The Kissing Parasite
In
northern countries, at Christmas time, anyone standing under the
mistletoe
must be kissed. In Costa Rica, this would lead to a lot of smooching:
there
are hundreds of mistletoe species in the neotropics. The characteristic
of
mistletoes is that they are parasites: they live off the branches of
trees.
Unlike ordinary epiphytes, mistletoes obtain all of their mineral
nutrients
and water by forming direct connections with water-conducting veins
(“xylem”)
inside the branches of host trees. That first kiss yields a lifelong
bond.
 Sticky
Seeds
All
mistletoes have extremely sticky seeds, which enable them to grab onto
to the branches of potential hosts instead of falling to the ground.
The
stickiness comes from specialized tissue called viscin. Mistletoes are
unable
to grow normal roots and depend completely on the host trees for
survival.
They
Get
Around
Seeds
from
this plant stick to the tail feathers of birds climbing up and
down the trees. The birds fly elsewhere, then rub their bottoms on the
bark
of the trees to remove the irritating seed, thereby forcing the seeds
into
tiny crevices in the bark where the seeds germinate. Once on the host
tree,
the seeds germinate and carry out a peglike root to tap into the tree's
vascular system. Mistletoe can only penetrate through young, thin bark.
The
mistletoe does not grow just anywhere it falls, however. One reason
seems
to be that the seed does not invade the tree — rather it is the other
way
around. The tree branch must grow and as it grows, the mistletoe is
able
to tap into its veins. No growth, or slow growth, seems to mean the
mistletoe
does not survive. They also need larger branches, and do better if they
are
protected from wind and seed predators like ants and mice. Scientists
are
still unsure as to why some trees are loaded with mistletoes while
others
have none.
A
Parasite,
Yes, But...
The
mistletoe removes water and essential nutrients from the host tree,
but mistletoe is not a complete parasite because it does make its own
food. It is not considered a major threat to trees but can cause some
growth loss and damage to branches.
|
