanist Luther Burbank about 1900, this popula- ticularly in the mistletoe-rich Severn Vale. The long-term tion has now spread through an area of over 200 km2 and to implications of this for mistletoe distribution are unknown, many different hosts. In contrast, the other population, on Vancouver Island, British Columbia, has spread to only a few For pollination, many species, including V. album, rely on trees in the immediate vicinity despite being established insects. This may seem an odd strategy for V. album, a winter- for some decades (Job Kuijt, pers comm). Similarly, in flowering dioecious species with tiny green unattractive Edinburgh, UK, mistletoe is confined to a few single plants flowers. Wind pollination seems more likely but examination in private gardens, with only small colonies in the Botanic of the flowers in February usually reveals small flies such Garden and the Dean cemetery. A local botanist, William as Dasyphora species, attracted by scent and nectar.
Paxton (buried close to the colony in the Dean) introduced Pollination mechanisms for some African and New Zealand mistletoe to some of these sites in the 1890s – but they have loranths are quite sophisticated, relying on particular bird not spread far in 100 years. In the Dublin Botanic Garden, a species to open the flowers and trigger the release of a cloud small 19th century colony has hardly spread despite a of pollen. These species rival the Orchidaceae in their spe- plethora of potential hosts nearby. By contrast, in the cialist animal-dependency (see Kirkup in Polhill, 1998).
Botanic Garden in Cambridge, also outside the usual range, Mistletoes, generally, attract herbivores. Berries are an mistletoe grows on a large range of different host trees.
obvious food source but foliage isalso attractive and more succu-lent than host foliage. Somespecies have developed host mim-icry to reduce grazing by herbi-vores; others (e.g., Mesquitemistletoe, a Texan Phoradendronspecies) may be used as a foragecrop by farmers.
including several hundred lepi-dopterans, are obligate mistletoespecialists. Examples include:many Australian Jezebels (Deliasspp); mistletoe browntail moth(Euproctis edwardsii); several Figure 7. Tetrad map of Viscum album distribution in Britain from the 1970s (left) confirmed in the 1990s (right) but with more records from the east and south east. Biologist (2003) 50 (6)
A s e a s o n a l r e v i e w o f m i s t l e t o e Recent studies of mistletoe colonies in Hamburg and satisfy this habit, is now considerable, with Britain relying Brussels, both on the edge of V.album’s continental distri- on imports from France, and much of America relying on bution, have confirmed that whilst spread may be very slow, material harvested in Texas. ‘Inconvenient’ customs, such the species is extremely persistent once established. The as removing a berry after each kiss and thus limiting the Brussels study (Olivier, 1998) details long-established populations in the city’s cemeteries. Mistletoe records here Mistletoe traditions in continental Europe can differ, with show the plant is confined to certain areas and habitats. In use at New Year rather than Christmas. New traditions can Hamburg, several populations established in about 1903 still arise. In the current plantlife campaign to nominate (Poppendieck and Petersen, 1999), have spread only within county flowers, there has been competition between a few hundred metres of their origin.
Herefordshire, Worcestershire and Gloucestershire to have Similar studies are now underway in London, as part of mistletoe as ‘theirs’. In the US, mistletoe is already the ‘state the Greater London Biodiversity Action Plan (London flower’ for Oklahoma. But the principal ‘new’ use for V. album Biodiversity Partnership, 2001). London has sporadic private may turn out to be in medicine – Getafix’s (the druid of the garden mistletoe records but larger historic colonies, centred Asterix cartoon) magic potion may have potential after all.
on Bushey and Home Parks (Richmond), and MyddeltonHouse and Forty Hall (Enfield). The long-established Park Mistletoe in medicine
populations are almost certainly introduced. The Enfieldpopulations were established by, or at least encouraged by, Sir John Colbatch, writing in 1720, suggested that ‘there must E A Bowles (1864–1954), the plant breeder and garden be something extraordinary about that uncommon beautiful writer, who lived at Myddelton House all his life.
plant, that the Almighty had designed it for further and morenoble uses than barely to feed thrushes or to be hung up Myths and traditional uses
superstitiously’. His experiments were on epilepsy, and he wasjust one of many mistletoe researchers throughout the 18th Mistletoe appears in legends in many cultures. There are and 19th centuries, particularly in German-speaking parts of many common themes; our kissing custom is probably a Europe. It was there that Rudolf Steiner made his extraor- remnant of an ancient fertility tradition, helped along by dinary predictions about the plant in the 1920s. He believed some British re-invention in the 18th and 19th centuries. In that, as a parasite, the plant should have medicinal value Greece, Aeneas was guided to the abode of the dead by against cancer, and he described methods of producing plucking the ‘Golden Bough’ of mistletoe. The Norse God, extracts for such use. Since then, there has been considerable Balder, was slain by an arrow of mistletoe, soon after every- use of mistletoe in both conventional and anthroposophic thing living or growing in the earth had sworn not to harm medicine, with many mistletoe products and extracts now him. In Britain, it is well known that the druidic priesthood used in cancer therapy as well as for hypertension, arte- valued mistletoe, both as a peace symbol and in medicine.
riosclerosis and arthritis (reviewed by Bussing, 2000).
They harvested it with a golden sickle, never letting it touch The components thought to be active in cancer therapy the ground and prized mistletoe on oak, their sacred tree, and some of the products available have been evaluated.
especially highly. Modern druids have resurrected their V album’s anticancer properties arise from a mix of cyto- reverence of mistletoe oaks, and actively seek them out.
toxic and immuno-modulatory compounds, including the Other religions are more suspicious, and the Church of Viscotoxins and mistletoe lectins. The viscotoxins (I, II III England still actively discourages mistletoe in its churches.
etc) are thionins that seem to exert cytotoxicity by rapid Myths change as they pass down through generations.
permeabilisation of cell membranes. The mistletoe lectins, The druid story is often reported as factual, despite having related in structure to ricin (from Ricinus) are Type 2 ribo- only Roman accounts and much ‘re-discovery’ of druidic cus- some-inactivating proteins (RIPs). Each consists of two toms in the 18th century by William Stukeley and others, but polypeptide chains (A and B). Several different types have as an evergreen parasite growing high on deciduous hosts, it been identified, known simply as ML I, II etc. These are also could appear to symbolise the life-force of the host through cytotoxic and are effective immuno-modulators, with uses the winter months. The fertility imagery is heightened by in site-directed immunosuppression. They have been exten- V.album’s shape: the paired leaves, berries and sticky white sively researched – both in terms of biological action and juice might just be said to resemble sexual organs.
structure (reviewed by Pfuller, in Bussing, 2000).
The British midland custom of hanging mistletoe all year The Viscotoxin and ML complement of V. album vary with and ceremonially replacing it each Christmas, whilst burning the season, host and subspecies, and so most proprietary the original, has now been lost. The kissing tradition, once mistletoe extracts use a blend from different sources and largely confined to Britain, is now established in all seasons. These extracts, manufactured by German and English-speaking countries, even if they do have to make do Swiss institutes (products include: Abnoba, Helixor, Iscucin with ‘incorrect’ local mistletoe species. Seasonal trade, to and Iscador) are mostly used in anthroposophic and com-plementary cancer therapy in continental Europe. Clinicaltrials involving the extracts (and isolated Viscotoxins andML) have not yet confirmed their value. Though many havereported improved quality of life and tumour regression,trial quality is often poor and the mixture of products usedmakes comparison difficult (see Bussing, 2000 and thewww.cancer.gov website).
Mistletoe is also available as herbal remedies, usually in the form of a tea, primarily for hypertension. The teas arewidely available in continental pharmacies but are rela-tively infrequent in Britain. Despite its medical potential,the most common consumer use of V. album in Britain may Figure 8. Mistletoe tortrix moth, Celyhpa woodian. From Bristol well be in shampoo, as it is an ingredient in several super- City museum collection. Photo by Raymond Barnett Biologist (2003) 50 (6)
A s e a s o n a l r e v i e w o f m i s t l e t o e Box 1. From berry to plant
Viscum album seeds tend to germinate in February and March after the berries have become attached tohost branches. Each berry is single-seeded but they are often polyembryonic and can produce more thanone hypocotyl (Figure 9). These green hypocotyls, which bend towards the host bark surface, are the onlynon-parasitic phase of the plant. They flatten to a sucker-shaped holdfast against the bark and then grad-ually penetrate to the host cambium. There are many studies of the mechanism of penetration – some arereviewed in Bussing (2000).
Vascular links can take several months to establish, and aerial growth is negligible in the first year.
During this time, an invasive organ known as a haustorium develops under the bark. This fuses with and grows in synchrony with thehost wood. Haustorial struc-ture varies between plants butoften distorts the host wood(becoming a large plant gall) portion of the infected branch.
In V. album the primary haustorium develops by stimulating the hostcambium and growing with it to form a wedge-shaped ‘sinker’ embed-ded in new host wood. This effectively grafts the mistletoe onto thehost (Figure 10). The primary haustorium also extends laterally, pro-ducing cortical strands under the host bark. These produce secondary Figure 10. Haustorial cross-sections: TS of the host (and LS of themistletoe), showing distorted host wood and fused cambial tissue. haustoria and new mistletoe shoots some distance away from theoriginal point of infection. Several growths on one branch may be linked by these cortical strands, which can often be traced where they pass under the bark.
The aerial portion of V. album plants has characteristic forked branches, each tipped with a distinctive pair of leaves (Figure 1). Each branch divides once each year, so that whilst initial growth is slow – only one pair of leaves after year 1 and two pairs after year 2, subsequent growthbecomes very rapid. The branching habit means that intact, unpruned V. album can form apparently spherical growths. Flowers develop on eachbranch in mid-winter, with male and female on different plants. Both sexes are small and green and often overlooked, even though present as buds(Figure 1) in Christmas mistletoe. The terminal flowers open whilst last year’s berries, by now positioned in branch axils, are still on the plant. Thenext season’s berries develop through the summer months as green berries, only ripening to white in winter.
Conservation, control and harvesting
Box 2. Growing your own
Publicity arising from the 1990s survey led to a widespread To succeed in growing your own mistletoe you’ll need to: belief that mistletoe in Britain is under threat. As we have • Be prepared – success rates are low and so you’ll need a lot of seen, results suggest this belief is erroneous, at least in dis- tribution terms. The issue of reduced population density • Time it right – success is much higher in February and March through orchard decline is gradually being addressed by • Be patient – mistletoe grows slowly in the first 4 years the inclusion of mistletoe (and mistletoe invertebrate) con-servation in restoration initiatives for traditional orchards.
First, secure a berry source. Berries collected at Christmas can be There are also conservation initiatives for some unusual stored in a shed until mid-February but it is far better to use fresh mistletoe colonies, including replacement of a lost colony in berries. Next, choose your host, bearing in mind V. album’s prefer- the Botanic Garden and introduction at a local arboretum ences: apple first, then poplars, limes, false acacia, hawthorn etc.
in Hamburg, with transplanted trees infected with mistletoe Most shrubs of the Rosaceae are suitable. Don’t choose your (Poppendieck and Petersen, 1999). In London, conservation favourite prize-winning fruit tree, as the mistletoe will distort growth of existing colonies and establishment of new ones is an objective of the local Biodiversity Plan. Trial plantings, Then prepare the berries. Stored berries will need rehydrating for a begun in 2002/3, are in sites chosen for ease of monitoring few hours in water. Whether fresh or stored, the seed needs to be and management, including Enfield Lock, Haringey squeezed out of the berry, along with a quantity of the sticky viscin.
Railway Fields Nature Reserve and Chelsea Physic Garden.
Try to plant at least 20 berries at once, as most will die or be eaten.
Biodiversity conservation is not the only reason for new Since V. album is diocieous, you’ll need at least two plants for future colony establishment. In France, researchers from Institut berries. I advocate ‘nature’s way’, emulating the mistle thrush and Hiscia (Switzerland) have a programme to increase blackcap, by smearing berries onto the branch. Cutting a slit, as V. album on selected oaks and elms. This is to ensure supplies occasionally recommended, can also open the host to other infec- of oak and elm mistletoe to contribute to cancer therapy tions. Stick each seed, with its own glue, to branches 1.5 metres or extracts. Globally, however, mistletoe conservation is an so up the tree and on the side or underside of a branch of at least infrequent issue. Mistletoe control is a much greater issue 20mm diameter. Mark each berry loosely with string to aid monitoring.
with many species seen as pests, reducing timber yields in Germination is fairly rapid and a short green hypocotyl should commercial forestry, or fruit yields in orchards and causing appear and bend to contact the host bark. At this stage, the tiny plants distortion of specimen trees. Mistletoe will reduce both are particularly susceptible to grazing invertebrates and birds, and yield and tree size, though it is only major infestations that prone to dehydration until they have made a host connection. If all threaten the survival of the tree itself. Chemical control is goes well, the hypocotyl should become erect and remain not easy, as herbicides can also damage hosts, and physical unchanged until the following Spring, when the first leaves appear.
control by pruning is often impractical or ineffective, assubcortical strands below host bark will regrow. However, Biologist (2003) 50 (6)
A s e a s o n a l r e v i e w o f m i s t l e t o e for V. album at least, traditional control by selling ‘prunings’ Websites
at Christmas can be an ideal way to reduce parasite impact,provide additional seasonal income and ensure continued www.ppws.vt.edu/IPPS
co-existence of both tree and parasite. Mistletoe prunings International Parasitic Plant Society promotes the study and from Hereford and Worcestershire orchards are tradition- understanding of parasitic plants. It provides a forum for information ally sold at the annual mistletoe auctions at Tenbury Wells.
Although local harvestings occur throughout Britain, muchof our Christmas mistletoe is imported from France, where www.science.siu.edu/parasitic-plants
The ‘Parasitic Plant Connection’ maintained by Dan Nickrent, SouthernIllinois University. An excellent site for all parasitic plants.
The more we find out, the less we seem to know about The ‘Mistletoe Center’, maintained by Brian Geils, RMRS Flagstaff Lab.
V. album. Surveys and analyses confirm previous distribu- Excellent resource site, with publications etc.
tions, but cannot explain the pattern. Mistletoe-dependent www.rmrs.nau.edu/publications/ah_709
insects are under-recorded, with no real knowledge of their Dwarf Mistletoes: Biology, Pathology, and Systematics. Agricultural importance or biology. Conservation initiatives are based more on the species’ cultural, than biodiversity, value. Thevarying European traditions are intriguing, with more seri- www.mistletoe.org.uk
ous mistletoe use in medicine in German speaking areas A site introducing mistletoes, maintained by J Briggs.
and, consequently, more publicity and published research www.lbp.org.uk/action/species/sapmistletoe.htm
in German. The recent English-language review (Bussing, Website for London Biodiversity Plan. Mistletoe page.
2000) gives a good review of current continental research –and demonstrates that there is a lot more to mistletoe than www.cancer.gov/cancerinfo/pdq/cam/mistletoe
just Christmas kissing. Our mistletoe traditions may be National Cancer Institute information on mistletoe extracts. contrived – but they’re fun, and mostly harmless.
Part of Birkbeck College School of Crystallography website, showing
Briggs, J (1995) Mistletoe – distribution, biology and the National http://plab.ku.dk/tcbh/lectin-links.htm
Survey. British Wildlife, 7, 75–82.
Thorkild’s Lectin Page. Maintained by Thorkild C. Bøg-Hansen, Briggs, J (1999) – Kissing goodbye to mistletoe? Plantlife/BSBI report University of Copenhagen. Information on lectins, including a link Bull, H G (1864) The mistletoe in Herefordshire. Trans Woolhope to the journal, Lectins, Biology, Biochemistry, Clinical Biochemistry.
Nat Field Club, 5, 59–108.
Bussing, A (Ed) (2000) Mistletoe: the genus Viscum (medicinal and asromatic plants industrial profiles) Harwood Academic Jonathan Briggs is an Environmental manager with
British Waterways. He has a long-standing interest in mistle- London Biodiversity Partnership (2001) The action. Volume 2 of the toe, co-ordinating the 1990’s survey project and ongoing London Biodiversity Action Plan. London Biodiversity Partnership.
smaller projects such as London Mistletoe Action Plan. Olivier J-F (1998) Cartographie de Viscum album a Bruxelles et Email contact for mistletoe matters: dans les environs Adoxa, 20/21, 1–14.
Perring, F (1973) Mistletoe. In: Plants wild and cultivated (Ed P Polhill, R and Wiens D (1998) Mistletoes of Africa Royal Botanic Poppendieck H and Petersen J (1999) Ein ausbreitungsbiologisches langzeit-experiment: Die einburgerung der mistel in Hamburg
und Umgebung Abh. Naturwiss. Verein Bremen, 44/2-3, 377–396.
Further reading
Becker, H and Schmoll, H (1986) Mistel – arzneipflanze, brauch- Box, J (2000) Mistletoe, Viscum album, on oaks in Britain.
Watsonia 23: 237–256.
Calder M and Bernhardt, P (Eds) 1983. The biology of mistletoes.
Common Ground (2000) Common Ground book of orchards.
Hawksworth, F G and Wiens, D (1996) Dwarf mistletoes: biology, pathology, and systematics. Agricultural Handbook 709. USDepartment of Agriculture Forest Service (now available viawebsite).v Murphy, C (Ed) (2001) Iscador: mistletoe and cancer therapy Biologist (2003) 50 (6)

Source: http://www.jonathanbriggs.co.uk/images/IoB_mistletoe_paper.pdf

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