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.
1
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
3
which then return to the soil. This is why putting dead leaves
or grass clippings on your garden is good for it.
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.
7
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 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-pizum).
8
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.
9
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
10
who have it. This process that favors the better adapted
organism is called natural selection.
Copyright © 2012 edHelper
1. 2. Name at least three things
that plants need to survive.
Plant adaptations increase the
chances that ______.
Plants will have more
flowers.
Plant species can survive
in their habitat.
Plants will keep growing
bigger and bigger.
Plants will have more
fruits.
3. 4. What is a tropism?
What is gravitropism or
Growing toward water geotropism?
Any change in a plant or
A plant's growth in
animal
response to gravity
Growing toward a taller
A plant growing toward
plant
sunlight
Plant growth in response
A plant growing toward
to a stimulus
water
A plant closing its leaves
5. 6. 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
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?
Phototropism
Thigmotropism
Gravitropism
Geotropism
7. 8. When peas or bean plants
grab onto a vertical support
and grow upward, this is an
example of ______.
Thigmotropism
Geotropism
Gravitropism
Phototropism
The process that favors the
better-adapted organism is
called ______.
Evolution
Tropism
Adaptation
Natural selection