Mistletoe is a flowering plant that does not take root in the soil, but lives on trees or shrubs and receives water and nutrients from its woody host plants. The European mistletoe (Viscum album) has the largest range of potential hosts among the over 1,000 mistletoe species worldwide. Three subspecies can be found in Central Europe:
- hardwood mistletoe (V. album ssp. album) on various deciduous trees such as apple, oak, elm, poplar, maple, lime, birch
- pine mistletoe (V. album ssp. austriacum) on pine trees
- fir mistletoe (V. album ssp. abietis) on fir trees.
The subspecies of European mistletoe differ in some morphological properties but especially with regard to their pharmacological properties.
Morphology and development
Mistletoe differs from other flowering plants in many botanic properties. Especially remarkable is the slow growth of the ball-shaped mistletoe bushes. Like all other mistletoe species, European mistletoe has no roots to absorb water and minerals from the ground. Instead, the mistletoe germ develops a so-called “haustorium” or “sinker”, through which it attaches itself to the branch of the host tree’s water transport system, which allows it to obtain water, minerals and certain organic substances.
In comparison to most other flowering plants, shoot development is strongly inhibited in European mistletoe. Instead of growing as many leaves as quickly as possible for optimal photosynthesis during a vegetational period, the buds merely grow one stem with just a couple of plain leaves. Large mistletoe bushes are therefore often over 10 years old.
Characteristic for the European mistletoe are its white berries which develop from inconspicuous flowers.
Mistletoe contains a variety of different mineral and organic substances, two of which are of particular pharmacological interest: viscotoxins and mistletoe lectins.
Since mistletoe is connected to the water transport system of its host tree, it absorbs the minerals dissolved in the water drawn up from the ground by each type of tree in its respective specific composition. Especially in spring, the organic substances drawn up by the tree in the ascending sap are absorbed by mistletoe. These include amino acids, low-molecular sugar as well as secondary metabolites of the plant, which form the basis for host-depending qualities of the mistletoe together with the minerals.
Viscotoxins and mistletoe lectins, however, are proteins produced by mistletoe. Their concentrations vary among the different mistletoe subspecies and depend on the host tree on which they grow. Viscotoxins reach their highest concentration in the young leaves during summer, while the highest concentration of mistletoe lectins can be found in the older stems during winter.
Cultivation of mistletoe
For a long time the cultivation of mistletoe on certain types of host trees only rarely found in nature, such as mistletoe-bearing oaks and elm trees, has been regarded as an ambitious challenge.
It is imperative for sustainable mistletoe cultivation on oak and elm trees to pick sites with a suitable climate and optimal soil conditions. This way selected trees can fully develop their disposition for mistletoe and can grow under the most natural and protected conditions possible.
At first, mistletoe seeds are placed on young branches along the treetop’s periphery, where mistletoe bushes are able to steadily grow and fully develop over many years. Later on, birds may take over the further spreading of the mistletoe.
Rhythms of mistletoe
The temporally and spatially inhibited growth of mistletoe is subjected to specific rhythms during the course of the seasons. While growing, the young mistletoe branches demonstrate synchronous swinging motions from late May until late June. These loosen the annual shoots from their original vertical orientation, after which they align with the center of the mistletoe bush.
At the same time the organs of new mistletoe branches are produced in the lateral buds which will develop the following spring. After the growth of a pair of leaves the vegetative development stops in mid-June with the growth of a flower head instead of additional leaves.
The berries reflect this change by accumulating nutritive tissue in the mistletoe seed until the end of June, followed by the development of mistletoe embryos in July. Rhythmic changes in the shape of the developing mistletoe berry indicate that the mistletoe’s growth also correlates with the path of the moon across the zodiac constellations.
Mistletoe has a complex relationship with the fauna: birds spread it, insects pollinate it and many natural antagonists regulate the mistletoe stock.
Mistletoe embryos cannot free themselves from the fruit. They rely on certain types of birds which feed on its berries during the winter and thereby free the sticky mistletoe seed. The mistle thrush and the Eurasian blackcap are the most important species that spread mistletoe. Waxwings also play a role.
Mistletoe is dioecious. Male and female mistletoe flowers develop on different mistletoe bushes. Since the wind is unable to carry mistletoe pollen to the stigma of the female flower, certain species of winter-active insects are responsible for the pollination of mistletoe.
Natural antagonists limit the spreading of mistletoe. Tits feed on mistletoe seeds during the winter. In spring, snails eat the budding mistletoe seedlings, but also young twigs. Even mice and deer have mistletoe on their menu.