CHAPTER 11 Plant Processes LESSON 2 Plant Responses What do you think? Read the three statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Key Concepts How do plants respond to environmental stimuli? How do plants respond to chemical stimuli? Before Statement After 4. Plants do not produce hormones. 5. Plants can respond to their environments. 6. All plants flower when nights are 10 12 hours long. Stimuli and Plant Responses Have you ever been in a dark room when someone suddenly turned on a light? How did you react when the light suddenly came on? You might have shut your eyes or covered them. Organisms can respond to changes in their environments in many different ways. In this lesson, you will learn how plants respond to environmental and chemical stimuli. Stimuli (STIM yuh li; singular, stimulus) are any changes in an organism s environment that cause a response. Many plant responses to stimuli occur slowly. In fact, they are so slow that it is hard to see them happen. The response might occur gradually over a period of hours or days. Light is a stimulus. The stems and leaves of many houseplants grow toward a window. The plants are responding to the light stimulus that comes through the window. This response occurs gradually over many hours. The response to a stimulus can be quick. For example, a Venus flytrap is a plant with unusual leaves that close when a stimulus, such as a fly, brushes against hairs on the leaf. The trap snaps shut, like jaws, when stimulated by an insect touching the leaf. The insect is trapped inside the plant. Identify the Main Ideas Write a phrase beside each paragraph that summarizes the main point of the paragraph. Use the phrases to review the lesson. 1. Determine Why is it sometimes hard to see a plant s response to a stimulus? Reading Essentials Plant Processes 183
Make a two-tab book to record what you learn about the two types of stimuli that affect plant growth. Environmental Stimuli Chemical Stimuli 2. Evaluate How is phototropism beneficial to a plant? Visual Check 3. Explain Use the figure to explain to a partner what happened to the plant. Environmental Stimuli Plants respond to their environments in a variety of ways. In the spring, some trees flower and new, green leaves sprout. In the fall, the same trees drop their leaves. Both are plant responses to environmental stimuli. Growth Responses Plants respond to different environmental stimuli. These include light, touch, and gravity. A tropism (TROH pih zum) is a response that results in plant growth toward or away from a stimulus. Positive tropism is growth toward a stimulus. Negative tropism is growth away from a stimulus. Light Look at the figure below. The plant s growth toward or away from light is a tropism called phototropism. A lightsensing chemical in a plant helps it detect light. Leaves and stems tend to grow in the direction of light. Recall that photosynthesis occurs in a plant s leaves, and photosynthesis requires light. By growing in the direction of light, the plant s leaves are exposed to more light. Roots generally grow away from light. This usually means that the roots of the plant grow down into the soil and help anchor the plant. Touch Thigmotropism (thihg MAH truh pih zum) is the name given to a plant s response to touch. You might have seen vines growing up the side of a building. Some plants have special structures that respond to touch. These structures are called tendrils. The tendrils wrap around or cling to objects, such as when vine tendrils coil around a blade of grass. This is positive thigmotropism. Roots display negative thigmotropism. They grow away from objects in soil, enabling them to follow the easiest path through the soil. Response to Light 184 Plant Processes Reading Essentials
Gravity A plant s response to gravity is called gravitropism. Stems grow away from gravity, so they show a negative gravitropism. Roots grow toward gravity, showing a positive gravitropism. When a seed lands in the soil and starts to grow, its roots will always grow down into the soil. The stem grows up. This will happen even when a seed is grown in a dark chamber. This shows that the response of the root and the stem can occur independently of light. Flowering Responses Flowering is a plant response to environmental stimuli. Some plants flower in response to the amount of darkness they are exposed to. Photoperiodism is a plant s response to the number of hours of darkness in its environment. Scientists once hypothesized that photoperiodism was a response to light. For that reason, the flowering responses are called long-day, short-day, and day-neutral. The names relate to the number of hours of daylight in a plant s environment. Scientists now know that plants respond to the number of hours of darkness. Long-Day Plants Plants that flower when exposed to less than 10 to 12 hours of darkness are called long-day plants. Long-day plants usually produce flowers in summer. During the summer, the number of hours of daylight is greater than the number of hours of darkness. Short-Day Plants Short-day plants begin to flower when there are 12 or more hours of darkness. A poinsettia is an example of a short-day plant. Poinsettias tend to flower in the late summer or early fall when the number of hours of darkness is increasing. Day-Neutral Plants The number of hours of darkness doesn t seem to affect the flowering of some plants. These plants are called day-neutral plants. These plants flower when they reach maturity and the other environmental conditions are right. Roses are day-neutral plants. Chemical Stimuli Plants respond to chemical stimuli as well as environmental stimuli. Plant hormones are substances that act as chemical messengers within plants. These chemicals are produced in tiny amounts. They are called messengers because the chemicals are usually produced in one part of a plant but affect another part of that plant. Key Concept Check 4. Identify What types of environmental stimuli do plants respond to? Give three examples. 5. State Why are plant responses named according to length of day? 6. Identify How is the flowering of day-neutral plants affected by exposure to hours of darkness? Reading Essentials Plant Processes 185
Auxins One plant hormone is auxin (AWK sun). There are many different kinds of auxins. Plant cells respond to auxins with increased growth. The growth of leaves toward light is a response to auxin. Auxins concentrate on the dark side of a plant s stem, and these cells grow longer. This causes the stem to grow toward the light. The figure below shows auxin on the left side of the seedling. It causes more growth on the left side, leading the seedling to bend to the right. Response to Auxins Light Visual Check 7. Explain Use the figure to explain to a partner how auxins have affected the growth of this seedling. Auxin Key Concept Check 8. Describe How do plants respond to the chemical stimuli, or hormones, auxin and ethylene? Ethylene The plant hormone ethylene helps fruit ripen. Ethylene is a gas that can be produced by fruits, seeds, flowers, and leaves. Have you heard the expression one rotten apple spoils the whole barrel? This is based on the fact that rotting fruits release ethylene. This can cause other fruits nearby to ripen and possibly rot. Ethylene also can cause plants to drop their leaves. Gibberellins and Cytokinins Rapidly growing areas of a plant, such as roots and stems, produce gibberellins (jih buh REL unz). These hormones increase the rate of cell division and cell elongation. This results in the increased growth of stems and leaves. Sometimes gibberellins are applied to the outside of plants to encourage plant growth. Fruit-producing plants can be treated with gibberellins to produce more fruit and larger fruit. Cytokinins (si tuh KI nunz) are another type of hormone. They are produced mostly in root tips. Xylem carries cytokinins to other parts of a plant. Cytokinins increase the rate of cell division. In some plants, cytokinins slow the aging process of flowers and fruits. 186 Plant Processes Reading Essentials
Summary of Plant Hormones Plants produce many different hormones. The hormones discussed in this lesson are groups of similar compounds. Often, two or more hormones interact and produce a plant response. Scientists are still discovering new information about plant hormones. Humans and Plant Responses Humans depend on plants for food, fuel, shelter, and clothing. Humans use plant hormones to make plants more productive. Some crops have become easier to grow because humans understand how the plants respond to hormones. For example, bananas and tomatoes can be picked and shipped while they are still green. They can then be treated with ethylene to make them ripen. 9. Identify How are humans dependent on plants? Math Skills A percentage is a ratio that compares a number to 100. For example, if a plant grows 2 cm per day with no chemical stimulus and 3 cm per day with a chemical stimulus, what is the percentage increase in growth? Subtract the original value from the final value. 3 cm - 2 cm = 1 cm Set up a ratio between the difference and the original value. Find the decimal equivalent. 1 cm = 0.5 cm 2 cm Multiply by 100 and add a percent sign. 0.5 100 = 50% 10. Use Percentages Without gibberellins, pea seedlings grew to 2 cm in 3 days. With gibberellins, the seedlings grew to 4 cm in 3 days. What was the percentage increase in growth? Reading Essentials Plant Processes 187
Mini Glossary photoperiodism: a plant s response to the number of hours of darkness in its environment plant hormone: a substance that acts as a chemical messenger within plants stimulus: a change in an organism s environment that causes a response tropism (TROH pih zum): a response that results in plant growth toward or away from a stimulus 1. Review the terms and their definitions in the Mini Glossary. Write your own sentence using one of the terms. 2. Complete the chart below to summarize what you have learned about environmental stimuli. Environmental Stimuli 1. phototropism 2. 3. Tropisms 3. How did writing a phrase next to each paragraph help you identify the main ideas? What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? 1. 2. 3. ConnectED Growth Responses Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 188 Plant Processes Reading Essentials