Some similarities to :
Witches' Broom which is a gall on
Hornbeam and also on Wild Cherry (Gean) trees trees usually caused by the fungus Taphrina betulina which look rather like bird's nests, but they are usually more dense than is Mistletoe.
Uniquely identifiable characteristics
Distinguishing Feature :
Mistletoe is Dioecious, with male and female flowers on separate plants, the sticky white berries only appearing (between November to December) on female plants. The berries are eaten by Mistle Thrush birds as well as by other birds, but they are so sticky that they also stick to the bird's beak as they try to eat them from where they are ready to be implanted on the next high branch of a tree that the bird visits. Because birds prefer to be high up in the trees, Mistletoes is to be mostly found near the top of the tree. But Mistletoe will only take on first or second year tree growth, and even then can take a whole year to 'root' and establish a connection with the host tree before it can obtain nutrients and fluids. Before that first year is up, it is on its own.
Mistletoe is Hemi-parasitic and grows on, obtaining some nutrients (but not all) from deciduous trees only, especially Malus (
Apple) at 40% of mistletoe occurrences, then in descending order of popularity (by the Mistletoe choice itself): Tilia (Lime), Crataegus (
Hawthorn), Populus (especially
Black Poplar), Salix (Willow), False Acacia and only rarely on Quercus (
Oak). But it can grow on virtually any tree, but these are all of much lower probability. It is grown commercially on mainly Apple trees in orchards for the Christmas trade in mistletoe for kissing beneath.
It grows from a seedling by 'gluing' itself to the bark of a tree by means of the sticky viscous liquid they contain. In the first year it will have grown 4 leaves on one branch. It branches into two once a year, the number of branches doubling each year, until it is a globular mass which grows slightly larger every year. In the last 15 years since 2000AD mistletoe seems to be spreading faster than usual.
It is capable of photosynthesis itself but draws water and minerals from its host vascular system at distinct swellings or gall where the two conjoin. It looks as though it buries itself under the bark of the tree, but this is incorrect; it grows on the surface of the bark, but initiates the tree to grow bark over and around itself, so that eventually it is growing from a large knob of altered bark tissue (aka haustorium). After gluing itself to the bark, the seedling puts tentacles into the tree by which means it can obtain some nutrients, and especially water, from the tree. Mistletoe does have green leaves, so it is able to generate some of its own nutrients by photosynthesis; but those it cannot it steals from the host tree, especially water.
Out of several sub-species found in the World, only the 'parent' grows in the UK, Viscum album. It contains a toxic protein and lectin called
Viscumin which has a high molecular weight. Other sources make mention of
viscotoxin as one of the toxins. They are, like Ricin, a RIP, a Ribosome Inactivating plant Protein which is another poisonous lectin, although the two target and bind to differing sites. Although Mistletoe is poisonous, fatalities are rare. The toxin is concentrated in the white berries but is said to be present throughout the whole plant.
Mistletoe does, of course, contain other chemical compounds, but because Mistletoes can grow on a wide variety of differing trees and is hemi-parasitic on them, obtaining some nutrients from them, the composition and proportions of these secondary metabolites varies. However, any toxicity of these compounds will probably pale into insignificance compared to the toxicity of the viscotoxins. A new acyclic monoterpene diglycoside has been found in Mistletoe, but it lacks a common name and the chemical name is very long.
Its greatest population density in the UK is in Somerset and Devon with a very broad spread around the home counties and a smaller outbreak on the Mid-Wales border. Virtually no presence north of South Yorkshire apart from one or two hectads. It grows mainly in gardens and orchards with a lot of parkland presence also. Roads, hedgerows, fields and woods taken together account for just 20% of the population.