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Name Lesson 4 Summary Use with pp. 110–113 Lesson 4: How do plants grow? Vocabulary tropism a way that plants change their direction of growth in response to the environment growth hormone a kind of chemical that affects plant growth A seed sprouts when conditions are right. A seed will not grow if it is too cold or too dry. Different plants have different needs. A plant’s DNA controls when and how it grows. Plants grow in many shapes. Some trees are tall and thin. Some pine trees are shaped like cones. Some bushes are round. A plant’s DNA controls the way its branches grow. DNA affects how fast a plant grows. The environment also affects how a plant grows. A plant will grow more quickly if the environment has good conditions. The plant will grow more slowly if the environment has bad conditions. Bad conditions can include dry soil and cold air. Tropisms Plants can change the direction they grow in. They do this to meet their needs. For example, a plant’s leaves may turn toward the Sun. A plant’s roots may grow toward water. Tropisms are ways that plants change the direction they grow. Tropisms often happen when cells on one side of a plant grow faster than on the other side. This makes the stem bend. There are three kinds of tropisms: 28 Chapter 4, Lesson 4 Summary • Gravitropism is a plant’s growth toward or against gravity. Gravity is the pull of the Earth. Roots usually grow downward. They grow toward the pull of gravity. Stems usually grow upward. They grow against gravity. • Phototropism is how a plant reacts to light. A stem might grow toward a light. Some plants turn their leaves toward light. • Thigmotropism is a plant’s growth due to touching an object. Thigmotropism can happen in stems or roots. Vine stems grow around posts or fences. This helps to support the plant. Roots may bend to grow away from rocks or hard soil. Growth hormones can make cells grow faster or larger. A growth hormone is a chemical. Some cells have more growth hormone than others. Water can also change the size of plant cells. Plant cells store water in vacuoles. Vacuoles can fill up with water and make the cell large. Vacuoles on one side of a plant might store more water than vacuoles on the other side of a plant. This can bend a plant. © Pearson Education, Inc. 5 DNA and Growth Quick Study Name Lesson 4 Checkpoint Use with pp. 110–113 Lesson 4 Checkpoint 1. Keisha plants a seed in moist, cold soil by her home. The seed does not sprout. What condition would you infer most likely needs to change for the seed to sprout? 2. Cause and Effect What is the effect of different species of plants having very different DNA? 3. What is a growth hormone? © Pearson Education, Inc. 5 4. Explain how plants or plant parts grow toward light or grow upward. Quick Study Chapter 4, Lesson 4 Checkpoint 29