1. Chemical Control Mrs: Jackie Maldonado

2. Chemical control in plants • Phototropism • Plants are not inactive • They respond to changes in the environment (light, water, gravity) • Have you ever notice that plants tend to grow toward the light? • It happens because an unequal stimulation called tropism

3. Chemical control in plants • Positive tropism- plant grow toward the stimulus • Negative tropism- plant grow away from the stimulus. • Darwin’s experiments

4. Darwin experiments • Results: The tip of the stem somehow detects the presence of lights and controls the grow of the stem below. • In 1926Frits Went carried out a famous experiment • Removed tips from many young stems and placed them on agar. • Placed the tips on agar which retain the substances from the tips • He placed a piece of the agar on the tip of the stem and the stem growth straight up. • Pieces with no substances were placed also on the tip but no straight grow occurs.

5. Auxins • Means to grow • Causes cells to elongate rapidly. • Light coming from one side causes auxins in the tip to move toward the darker side • Cells in the darker side elongate more quickly than cells in the lighted side. • Because the unequal growth of cells the stem seems to bend toward the light.

6. Auxin • Is a hormone • Causes a decrease in the pH of stem cell walls making them more acidic • An enzyme in the cell walls separate cellulose molecules from one another allowing cell elongation.

7. Others effects of auxins • Geotropism • Growth response to the force of gravity • Plants present negative geotropism. • Roots grow in the direction of gravity- positive geotropism • Root growth is regulated by auxin • If a root is place sideways, auxins collects along its lower side inhibiting growth. Upper side cells elongate and the root turns downward.

8. Auxin and trees growth • Apical meristem- tip of the tree has a large concentration of auxins • Auxin inhibits the growth of the lateral meristem. • Results- cone shape • This is called Apical dominance= inhibition of lateral bud development by auxins

9. HW • 1. At the time of Went’s experiments, it was taught that the tip of the stem informs the rest of the stem by some electrical means. How did Went’s work show that it is a chemical that passes from the tip to the rest of the stem and not an electric current? • 2. Why do you think many orchard keepers spray developing fruits with auxin? • 3. How is cone shaped growth in a plant an adaptive advantage? Think about photosynthesis.