Plant Behaviors and Evolution
By Cindy Grigg
When you look at a plant, you may not see much happening. But, like
people and animals, plants have needs. Living things can live on Earth because
Earth has certain things that allow them to grow and reproduce. Plants need light,
air, water, and space.
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Plants also need nutrients, or certain substances found in soil, to live. Some
nutrients come from rocks which are filled with minerals. Minerals are not living,
but they contain compounds that plants use as nutrients. These nutrients become
part of the soil when rock minerals break up into very small pieces and dissolve. A
plant takes in nutrients through its roots along with water.
2
Plants also get nutrients from other living things. When plants and animals die,
they decay and release their nutrients which then return to the soil. This is why
putting dead leaves or grass clippings on your garden is good for it.
3
Plants that live in different habitats have different traits that allow them to
survive there. Over time, adaptations, or changes, in plant parts have happened.
They continue to happen to increase the chances that plant species can survive in
their habitat. Adaptations are what make the sizes and shapes of plant parts
different.
4
Plants can't move like animals can, but they can respond to a stimulus, or a
change in the environment. Plant growth in response to a stimulus is called a
tropism. Plants respond to stimuli such as light, gravity, water, and touch.
5
When you plant a seed in the soil, you don't have to worry about which
direction the seed faces. Why? Because plant roots respond to gravity. They will
always grow down toward the center of the earth. Stems always grow upward,
away from the pull of gravity. Plant growth in response to gravity is called
gravitropism (grav-ih-TROH-piz-um) or geotropism.
6
If you place a plant near a window, you will notice that, eventually, most of the
leaves will be facing the sun. The leaves turn when cells on one side of the stem
grow longer than cells on the other side. This change in the growth of a plant in
response to light is called phototropism. Phototropism is important because plants
need light to carry out photosynthesis.
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Some plants respond to touch. For example, the Mimosa tree will close its
leaves when you run your finger over them. Another plant that does this is the
commonly known prayer plant. When touched, it folds its leaves together like
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hands folded in prayer. Vines of beans, peas, and other plants will grab onto and
grow upward along a vertical support to maximize the amount of sun they receive.
The response of a plant to touch is called thigmotropism (thig-mo-TROP-piz-um).
Plant adaptations have occurred over thousands and millions of years. These
adaptations started as very small changes. A plant may have been changed
somehow to grow a little taller. This change would have helped it to reach into the
sunlight a little higher than other plants around it. When survival is the goal, even
the smallest advantage is important. This little difference in height might have
helped the plant have a better chance of surviving and reproducing than the shorter
plants around it.
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When the plant reproduced, the characteristic would have been passed on to its
offspring. As generation after generation passed, there may have been another
change that allowed another one of these offspring to reach even higher. This is
how the process of evolution works. These slow, gradual changes over time give
certain organisms a characteristic that lets them survive to reproduce better than
others of its species. These organisms produce offspring that are better able to
survive. The organisms without the characteristic gradually die off and are
replaced by the ones who have it. This process that favors the better adapted
organism is called natural selection.
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Name _____________________________
Date ___________________
Plant Behaviors and Evolution
1. Name at least three things that plants 2. Plant adaptations increase the
need to survive.
3. What is a tropism?
Plant growth in response to a
stimulus
Growing toward water
chances that ______.
Plant species can survive in
their habitat.
Plants will have more fruits.
Plants will keep growing bigger
and bigger.
Plants will have more flowers.
4. What is gravitropism or geotropism?
A plant closing its leaves
A plant growing toward water
A plant growing toward
Growing toward a taller plant
Any change in a plant or animal
5. In gravitropism ______.
Roots always grow toward the
center of the earth
Stems always grow away from
the center of the earth
Both a and b are correct
None of the above
sunlight
A plant's growth in response to
gravity
6. You put a plant in a sunny window.
In a few days, you notice that most
of the leaves are now turned toward
the window. Which tropism is this an
example of?
Geotropism
Thigmotropism
Phototropism
Gravitropism
Name _____________________________
Date ___________________
Plant Behaviors and Evolution
7. When peas or bean plants grab onto a 8. The process that favors the better-
vertical support and grow upward,
this is an example of ______.
Gravitropism
Geotropism
Phototropism
Thigmotropism
adapted organism is called ______.
Natural selection
Tropism
Evolution
Adaptation
Plant Behaviors and Evolution - Answer
Key
1 Plants need light, air, water, nutrients, and space.
2
Plant species can survive in their habitat.
3
Plant growth in response to a stimulus
4
A plant's growth in response to gravity
5
Both a and b are correct
6
Phototropism
7
Thigmotropism
8
Natural selection