From Seed to Seed:
Plant Science for K-8 Educators

Unwelcome Visitors.

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. There, the seed germinates and produces rootlike 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.

Generally, mistletoe doesn't kill its host tree, although 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.

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 species) 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 rootlike organs that penetrate its hosts. In contrast to mistletoe, dodder has very little chlorophyll, and fills all of its food, nutrient, 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 seeds germinate 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 half a mile in length! Dodder is a troublesome parasite found all over the United States 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. Students can identify these plants quite easily during a forest field trip. 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 nutrients 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 only a few examples of plant-plant relationships. And these are just examples of the connections that we know about-there are many more out there, waiting to be discovered.

Since we have already spent some time learning about plant-animal relationships, we'll be brief in this next section.