Biological odd couples: Plants are more social than they look

Many Alaska plant species form unusual partnerships with other organisms

Posted: Sunday, November 26, 2006

We all learn in high school biology that land plants draw nutrients and water from the soil, and their green pigments use the carbon from carbon dioxide in the air to build carbohydrates in a process known as photosynthesis.

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We probably heard (if we could just remember...) that plants generally are called "autotrophs," meaning that they feed and nourish themselves and don't depend on consuming other organisms. But the real world is not so simple - plants are often not as independent as that.

Lots of land plants have essential associations with special fungi called 'mycorrhizal' fungi. There are many kinds of these fungi, and well over 80 percent of vascular plants (trees, shrubs, herbs, ferns) interact with them; only a few groups, such as sedges, typically lack them.

The fungi live in the soil and grow next to, around, and even into the roots of the plants. It's a symbiotic relationship from which both partners benefit. The fungi get carbohydrates that the plant synthesizes, and the plant gets an enhanced supply of nutrients such as minerals from the fungi, which extract them from the soil.

Especially in poor soils, plants grow better if they have their mycorrhizal associates. Local examples are legion: blueberries, orchids, willows, pines, spruces, and even club mosses.

Perhaps the moral is that we shouldn't kick over toadstools on a walk through the woods - they may be the reproductive bodies of essential mycorrhizal fungi.

Still other land plants have rather poor root systems but supplement their "income" by capturing insects. Local examples include the sundews and the butterwort, which grow in muskegs.

These plants are green and can support themselves without captured insects, but they usually grow and reproduce better with the bugs.

The leaves of sundews and butterwort secrete digestive enzymes that dissolve insects, and nutrients from the insect are absorbed into the plant.

Insect-eating plants are thought to be especially characteristic of nutrient-poor habitats, but there is growing evidence that lots of plants, in a variety of habitats, use sticky parts to capture insects and may be able to digest them.

Over a dozen species in the Pacific Northwest show this ability to some degree. Long ago, one of my Midwestern graduate students fed radioactive fruit flies to two species that were not previously known to be insect-eating, but both had sticky traps near the flowers.

The students found that the radioactive markers on the nutrients from the flies were transferred to the developing seeds, and seed production was improved.

I suspect that a number of additional plants will be found to be insect-eating, and that the traditional view of such plants living only in poor habitats will have to be revised.

The situation is complicated by the fact that many plants use sticky traps as defenses against plant-eating insects, so sticky traps have more than one possible function. It will be especially interesting if we find that some plants use sticky traps both for defense and for nutrition.

There are also plants that aren't green at all. They lack the green pigment that is part of the internal machinery of photosynthesis, so they depend on other plants for nourishment; in short, they are parasitic.

We have several local examples. Ground cone is parasitic on alder roots and occasionally on other species. All we see above the ground is the flowering stalk, on which the small maroon-brown leaves embrace small dark flowers that are pollinated by flies.

Bears dig up and eat the underground parts of ground cone. Dwarf mistletoe commonly grows on the branches of hemlock trees, creating the disorganized-looking tangles called witches' brooms.

Its sticky seeds are dispersed when the seed capsule forcibly ejects them; they adhere to other branches by means of the sticky covering.

The next three species of non-green, essentially leafless plants are usually considered to derive nutrients from decaying plant material, but the real relationship may be more complex - they may use fungi to draw energy from trees.

Two species of coralroot orchids grow here: a pink species in the conifer forest, and a yellowish species usually among alders in early vegetation succession. Pinesap is relatively rare around here, growing under conifer trees.

Many trees and even herbs make root grafts with each other, so that nutrients are transferred among individuals and even species via the root connections. So plants living in poor conditions, even cut stumps, often can live and grow by feeding off the neighbors.

The bottom line is that land plants are not necessarily such nutritionally independent beings as we may have believed; instead, they are part of a widespread, interdependent, nutritional network of organisms.

As one eminent biologist noted: the exceptions PROBE the rule.

• Mary Willson is a retired professor of ecology and a Trail Mix board member.

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