Download Lesson 4: How do plants grow?

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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:
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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.
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DNA and Growth
Quick Study
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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
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