Mistletoe. Many gardeners (and holiday enthusiasts) are shocked to
learn that their traditional "kissing" mistletoe is actually a parasitic plant. The life of a typical mistletoe plant begins when a bird eats a
berry and drops the seed, undigested, high in the branches of a tree, usually a hardwood
like apple or oak. There, the seed germinates and produces root-like structures. These
structures penetrate the bark of the tree until they reach the secondary xylem (wood). The
mistletoe then begins absorbing water and minerals from its host.
Unlike some parasites, mistletoe manufactures its own food
with its mass of small green leaves. (Organisms like this, that are only partially
dependent on their hosts, are sometimes called partial parasites, or hemi-parasites.)
Generally, mistletoe doesn’t kill its host tree, thought it may weaken it, especially
in regions where water is scarce and the tree is unable to fill both its own and the
mistletoe’s needs. And a large mass of mistletoe may shade the tree’s leaves and
reduce its capacity for photosynthesis, further weakening it.
Mistletoe (Phoradendron spp.) is often referred to
as leafy mistletoe, to distinguish it from dwarf mistletoe, a more damaging parasite.
Leafy mistletoe is found mostly in warmer regions, but may be found as far north as parts
of New Jersey and Oregon (about 40 degrees N latitude).
Dodder. Some plants have lost most or all
of their capacity to photosynthesize, and are completely dependent on their hosts for food
and water. Dodder (Cuscuta spp.) is one such example. Dodder is a stem and leaf
parasite consisting of a long, thin, yellow or orange stem. It coils tightly around nearby
plants, forming root-like organs that penetrate its hosts. In contrast to mistletoe,
dodder has very little chlorophyll, and fills all its food, mineral, and water needs via
its host, making it fully parasitic. Some species of dodder are highly selective, found
only on certain types of plants.
Dodder is an annual; its seed germinates in the soil, but
once the shoot has found a suitable host its connection to the soil deteriorates. A single
dodder stem can reach up to a half mile in length! Dodder is a troublesome parasite found
all over the U.S. except northern New England and the northern Great Lakes region.
And last but not least, let’s look at a complex,
three-way association.
Indian pipe. Completely lacking
chlorophyll, Indian pipe (Monotropa uniflore) sends up ghostly white shoots with
nodding, cup-shaped flowers. For years scientists believed that the plant was a saprophyte
more akin to fungi than to green plants. It is now known that the plant is instead wholly
dependent on its association with soil-dwelling, mycorrhizal fungi. Mycorrhizal fungi
typically form a symbiotic association with the roots of a photosynthetic plant. In this
case, the fungus draws carbohydrates and minerals from a green plant (often a spruce or
pine tree) for its own needs, but also acts as a bridge, transferring these materials to
the Indian pipe as well!
We’ve looked at just a few examples of plant-plant
relationships. Hopefully, you are getting a taste of the interconnectedness of all living
things. And these are just examples of the connections we know about—there are many
more out there, waiting to be discovered.
Since we’ve already talked at different points in the
course about plant-animal relationships, we’ll be brief in this next section.
