It's the season not only to bring trees and other greens indoors, but also to kiss under the mistletoe. The origins of this tradition are ancient, rooted in Celtic and Old Germanic traditions. In Europe, mistletoe is mostly found in oaks, and was thought to be found only in lightning-struck oaks. So mistletoe was imbued with the power of lightning and therefore with sexual potency. Or so goes the story.
There is a simpler explanation. In the winter when all the hardwood trees are bare, mistletoe remains green, a harbinger of spring. We take holly, fir, mistletoe and other trees into our houses to remind us that winter will not last forever.
In Kentucky, mistletoe is found most often on walnut trees, though over 100 other hardwood trees can be hosts. Throughout the Bluegrass, the many walnuts are decorated with mistletoe. It occurrence is patchy, with many walnut groves bearing no mistletoe. Our mistletoe is Phoradendron leucarpum. It is a mildly toxic plant (probably only the berries). Although European mistletoe (Viscum spp.) has some medicinal and possibly anti-cancer properties, there is no known medicinal use of Phoradendron.
Mistletoe is a hemi-parasite (partial parasite). It is a green plant capable of supporting itself by photosynthesis. It is anchored by haustoria (specialized roots) into the xylem of its host tree, not the phloem. Mistletoe is able to obtain whatever is in the xylem sap of the host tree - mostly water and mineral nutrients, but also plant hormones such as cytokinins, which are produced in the tree roots. In addition, any organic carbon in the xylem sap, such as sugar transported in the spring, is available to the mistletoe. According to some research, mistletoe may get a lot of its carbohydrates from the xylem sap of the host plant rather than from photosynthesis. Mistletoes generally transpire much more water per centimeter of leaf area than the host plant, and this may help them compete with their host plant.
Generally, we consider mistletoe to be harmless at low densities. However, when a tree canopy is heavily loaded with mistletoe, its form is affected and sometimes its growth slows.
The relationship between mistletoe and its host plant has not been studied much, and is probably a lot more complicated than we currently know.