IS AMERICA'S MOST POPULAR PARASITE
by Whit Gibbons
As one of my daughters told me, in the spirit of the holiday season,
"Why don't you write about mistletoe, holly, or reindeer."
I've written about holly and reindeer in recent years, so let's return
to mistletoe, the only plant associated with Christmas that has flowers
pollinated by insects, has seeds transported by birds, and takes its
water and minerals from trees.
"Mistletoe" refers to any of more than 200 species of semi-parasitic
shrubs found worldwide. Mistletoe lives throughout the southern United
States, from the Atlantic Coast to California, and on every continent
Having true parasitic properties, mistletoe is devoid of roots. Instead,
the dark green shrub has extensions called holdfasts that grip the host
tree, from which the root-like anchors suck water and nutrients. Thus,
mistletoe is only found on living trees, which are essential to the
mistletoe's survival. In contrast, Spanish moss uses trees dead or alive,
but only for support, extracting water and nutrients from the atmosphere.
In the South, tiny yellow flowers bloom on the evergreen mistletoe from
fall into winter. The familiar white berries begin to form soon after
pollination and resemble little packets of glue around tiny indigestible
seeds. A mistletoe plant can be either male or female, and, like a holly
tree, only the female plant has berries. Although eating mistletoe berries
may potentially be lethal for humans, birds seem to be immune to any
The immunity of birds to mistletoe's poisonous qualities is essential
to the welfare of the plant. The dispersal and propagation of mistletoe
is largely dependent on birds that eat the berries but do not digest
the seeds. Ecological studies suggest that seeds are most likely to
survive and grow if a bird deposits them on the same species of tree
on which the parent plant lived. A spring migration flock of cedar waxwings
can result in newly developing mistletoe plants being far away from
where the seeds were eaten.
Mistletoe thrives in bright sunlight in the uppermost branches of big
oaks and is absent from pines and evergreen hardwoods such as magnolias
with needles and leaves that hamper direct sunlight.
A parasitic life style is unusual among flowering plants, but many aspects
of mistletoe biology are well understood. Although the effort to obtain
water and minerals, or even space itself, is intense and highly competitive
among most plants, mistletoe does not encounter such problems. Tree
limbs, a ready source of water and minerals for this unusual little
plant, are available throughout the South, and its absence from the
uppermost branches of a tall oak is probably because no bird has dropped
a seed there.
Mistletoe has been singled out over the ages as possessing unique qualities,
perhaps, in part, because of its many unusual ecological properties.
According to Scandinavian legend, mistletoe was the only organism in
the world from which Odin's son Baldur was not protected, and a mistletoe
dart was the cause of his death. Mistletoe is also associated with the
Druids, the mysterious, oak-worshiping sect that inhabited the British
Isles centuries ago. The Druids considered mistletoe a plant of honor
and power. According to legend, when the parasitic plant was found growing
in an oak tree, the Druids performed strange sacrificial ceremonies
at the tree on the sixth day after a full moon. The Druids reportedly
used a golden sickle when harvesting mistletoe from a sacred oak.
The use of mistletoe as a romantic lure stems from England at least
as early as the 1500s. In 1520, William Irving wrote that a young man
should pluck a berry each time he kisses a young girl beneath the mistletoe.
A version of the tradition persists today in secular Christmas decorations.
However, although the berries appear just in time for Christmas, mistletoe
is not used in churches. One possible reason for not including mistletoe
in today's religious decorations is its association with the Druids.
Mistletoe serves as an example of how an organism can receive attention
because of its extraordinary ecology. To be sure, the influence of such
a lowly and inconspicuous plant on human actions is impressive.
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