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Background

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When plants grow up and the roots grow down, we call this gravitropism. It is how the plant responds to gravity. Some plants are sensitive to touch, such as the mimosa plant. This can be a response to prevent animals from eating the leaves. Plants also grow towards the sun as well as can grow towards water sources. All of these are responses to environmental conditions. Plants can also reach to environmental stresses, such as heat, drought, and wind. Unless the plant is tolerant, these stresses can stunt the growth of the plant or cause it to die.

Vocabulary

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Gravity: the force that causes two particles to draw towards each other. In general, it is why things stay on the ground instead of float away.
Stress: an emotional and physical response to threats from the outside world.

Attention Grabber

Plan ahead and have a plant that is in the windowsill and growing towards the sun. Show the plant to the students and ask them what they notice about how it is growing. It is growing towards the sun so it can get as much sunlight as possible.

Share how plants also can grow their roots towards water. Why would they do that? But what about when they are seeds? They are underneath the soil, so they cannot see the sun.

How do they know that the leaves should grow up towards the sun and the roots should grow deeper in the soil? Let’s perform an experiment to see what goes on.
 

Garden Activity   

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• Have students start bean seedlings and observe.  
• During planting, have students place the seed on all different sides (see images below).  Assign students different ways of putting the seed in the soil and mark the pot with an arrow.
• Keep track of the growth using a garden journal.  (This could include using rulers to measure the plants; a good way to incorporate math.)
• Up root a few of the plants that were planted differently.  All of the roots will grow down and the leaves will grow up.  Ask the students, “How does the plant know which way to grow?”  Answer: The plant senses gravity and that is why it grows in this direction.  
• Bring in a plant that is sensitive to touch. Some good examples are a mimosa plant or Venus flytrap.  This shows that plants can be sensitive to being touched.  In nature, this could help the plant from being eaten, or help it use pollinators like bees.

Wrap Up 

• As a class, discuss the observations and conclusions. The plant cells (because of the water in the cells) know which way is down and which way is up.

What's Next?

• Look at pictures of plants that are subject to other environmental stresses such as wind, drought, and heat.