Download Key Concept Summaries

Name ___________________________________ Date _____________ Class ______________
Plant Responses and Growth
Key Concept Summaries
What Are Three Stimuli That Produce Plant Responses?
Plants respond to stimuli by growing toward or
away from the stimulus, a response called a
tropism. Growth toward a stimulus is a
positive tropism; growth away is a negative
tropism. Touch, gravity, and light are three
important stimuli that trigger growth
responses, or tropisms, in plants.
A plant’s response to touch is called
thigmotropism. A vine that curls around an
object it touches shows a positive
thigmotropism. Geotropism is a plant’s
response to the force of gravity. As they grow
downward, roots show a positive geotropism.
Stems growing up and against the force of
gravity show a negative geotropism.
Phototropism is a plant’s response to light.
Leaves, stems, and flowers that grow toward
light show a positive phototropism. Plants
respond to stimuli by producing hormones,
chemicals that affect how a plant grows and
develops. Auxin is a hormone that speeds up
the rate at which a plant’s cells grow and
controls a plant’s response to light. As auxin
builds up on the shaded side of a stem, cells on
that side grow faster and bend the stem toward
the light.
How Do Plants Respond to Seasonal Changes?
Plants respond to the changing seasons
because the amount of light they receive
changes. The amount of darkness a plant
receives determines the time of flowering in
many plants. A plant’s response to seasonal
changes in the length of night and day is called
photoperiodism. Some plants will only bloom
when the night lasts a certain length of time.
Plants can be grouped according to critical
night length, or the number of hours of
darkness that determines whether or not a plant
will flower. Short-day plants flower when
nights are longer than a critical length. They
bloom in fall or winter. Long-day plants
flower when nights are shorter than a critical
length. They bloom in spring and summer.
Day-neutral plants have a flowering cycle
that is not sensitive to periods of light and
dark. They can bloom year-round, depending
on weather.
Some plants prepare for winter by going
into a state of dormancy, or a period when
growth or activity stops. Dormancy helps
plants survive freezing temperatures and
the lack of liquid water. Cooler weather and
shorter days may trigger a plant to prepare to
become dormant.
Name ___________________________________ Date _____________ Class ______________
Plant Responses and Growth
Review and Reinforce
Understanding Main Ideas
Answer the following questions in your notebook.
1. List three stimuli that produce responses in plants and give the name for each
response.
2. What is geotropism? How can a plant show both a positive and a negative
geotropism?
3. What is photoperiodism? What is its effect in many plants?
4. What happens when a plant enters a state of dormancy?
Building Vocabulary
Match each term with its definition by writing the letter of the correct definition in
the right column on the line beside the term in the left column.
5. _____ critical night length
6. _____ auxin
7. _____ short-day plant
8. _____ long-day plant
9. _____ hormone
10. _____ day-neutral plant
11. _____ tropism
a. a hormone that controls a plant’s
response to light
b. a plant that flowers when the nights
are shorter than a critical length
c. a plant’s growth response toward or
away from a stimulus
d. a chemical that affects the growth and
development of a plant
e. a plant whose flowering cycle is not
sensitive to periods of light and dark
f. a plant that flowers when the nights
are longer than a critical length
g. the number of hours of darkness that
determines whether or not a plant will
flower
Write About It
12. In your notebook, explain three different types of response a plant may have to
light—its direction or the length of time it lasts.