European and North American mistletoes are most commonly associated with the Christmas kissing tradition, but mistletoes are found widely across the world and belong to the Santalales order of flowering plants. Only Santalales that act as parasites to the branches or stems of other plants are considered mistletoes, excluding some Santalales that grow off roots on the ground. In other words, if you can't hang, you're not a mistletoe.
There are plenty that can hang, though. The Loranthaceae family alone has about 77 genera and 1,000 species, which parasitize trees, bushes and even cacti.
Mistletoes produce nutritious berries as part of their reproduction strategy. These berries contain seeds surrounded by a glue-like substance called viscin, which can survive the digestive system of birds and mammals. When the birds and small mammals that consume the fruit poop it back out, the seeds will stick to branches or trunks and begin to penetrate through the bark and into the host's root system to steal resources.
They may also parasitize each other. Recent research published in Oikos by Francisco Fontúrbel, a botanist at the Catholic University of Valparaiso in Chile and his colleagues has revealed that one species found in the Chilean Andes, Tristerix corymbosus, doesn't usually reach past the elevations of its preferred host trees.
These mistletoes can also parasitize another mistletoe species, Desmaria mutabilis. Since Desmaria mutabilis' hosts are found at elevations of 4,000 feet and above, Tristerix corymbosus manages to extend its reach up the mountain by growing on top of the mistletoes that can parasite upper elevation trees, "one parasite of another parasite," Fontúrbel says.
Mistletoes can bring the kiss of death to their host plants in the right conditions. In South American cities, for example, some people view them as pests, since mistletoes can kill trees outright by draining their fluids. But Fontúrbel says that mistletoes usually don't derive a benefit from killing their hosts, as this spells their own end as well. Most often, parasites are only deadly for their hosts in times of drought, when they take too much water, or for introduced species often found in cities, which haven't evolved alongside them. "They rarely kill native trees," he says.