1. Plant Responses
6.L.2.2 Explain how plants respond to external stimuli (including dormancy and forms of tropism) to enhance survival in an environment.

2. What are plant responses?
A stimulus is anything in the environment that causes a response in an organism.
A stimulus may come from outside (external) or inside (internal) the organism.
All living organisms, including plants, respond to stimuli.
A Venus’s-flytrap has
three small trigger hairs on the
surface of its toothed leaves. When
two hairs are touched at the same
time, the plant responds by closing
its trap in less than 1 second.

3. Tropisms
Plants can change their growth in response to their environment. These changes are called tropisms. Positive tropism - a plant growing toward a stimulus. Negative tropism – a plant growing away from a stimulus.

4. Tropisms (Cont.) Plants can exhibit the following kinds of tropisms:
Phototropism - the way a plant grows or bends in response to light.
Geotropism - the way a plant grows or bends in response to gravity.
Hydrotropism - the way a plant grows or bends in response to water.
Thigmotropism - the way a plant grows or bends in response to touch.

5. Phototropism
Positive phototropism - a plant growing or bending toward the light.
When a plant responds to light, the cells on the side of the plant opposite the light get longer than the cells facing the light. Because of this uneven growth, the plant bends toward the light.

6. Geotropism
The downward growth of plant roots is a positive response to gravity or positive geotropism.
A stem growing upward is a negative response to gravity or negative geotropism.

7. Hydrotropism
Hydrotropism Turning or bending towards moisture, as roots.
The most common example is that of plant roots growing in humid air bending toward a higher relative humidity level.

8. Thigmotropism
Thigmotropism - is the directional response of a plant organ to touch or physical contact with a solid object. The plant’s stem bends and twists around any object it touches.

9. Plant Hormones
Hormones control the changes in growth that result from tropisms and affect other plant growth. Ethylene:
Many plants produce the hormone ethylene gas and release it into the air around them. Ethylene is produced in cells of ripening fruit, which stimulates the ripening process.
Fruits such as oranges and bananas are picked when they are unripe and the green fruits are exposed to ethylene during shipping so they will ripen.

10. Plant Hormones (Cont.) Auxin:
A plant growth-regulating substance found in plants that stimulates cell elongation in plant tissues. It promotes root formation and bud growth. It also causes plant leaves and stems to exhibit positive phototropisms.

11. Plant Hormones (Cont.) Gibberellins:
Are plant growth substances involved in promotion of stem elongation, mobilization of food reserves in seeds and other processes. Its absence results in the dwarfism of some plant varieties.
Cytokinins:
Are a class of plant growth substances (plant hormones) active in promoting cell division, and are also involved in cell growth.

12. Plant Hormones (Cont.) Abscisic Acid:
The substance that keeps seeds from sprouting and buds from developing during the winter. It also causes the stomata to close and helps plants respond to water loss on hot summer days.

13. Photoperiods
Some plant species produce flowers at specific times during the year. Ex.) Sunflowers bloom in the summer, and cherry trees flower in the spring.
Photoperiodism is the effect of day and night length on plant flowering. Some plants are long day, requiring hours of sunlight per day to flower. Others are short day requiring only hours. Others are day neutral and unaffected by day length.

14. Darkness and Flowers
Long-day plant is a plant that generally requires short nights—less than 10 to 12 hours of darkness—to begin the flowering process. Ex.) Spinach, lettuce, and beets
Short-day plant is a plant that generally requires long nights—12 or more hours of darkness—to begin the flowering process. Ex.) Poinsettias, strawberries, and ragweed

15. Day-Neutral Plants
Day-Neutral Plant is a plant that doesn’t require a specific photoperiod and can begin the flowering process over a range of night lengths.

16. Questions
A plant is placed inside near a window. Over time, it grows toward the window. Why does the plant grow toward the window? A. to receive more sunlight B. to receive more fresh air C. to get away from the air conditioning D. to get away from the light in the room

17. A student places a small plant on a shelf for three weeks.
What has most likely happened to the plant?
A. The plant was growing toward the source of light.
B. The plant was too heavy to be supported by its stem.
C. The plant was blown by air from a vent in the house.
D. The plant was moved toward the window by the student.

18. Which is the primary stimulus that makes a stem from a seed grow up and out of the ground, instead of down into it?
A. air
B. gravity
C. light
D. pressure

19. The End