Plant of the Week – 4th January 2021 – Viscum album Common Mistletoe

Mistletoe, Viscum album, belongs to the family Santalaceae (the sandalwoods) and is one of only two species of this family occurring in Britain (Stace, 2019). Zuber (2004) has written a very detailed description of this species in the European context. An epiphyte, attached to the branches of host trees, it is also a hemi-parasite, taking some of its nutrients from the host, and is dioecious (male and female flowers are borne on separate plants). Leaves are in opposite and alternate pairs, and the developing bush is fan-shaped at first, becoming a globe after a few year’s growth. The flowers are inconspicuous yellowish-green, developing in the early summer of one year, and opening the following spring. The white fruits which we see at Christmas, are berries which typically occur in threes and contain the sticky mucilaginous substance viscin.

Mistletoe (Viscum album) on apple trees, Malvern, Herefordshire. © Chris Jeffree
Viscum album on pine, Cazorla, Spain. © Chris Jeffree

These berries are attractive to birds, which are the main vectors of the seeds. In the UK, blackcaps and members of the thrush family, particularly the eponymous mistle thrushes, are important vectors. The sticky berries may be wiped onto branches or pass through their gut and the seeds are deposited in their faeces. Seed dispersal mainly occurs in late winter when migratory birds such as thrushes return north. Germination takes place shortly afterwards, but the first leaves may not appear for a few years. A holdfast develops from the hypocotyl of the germinating seed, and the mistletoe becomes parasitic once haustoria develop. These haustoria are embedded in the xylem of the host, but do not penetrate xylem cells. Cortical strands contain chlorophyll and extend through the parenchyma and phloem of the host, they possess phloem and xylem but connections between the phloem of the host and the parasite are not known. A drawing of the life cycle of Viscum album is available in Zuber (2004) with further details in Nierhaus-Wunderwald and Lawrenz (1997).

Mistletoe seedlings (not V. album) germinating beneath a plastic signboard, Belize Botanica Garden, San Ignacio. © Chris Jeffree

Apart from being a hemi-parasite, this species is a bit of a botanical oddity in terms of the amount of nuclear DNA, and the structure of its epidermal cells and mitochondria. It is a diploid Eudicot species, with 20 chromosomes, and its cells contain an exceptionally large amount of nuclear DNA – 205 pg per diploid nucleus (compared with 0.321 pg DNA in Arabidopisis thaliana – the lab rat of plant scientists, which has 10 chromosomes). Apart from in polyploid species, the DNA amount does not reflect chromosome number, but rather the amount of repetitive DNA in the genome. Unlike the leaves of other flowering plants, the epidermal cells contain chlorophyll (Zuber, 2004). The mitochondria of Viscum album have reduced cristae (folds in the inner membrane where many important chemical processes occur). Recent studies show that these mitochondria have lost genes which encode Complex 1 of the five complexes involved in oxidative phosphorylation, and the mitochondrial respiratory system has been reorganised (Petersen et al. 2015). Despite this lack of a crucial part of the respiratory system, Viscum seems to thrive, and the key to this probably lies in its hemi-parasitic habit (Busch 2018, Fonseca-Pereira et al. 2018, Senkler et al. 2018).

Native to Britain, it occurs mostly in southern parts of England and the Welsh borders and becomes increasingly rare northwards. Stace (2019) describes it as introduced in Scotland and Ireland, where there are few locations. However, the BSBI maps show several locations in Edinburgh, and one place where it can be seen (when we’re allowed to travel) is from the car park of the Scottish National Gallery of Modern Art (Modern 2).

Distribution of Viscum album in Britain and Ireland 1987-2020 © Botanical Society of Britain and Ireland
Mistletoe, Scottish National Gallery of Modern Art (Modern 2). © Chris Jeffree

The main distribution of this species is in Europe, centred on France, but variously recorded elsewhere. It is considered as introduced in North America and Ireland.  Jeffree and Jeffree (1996) modelled the geographic range of Viscum album and its response to climate warming and concluded that with the predicted increasingly oceanic climate Viscum album was unlikely to extend northwards in Britain and that the existing range of the species might diminish if winters became warmer.

Distribution of Viscum album L. in GBIF Secretariat (2019). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2020-12-31

In Britain, it grows on several species of tree, especially Rosaceae such as apple, blackthorn, hawthorn and rowan, and also on other genera such as poplar, lime and willow. In Europe three subspecies have been recognised, which grow on different species of host trees. Viscum album ssp. album grows on hardwoods, V. album ssp. abietis on fir trees (Abies) and V. album ssp. laxum grows on pines and spruce. Further east there are forms of V. album which have coloured fruits (Kew Species Profiles). Searching through the online archives of the Botanical Society of Scotland publications (to which BSS full members have free access), I found a report from the meetings of the Botanical Society of Edinburgh in June 1894 of a variegated form growing on a thorn in the Dean Cemetery, Edinburgh (Grieve, 1896). I wonder if its descendants are still there?

Botanical Society of Edinburgh meeting, 1894

In Celtic lore, it was widely used by the Druids (Henderson, 1994). According to Bostock (1855), Pliny the elder, in the first century AD, wrote of a sacrificial ceremony involving mistletoe and oak trees conducted by Druids, in which a white-robed priest cut mistletoe from an oak tree with a golden sickle and two white bulls were sacrificed, in the belief that an infusion of mistletoe, would impart fecundity to barren animals, and that it was an antidote for all poisons. Notwithstanding, Pliny’s account, readers should be warned that all parts of mistletoe are poisonous.

It has been used historically in medicine for treating arthritis, high blood pressure, epilepsy and infertility. Rudolf Steiner hypothesised that mistletoe could cure cancer, based on the observation that the plant was a parasite which eventually killed its host, a process which he claimed paralleled the progression of cancer (BMJ, 2006). In modern medicine, derivatives of it have been approved for use in some countries for the treatment of cancers. In the UK it is available as a complementary therapy, but no drugs have been approved for use by the NHS.

The Romans associated mistletoe with peace, love and understanding and hung it over doorways to protect the household and as part of the December Saturnalia festival. By the 18th century mistletoe had been incorporated into Christmas celebrations around the world, with the custom of kissing under the mistletoe. Apart from extracts for medicinal purposes, it is difficult to find other uses of processed plants, although a mistletoe brandy ‘Biska’ made with macerated mistletoe leaves is available in Croatia.

Worldwide, there are over a thousand mistletoe species, in wide-ranging habitats. All of them are parasites. While our Viscum album has inconspicuous blooms, some other species are much more showy.

Psittacanthus sp., Loranthaceae, a central American mistletoe that grows as a hemiparasite on a number of hardwood and coniferous species of trees, in this instance Bursera simaruba, the tourist tree, also known as gumbo-limbo. Pollinated by humming-birds and passerines. © Chris Jeffree

Are hemi-parasitic mistletoes detrimental or beneficial plants? It depends on who you ask. Foresters worry about damage to the host trees (e.g. Lech et al., 2020), whereas Watson (2001) concludes that they are important keystone species of immense importance to wild-life, biodiversity and ecosystem services.

References

BMJ (2006) Editorial: Mistletoe as a treatment for cancer. BMJ 2006;333:1282. https://doi.org/10.1136/bmj.39055.493958.80

Busch KB. 2018. Respiration: Life Without Complex I. Current Biology 28(10) R616-618. https://doi.org/10.1016/j.cub.2018.04.030

Fonseca-Pereira P da, Batista Silva W, Araújo WL, Nunes-Nesi (2018) A How Does European Mistletoe Survive Without Complex I? Trends in Plant Science, 23(10) 847-850. https://doi.org/10.1016/j.tplants.2018.07.008

Grieve, J. (1896) Note on the occurrence of a variegated form of the common mistletoe (Viscum album), Transactions of the Botanical Society of Edinburgh, 20(1-4) 227-230, DOI: 10.1080/03746609609468842 To link to this article: https://doi.org/10.1080/03746609609468842

Henderson, HM (1994) The Physicians of Myddfai: The Welsh Herbal Tradition, Botanical Journal of Scotland, 46:4, 623-627, DOI: 10.1080/13594869409441773 https://doi.org/10.1080/13594869409441773

Jeffree CE and Jeffree EP (1996) Redistribution of the potential geographical ranges of Mistletoe and Colorado Beetle in Europe in response to the temperature component of climate change. Functional Ecology, 10 (5) 562-577. https://www.jstor.org/stable/2390166

Kew Species Profiles Viscum album L. | Plants of the World Online | Kew Science http://creativecommons.org/licenses/by-nc-sa/3.0

Lech P, Żółciak A and Hildebrand R (2020)  Occurrence of European Mistletoe (Viscum album L.) on Forest Trees in Poland and Its Dynamics of Spread in the Period 2008–2018.

Forests 2020, 11, 83; https://doi.org/10.3390/f11010083

Nierhaus-Wunderwald D and Lawrenz, P. (1997) Zur Biologie der Mistel. Birmensdorf, Eidg. WSL Research Institute. Merkblatt Praxis 28: 8pp. view (lib4ri.ch)

Petersen G, Cuenca A, Møller IM and Seberg, O. (2018) Massive gene loss in mistletoe (Viscum, Viscaceae) mitochondria. Sci. Rep. 5, 17588; doi: 10.1038/srep17588

Pliny in ‘Pliny the Elder, The Natural History’ by John Bostock The Natural History. Pliny the Elder. John Bostock, M.D., F.R.S. H.T. Riley, Esq., B.A. London. Taylor and Francis, Red Lion Court, Fleet Street. 1855.

Scotsman newspaper (2020) https://www.scotsman.com/news/environment/secrets-mistletoe-be-uncovered-scottish-scientists-3071947

Stace, C. A. (2019). New Flora of the British Isles (Fourth ed.). Middlewood Green, Suffolk, U.K.: C & M Floristics. ISBN 978-1-5272-2630-2.

Senkler J, Rugen N, Eubel H, Hegermann J, Braun H-P. 2018 Absence of Complex I Implicates Rearrangement of the Respiratory Chain in European Mistletoe. Current Biology 28 (10), 1606-1613. https://doi.org/10.1016/j.cub.2018.03.050

Watson, DM (2001) Mistletoe—A Keystone Resource in Forests and Woodlands Worldwide. Annu. Rev. Ecol. Syst. 32:219–49. https://www.annualreviews.org/doi/10.1146/annurev.ecolsys.32.081501.114024

Zonneveld BJM. (2010) New record holders for maximum genome size in eudicots and monocots. Journal of Botany Article ID 527357 https://doi.org/10.1155/2010/527357

Zuber D. (2004) Biological Flora of Central Europe – Viscum album L. Flora 199, 181–203. Biological flora of Central Europe: Viscum album L. -ScienceDirect

Julia Wilson, all photographs © Chris Jeffree

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: