Sherri Berbereia, age 10, of Visalia, California, for her question:
Why does mistletoe grow on trees?
Ages ago, people thought that mistletoe was a lucky plant. If a boy saw a girl standing under a bunch of its pretty leaves, he claimed the right to a kiss. This was during the merry Christmas season, when bunches of mistletoe were hung over the doors. Some people still practice this merry old mistletoe custom.
Most plants poke their roots into the soil and lift their leaves in the sunshine to manufacture their own food. Most, but not all plants make their own livings, so to speak. The dainty mistletoe is one of the plants that cannot make do for itself. It must feed on the nourishing sap manufactured by other plants. We call such a plant a parasite and the plant it feeds upon is called a host.
The roots of the tufty, pale green mistletoe dig down through the bark of a tree. They reach the woody cells that carry the sap and soak up this nourishing mixture of dissolved plant foods. The mistletoe is unable to make this necessary food for itself. However, its pale green leaves can and do manufacture some of the food it needs. But to do this they must have air and sunshine.
This is why mistletoe perches on boughs high above the ground, where there is sunshine and circulating air. The tree on which the parasite plant lives provides most of its food, plus a perch where its pale leaves can manufacture the rest.
Some people think that mistletoe grows only on sturdy oak trees. Actually it flourishes on many different trees, both evergreen and those that lose their leaves in winter. In the American woodlands it may grow on poplar and sycamore, on willow and maple and mountain ash, on evergreen firs and sometimes on the boughs of smallish hawthorn and locust. Sometimes it grows on wild crab apple and on orchard apple trees.
Everything in this pale green parasite seems to grow in pairs. The main stalk sprouts a pair of twigs, a twig sprouts a pair of stems and a stem sprouts a pair of fat oval leaves. Even the round milky white berries tend to cluster in twos.
In winter, the waxy white mistletoe berries are greatly enjoyed by the birds, especially by certain thrushes. The berries tend to be rather tacky and often stick to a bird's bill. Chances are, he flies off and cleans his bill on some other tree. A few seeds are wiped off onto the rough bark. A lucky seed may fall into a crevice, where it can set down root suckers into the wood and start a new plant. If the bird returns later, he finds a new feast of berries.
A tuft or two of mistletoe doesn't do a great deal of harm to a strong healthy tree. But, after all, the little plant is a parasite that takes plant food manufactured for the host. Sometimes the mistletoe spreads from bough to bough, stealing quantities of plant food and shutting out light and air. An overgrowth of mistletoe of this sort sooner or later kills the host tree.