Lesson Overview

1. Lesson Overview 24.4 Plants and Humans

2. Worldwide Patterns • Many scholars trace the beginnings of human civilization to the cultivation of crop plants. • Once people discovered how to grow plants for food, the planting and harvesting of crops tended to keep them in one place for much of the year, leading directly to the establishment of social institutions.

3. Worldwide Patterns • Thousands of different plants—nearly all angiosperms—are raised for food in various parts of the world. • Despite this diversity, much of human society depends on just a few crop plants, such as rice, wheat, soybeans, and corn for the bulk of its food supply. These crops are also fed to livestock.

4. Worldwide Patterns • The food we eat from most crop plants is taken from their seeds. • For nutrition, most of humanity worldwide depends on the endosperm of only a few carefully cultivated species of grass. • Roughly 80 percent of all U.S. cropland is used to grow just four crops: wheat, corn, soybeans, and hay. Of these crops, three—wheat, corn, and hay—are derived from grasses.

5. New Plants • The efficiency of agriculture has been improved through the selective breeding of crop plants and improvements in farming techniques. • The corn grown by Native Americans, for example, was developed more than 8000 years ago from teosinte, a wild grass found in Mexico.

6. New Plants • Selective breeding has produced modern-day corn. • The changes caused by selective breeding can be very dramatic. Modern corn has greatly exaggerated kernels compared to its ancestor, teosinte.

7. New Plants • In more recent times, other familiar crops have been the product of selective breeding. • Sugar beets, the source of most refined sugar from the United States, were produced from the ordinary garden beet using selective breeding. • Plants as different as cabbage, broccoli, and Brussels sprouts have been developed from a single species of wild mustard.

8. Changes in Agriculture • Improvements in farming techniques have contributed to dramatic improvements in crop yields, as shown in the graph.

9. Changes in Agriculture • Some of the most important techniques have involved the use of fertilizers and pesticides. • Fertilizers are labeled with three numbers that reflect the percentage by weight of three elements: nitrogen (N), phosphorus (P), and potassium (K).

10. Changes in Agriculture • Fertilizers and pesticides must be used with great care. Overfertilizing can kill crop plants by putting too high a concentration of salts into the soil. • The intensive use of fertilizers can also affect the ground water. When large amounts of nitrogen- and phosphate-containing fertilizer are used near wetlands and streams, runoff from the fields may contaminate the water. • Chemical pesticides are poisons, and they have the potential to harm wildlife and leave dangerous chemical residues in food.

11. Fiber, Wood, and Medicine • Plants produce the raw materials for our homes and clothes, and some of our most powerful and effective medicines. • The succulent plant Aloe vera contains many chemicals that soothe and moisturize the skin.

12. Fiber, Wood, and Medicine • The acoustical properties of spruce wood make it ideal for use in musical instruments.

13. Fiber, Wood, and Medicine • Cotton is used in many products including thread, bandages, carpet, and insulation. Cotton fibers are outgrowths of the seed coat epidermis.