| At a
recent meeting Roland’s plant of Euphorbia horrida with red
flowers down the side caused some interest. A closer look showed
that they were not flowers but fruits, and an even closer look
showed they were not Euphorbia fruits at all. Typical Euphorbia
fruits are a three-seeded capsule that fling the seeds out a yard as
they dry out. These were berries and Roland told us they were the
fruits of Viscum minimum, a mistletoe that is a parasite of
Euphorbias. Roland tells us more about these curious plants…
Some years ago I purchased a slightly tatty Euphorbia horrida with strange growths between the ribs. It was offered at the auction of Keith Grantham’s plants following his untimely death. This was a plant parasitized by Viscum minimum – an East African Mistletoe. Mistletoes are found in two plant families: The Loranthaceae (Showy Mistletoes) and the Viscaceae (Mistletoes). They are amongst nineteen families comprising more than four thousand species of parasitic plants. |
Top; The germinating parasite seed has formed a shoot and a holdfast on the surface of the host - it is ready to invade! Below: The short parasite growths and the small three- or four - petalled flower
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Top; Developing fruits on Viscum minimum |
I had managed to parasitize a Crab Apple (Malus ‘Golden Hornet’) with the European Mistletoe (Viscum album) in my own garden about six years ago, though this was after many attempts with the berries left from Christmas decorations. My success rate was probably about 1% over quite a few years! My new succulent plant was bearing greenish fruits that ripened quickly and became orangey-red over the next two months. So how should I handle them? Eager to ensure the long term survival of the parasite I experimented by nicking the stems of some columnar Euphorbia plants, stopping the latex flow from the wound with a misting of rainwater, and inserting the washed seeds just below the epidermis. The seeds turned brown and desiccated; there was no germination. |
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Determined to succeed with the following year’s fruits I spent time on research. I referred to a booklet on English mistletoe [†1] which gave instructions on how to parasitize native trees (Malus, Sorbus etc...). I also obtained a copy of the article by Frank Horwood [†2], in which he described how to parasitize Euphorbia plants. In fact the techniques are identical. The fruits are stuck onto the epidermis between the ribs of the Euphorbia; the mucilage should not be washed off! The seeds will germinate by producing a green shoot which turns towards the Euphorbia stem and when it touches forms a swollen modified root called a haustorum. This acts as a holdfast, and from it the parasite invades the host. Seeds that fail to germinate and those that do but fail to invade the host plant, merely turn brown and fall off. Almost one year later the parasite shoots erupted from the host plant’s ribs. The short stems and small leaves expanded and a yellowish three-petalled flower opened. The plants appear to be self fertile and the fruits swell quickly and change colour from green to red. Then the process can start again. |
Mature fruits of Viscum minimum on Euphorbia horrida. |
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A fine plant of the native mistletoe (Viscum album) on a Sorbus in the garden at Burford House, Tenbury. |
The European Mistletoe can
grow into large plants, though it is slow. During August 2006 I was
fortunate to find a very fine old plant on a Sorbus tree in
Burford House Garden in Shropshire. I have high hopes for the plant
on my own Crab Apple, but Viscum minimum is of much smaller
stature. Indeed when the host experiences droughty conditions the
Mistletoe reacts by causing the visible portion outside the host (ectophyte)
to dry and fall; whereas the portion inside the host (endophyte)
remains healthy and waits for better conditions to grow again.
The ISI distributed parasitized Euphorbia polygona ‘Snowflake’ plants as ISI96-60. Various sources suggest that Viscum minimum parasitizes a limited number of columnar Euphorbia hosts including E. horrida, polygona and submammillarias amongst pedunculate, thorned euphorbias. It is suggested that some parasites eventually kill their host plants, or portions of them above the points of entry. |
I found two further references to Mistletoes parasitizing Cacti. In [†3] the authors highlight Tristerix aphyllus infecting Trichocereus chilensis in Argentina. In [†4] the authors describe Ligaria cuneifloia infecting Corryocactus brevistylus north of Arequipa in Peru. There are a number of websites worth investigating that are referenced at the end of this piece; notably the very detailed parasitic-plants resource. There is undoubtedly a lot more to discover about these uncommon plants; I am pleased to have had some limited success in maintaining one in the UK.
Roland
References
[†1]. ‘Mistletoe’ Wheeldon J N – obtainable via: www.mistle.co.uk
[†2]. ‘Two Parasites of Euphorbia’ (Viscum minimum & Hydnora africana), Horwood F K, The Euphorbia J 1 [1983] pp45-48 (reprinted from NCSS 27.4 pp96-98).
[†3]. ‘A Cactus Odyssey, Journeys in the wilds of Bolivia, Peru and Argentina’, Mauseth J D, Keisling R, & Ostalaza C, Timber Press, Oregon [2002] pp206-210 and illustration.
[†4] ‘A Mistletoe that attacks Cacti’, Mauseth J D et al, Cactus & Succulent J (US) 78.2 [2006] pp88-91
www.parasiticplants/science.siu.edu
www.rbgkew.org.uk/herbarium/ftea/visca.htm
