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Mycorrhizae Root Nodules

Human Relevance of Roots Soils

Parent Material Climate

Living Organisms and Organic

Composition Topography

Soil Texture and Mineral

Composition Soil Structure Water in the Soil Soil pH Summary

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This chapter discusses roots, beginning with the functions and continuing with the development of roots from a seed. It covers the function and structure of the root cap, region of cell division, region of elongation, and region of maturation (with its tissues). The endodermis and pericycle are also discussed.

Specialized roots (food-storage roots, water-storage roots, propagative roots, pneumatophores, aerial roots, contractile roots, buttress roots, parasitic roots) and mycorrhizae are given brief treatment. This is followed by some observations on the economic importance of roots.

After a brief examination of soil horizons, the chapter concludes with a discussion of the development of soil, its texture, composition, structure, and its water.

Some Learning Goals

1. Know the primary functions and forms of roots.

2. Learn the root regions, including the root cap, region of cell division, region of cell elongation, and region of maturation (including root hairs and all tissues), and know the function of each.

3. Discuss the specific functions of the endodermis and the pericycle.

4. Understand the differences among the various types of specialized roots.

5. Know at least 10 practical human uses of roots.

6. Understand how a good agricultural soil is developed from raw materials.

7. Contrast the various forms of soil particles and soil water with regard to specific location and availability to plants.

ou have at least seen pictures of the destruction ' caused by a tornado as it passed through a village or a city, but have you seen what a twister can do to a forest? Large trees may be snapped off above the ground or knocked down, and branches may be stripped bare of leaves. Unless the soil in the area happens to be thin, sandy, or loose, however, you will probably see relatively few trees completely torn up by the roots and blown elsewhere. In the tropics, it is indeed rare to find healthy palm trees uprooted even after a hurricane.

Roots anchor trees firmly in the soil, usually through an extensive branching network that constitutes about one-third of the total dry weight of the plant. The roots of most plants do not usually extend down into the earth more than 3 to 5 meters (10 to 16 feet); those of many herbaceous species are confined to the upper 0.6 to 0.9 meter (2 to 3 feet). The roots of a few plants, such as alfalfa, however, often grow more than 6 meters (20 feet) into the earth. When the Suez Canal was being built, workers encountered roots of tamarisk at depths of nearly 30 meters (100 feet), and mesquite roots have been seen 53.4 meters (175 feet) deep in a pit mine in the southwestern United States. Some plants, such as cacti, form very shallow root systems but still effectively anchor the plants with a densely branching mass of roots radiating out in all directions as far as 15 meters (50 feet) from the stem.

Besides anchoring plants, roots absorb water and minerals in solution, mostly through "feeder" roots found in the upper meter (3.3 feet) of soil. Some plants have roots that, as well as anchoring and absorbing, store water or food, or perform other specialized functions.

Some aquatic plants (e.g., duckweeds and water hyacinths) normally produce roots in water, and many epiphytes (nonparasitic plants that grow suspended without direct contact with the ground—e.g., orchids) produce aerial roots. The great majority of vascular plants, however, develop their root systems in soils. The soils, which vary considerably in composition, texture, and other characteristics, are discussed toward the end of this chapter.

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