Roots Play Key Roles in the Ecology and Evolution of Plants

Soil teems with life: on average, about 0.1% of the weight of soil is living organisms. Although this percentage may not impress you, consider that nearly 170,000 species of soil organisms have been identified. One kilogram of fertile soil contains about 2 trillion bacteria, 400 million fungi, 50 mil-

How Roots Stabilize Plants

FIGURE 7.20

Modified roots. Some tropical trees produce planklike buttress roots at the base of their trunk. Buttress roots help stabilize and support the tree.

FIGURE 7.20

Modified roots. Some tropical trees produce planklike buttress roots at the base of their trunk. Buttress roots help stabilize and support the tree.

lion algae, 30 million protozoa, nematodes, other worms, and insects. Together, these organisms exert a tremendous influence on soil and the plants it supports.

Organisms mix and refine the soil. For instance, one earthworm can digest more than a ton of soil in 1 year. This is why Aristotle called earthworms "the intestines of the earth": they aerate and refine the soil by processing it through their guts. Organisms also add humus (decaying organic material) to the soil. Plants, especially grasses, are the primary source of organic matter in soil. A 4-month-old rye plant produces almost 11,000 kilometers (a length greater than the round-trip distance between Miami and Seattle) of roots and root hairs, most of which eventually die and become humus. Because humus contains large amounts of nutrients, the biological activity of soil is usually an excellent indicator of its fertility.

Organisms can affect the availability of nutrients to plants. Roots of lupine (Lupinus albus) growing in

Indicator Plants For Minerals

FIGURE 7.21

Scanning electron micrograph of a root of a lodgepole pine (Pinus contorta) associated with filaments of mycorrhizal fungus.

FIGURE 7.21

Scanning electron micrograph of a root of a lodgepole pine (Pinus contorta) associated with filaments of mycorrhizal fungus.

phosphorus-deficient soil can decrease the pH of the soil and thus increase the availability of other nutrients to plants. Similarly, plants and microbes secrete enzymes into the soil that liberate phosphate from mineral particles, making more phosphate available to plants.

Some organisms make the soil inhospitable for plants. For instance, many plant pathogens, such as nematodes, live in the soil. Furthermore, some plants produce chemicals that inhibit the growth of other plants; as an example, the soft-leaved purple sage (Salvia leucophylla) produces chemicals that inhibit the growth of nearby plants. As a result, these sage plants are often surrounded by bare zones in which no other plants grow. Some scientists refer to this interaction as a form of "chemical warfare" called allelopathy.

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