Chapter Summary

1. Roots anchor plants and absorb water and minerals in solution. A germinating seed radicle becomes the first root. Taproots with branch roots, or adventitious roots that become a fibrous root system develop from the radicle or the stem just above it. Many plants have combinations of both systems.

2. Four zones or regions of young roots are recognized: (1) A protective root cap that also aids in the perception of gravity. (2) A region of cell division. Its apical meristem subdivides into a protoderm, which produces the epidermis; a ground meristem, which produces the cortex; and a procambium, which produces primary xylem and primary phloem. (3) A region of elongation in which the cells produced by the apical meristem become considerably longer and slightly wider. (4) A region of maturation in which the cells mature into the distinctive cell types of primary tissues.

3. Some of the epidermal cells in the region of maturation develop root hairs; the root hairs greatly increase the absorptive surface of the root. The tissues that mature in this region are similar to those of stem tips, but pith is absent in most dicot roots and originates from the procambium in monocot roots.

4. The cortex has an endodermis with suberized Casparian strips at its inner boundary.

5. Next to the endodermis toward the center of the root are parenchyma cells constituting the pericycle. Branch roots and the vascular cambium arise in the pericycle.

6. In dicot roots, the primary xylem usually first forms a solid core with two to several arms in the center of the root; a pith may be present in monocot roots.

7. Primary phloem first is produced in discrete patches between the primary xylem arms, but the tissues eventually appear as concentric cylinders. In woody plants, a cork cambium usually arises in the pericycle and produces cork tissues similar to those of stems. Roots may graft together naturally. There are no leaves in roots.

8. Specialized roots include those for food- or water-storage; pneumatophores; aerial roots (velamen roots, prop roots, photosynthetic roots, and adventitious roots); contractile roots; and buttress roots. Haustoria are peglike parasitic roots. Mycorrhizae are mutualistic associations between roots and fungi. Some plants have nitrogen-fixing bacteria in nodules on their roots.

9. Root crops include sugar beets, beets, turnips, rutabagas, parsnips, carrots, sweet potatoes, yams, and cassava. Several spices are obtained from roots. Other uses of roots include the production of alcohol and the extraction of dyes, drugs, insecticides, and poisons.

10. Soils contain a mixture of ingredients, including sands, rocks, silt, clay, humus, dead organic matter, plant roots, small animals, microorganisms, plus air and water, within pore spaces of various sizes.

11. A vertical column of soil exhibits horizons; the topsoil is divided into an upper A horizon and a lower E horizon. The B horizon (subsoil) usually contains more clay and is lighter in color than the topsoil. The C horizon (bottom portion) constitutes the weathered soil parent material.

12. Living organisms in the soil decompose organic matter, the source of most important plant nutrients. Animals also cultivate the soil. Soil erosion is affected by topography.

13. Soil texture pertains to the relative proportions of sand, silt, and clay. More than half of the composition by weight of mineral matter is oxygen.

14. Soil structure refers to the arrangement of the soil particles into aggregates. Good soils are highly granular and have pore spaces that constitute about half the total volume.

15. Water in the soil occurs as hygroscopic water, gravitational water, and capillary water.

16. The field capacity of the soil is the amount of water that remains after the rest of the water has drained away by gravity. Soil reaches the permanent wilting point when plants wilt permanently because they can no longer extract enough water from the soil for their needs. Available water is soil water between field capacity and the permanent wilting point.