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Roots: A wide world under our very feet

Posted: Sunday, January 27, 2008

When I think of the roots of plants in an off-hand way, three things come to mind immediately: 1) those treacherous, wet, slimy tree roots that separate mudholes on certain poorly maintained trails around here; 2) root wads of up-ended trees, which host a bevy of colonizing young trees and shrubs, and when washed out onto the tideflats-provide convenient perches for eagles; and 3) human food crops such as carrots, beets, and (ugh) parsnips.

If I bother to think a little more, however, I begin to recognize just what amazing, sophisticated structures they are. And what they do affects many other organisms in their ecosystems.

Plant roots can be said to have three main functions:

A) Anchorage. Roots anchor plants to the ground, sometimes penetrating many yards deep and commonly spreading out beyond the perimeter of the plant's top parts. The anchoring function is such common-place knowledge that we notice it more when it has failed.

B) Storage. Some plants store considerable amounts of carbohydrates (from photosynthesis of their green parts) in their roots. We are not the only animals that make use of storage roots such as carrots. Bears sometimes roto-till the ground to obtain lupine roots or, in the Interior, the roots of sweet-vetch. Some lupines and sweet-vetches are poisonous to humans, however. Both humans and bears dig the root systems of chocolate lilies (a.k.a. Indian rice) to get the nutritious little "grains" attached to the roots.

C) Obtaining water and minerals from the soil. In general, roots take up water and minerals via multitudes of fine root hairs that grow out from the main roots. A given root system may have billions of root hairs at a time, totaling thousands of miles in length if placed end-to-end, and providing hundreds of square yards of absorbing surface. Root hairs develop just behind the root tip. They are short-lived and are replaced as the root grows, so they are continually being exposed to new areas of soil.

In addition to this basic function, plants have evolved some amazing special adaptations to improve mineral uptake. Two of these involve mutualistic relationships with other organisms.

The first of these special arrangements occurs between most woody plants (including lots of those around Juneau) and special fungi called mycorrhizae ("fungus-root"), which I've mentioned before in this column. Some of these fungi actually penetrate the plant root and others merely invest the outside of the root. Both kinds of mycorrhizae augment the plants' supply of water and minerals, and in some cases they move nutrients from one plant to another. The fungi depend on small mammals for dispersal of their spores - mammals such as squirrels eat the spore - bearing part of the fungus and excrete the spores intact and germinable.

The second special arrangement occurs between plants and two different kinds of bacteria. In both cases, the bacteria induce the plant to produce nodules on the roots, in which the bacteria then live. The bacteria "fix" nitrogen, taking nitrogen from the air and putting into a form (nitrate) that plants can use. Plants with better supplies of nitrogen often grow better, and the leaves they shed annually augment the nitrogen in the soil, where other plants can then use it. Local examples of nitrogen-fixing plants include lupines and alders and sweetgale.

The third special adaptation is found especially in plants that grow in soils where phosphorus is in short supply. These plants form "cluster roots" on their main root systems - a sort of on-demand phosphorus pump, produced only when needed. Cluster roots look like bottlebrushes, a dense bristling of fairly short rootlets that specialize in the uptake of phosphorus, although other minerals may be gathered as well. They also increase the availability of certain minerals to neighboring plants and change the community of soil bacteria. Local examples of plants that can make cluster roots include alders and sweetgale (a shrub of wet meadows). Curiously, lupines of the Old World form cluster roots but reportedly our lupines do not.

Woody plants sometimes make grafts between their roots, thus transferring nutrients from one root system to another. This is well known to occur among members of the same species, but also occurs between species.

I might mention, as an afterthought, that some plants living in distant places do weird and wonderful things with their roots. Some climbing vines make aerial roots that cling to available surfaces. Mangroves grow aerial roots that stick up out of the mud to aerate the normal roots, which need air for respiration. Those of us from the Midwest have seen the prop roots at the base of corn plants; the prop roots emerge from the lower part of the stem and help support the plant. Perhaps the most exotic arrangement is found in a tropical epiphyte (a plant that grows on other plants). This queer plant makes almost-closed cups of its leaves, and ant colonies live in these cups. The cups also catch some rainwater and debris, and the ants add to the nitrogen supply by dumping the remains of their insect prey there. The plant sends roots into this cup of damp and decomposing matter to obtain the necessary nutrients.

Thus, those mundane, (usually) underground parts of plants turn out to be a lot less boring than one might think at first glance. What goes on underground beneath our feet is far more complex and important than we normally bother to think about. Indeed, some of these remarkable adaptations are both ecologically important and evolutionarily interesting. We are left with questions such as, why do certain plants have these features, but others growing in seemingly similar circumstances, lack them?

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



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