Biology Reference
In-Depth Information
Mistletoe
Mistletoes are seed plants belonging to the family
Viscaceae. They are semiparasites, manufactur-
ing food but depending on a host plant for water
and mineral salts. There are three genera in North
America:
Phoradendron
and
Viscum
which are
true mistletoes,
opposite leaves and round, jointed stems, and are
dioecious with inconspicuous petal-less flowers.
They occasionally become so large or numerous
that the weight of the parasite breaks branches of
the host. Growth is slow at first, but in 6 to 8 years
the tufts may be 3 feet across. The aerial part does
not live much longer than that, but the haustoria
live as long as the tree, producing new bunches
from adventitious buds.
Because they manufacture their own food, mis-
tletoes require a lot of sun, which may be one
reason why they flourish so in the Southwest.
Leafy mistletoes are relatively harmless in some
situations; in others they handicap shade and forest
trees, and occasionally kill hackberries and oaks.
There are a few leaf spots and other fungus dis-
eases that keep mistletoes from getting too abun-
dant. They are harvested for Christmas greens with
a curved mistletoe hook, which can be used to
keep aerial portions cut off valuable trees. Break-
ing off or cutting off the bunches, however, may
lead to more shoots in an ever-widening area.
Dwarf mistletoes are far more injurious, espe-
cially to forest trees, and much less conspicuous.
In western coniferous forests they rank next to
heart rots in importance, reducing the quality and
quantity of timber and paving the way for bark
beetle infestations. Infected branches should be
pruned out; if the trunk is infected, the tree should
be felled and removed.
and
Arceuthobium
,
dwarf
mistletoe.
The mistletoe seed is naked embryo and endo-
sperm invested with a fibrous coat and borne in
white, straw-colored, pink, or red
fruits - “berries” - embedded in a sticky gelati-
nous pulp enabling them to cling to bark of trees
or stick to feet and beaks of birds, which dissem-
inate them.
The seeds can germinate almost anywhere but
penetrate only young thin bark, by means of
a haustorium sent out from a flattened
disc. Branches of the haustorium extend up and
down and around the tree and occasionally pro-
duce secondary haustoria. The number of annual
rings on a tree between the tip of the primary
haustorium and the bark tells the age of the mis-
tletoe. Many are 60 to 70 years old, and one has
been reported as living 419 years. The aerial
portions of mistletoes are leafy, evergreen tufts
of shoots on the stems of host plants, most con-
spicuous on hardwoods after leaf fall (see
Fig. 1
).
The stems and leaves contain chlorophyll and are
green but often with a yellowish, brown, or olive
cast, depending on the season. All species have
