Animals aren't the only things that can respond to the environment. While plants may seem inanimate at times, they, too, can respond to the environment in order to better survive.
When you hear a loud noise, you may respond by jumping. External stimuli result in a response. We easily see this in animals, but it also happens in plants. We generally think of plants as passive. They simply - and much of the time, very slowly - grow. However, there is much more to plants than we perceive.
The way sunflowers turn in the direction of the sun throughout the day is an example of phototropism
When animals, including humans, respond to a stimulus (which is something that causes a response), we call it a reaction. For example, if someone touches your arm, you generally turn to react. In plants, the response to a stimulus is known as a tropism. This plant movement toward or away from a stimulus can come in many forms. Before we look at a few, let's better understand the word tropism. Like many words in science, tropism comes from a Greek word. Tropos means 'to turn'. Therefore, a tropism is a turn towards or away from a stimulus. When the movement is towards the stimulus, it is called positive tropism. Likewise, when the movement is away from the stimulus, it is called negative tropism. While there are several forms of tropism, we'll just focus on three key types: phototropism, geotropism and thigmatropism.
Phototropism is generally the tropism that makes the most sense. We know that plants grow towards the sun, so they can make food through photosynthesis. This movement in response to sunlight is called phototropism. Let's break this word apart. Tropos means 'to turn,' and photo means 'light.' Therefore, phototropism is a turn towards or away from light.
While most phototropism involves plants just growing toward the sun, some plants actually follow the sun throughout the day. For example, young sunflowers will orient their flowers to the sun. In the morning, they point east towards the rising sun. They then gradually follow the sun throughout the day, eventually pointing west towards the setting sun.
The downward growth of roots and the upward growth of shoots are examples of geotropism
Let's now look at geotropism. We know that tropos means 'to turn,' so we already understand the second half of the word. Geo many sound familiar because of words such as 'geography' and 'geology.' Geo means earth; therefore, geotropism is movement towards or away from the earth. You may see geotropism used interchangeably with gravitropism, which is the movement towards or away from gravity.
Examples of geotropism include the downward growth of roots and the upward growth of shoots. Roots have specialized cells that sense gravity and therefore have a positive tropism by growing toward the pull of gravity. Opposite of this is the growth of the shoot, which is the aboveground portion of the plant. The shoot exhibits negative geotropism by growing away from the earth. You can see these growth patterns with lateral roots and branches: lateral roots will curve down, while lateral branches will curve up.
The last type of tropism we will look at is thigmatropism. Thigma means 'touch' and again, tropos means 'to turn.' Therefore, thigmatropism is the movement towards or away from touch. Plants can be affected by touch both positively and negatively. For example, if you repeatedly touch a young, growing plant, this can actually cause the plant to grow less. This is useful for plants growing close together so that they don't outgrow the limited space.
The way vines respond to touch is an example of thigmatropism
We can also see a response to touch in vines and other climbing plants. Vines will grow straight along the ground until they touch something to grow on, such as a tree or a wall. This positive thigmatropism allows vines to find a support on which to grow. Another example of positive thigmatropism can be seen in the Venus flytrap. These plants will close leaves around unsuspecting insects, which are then digested for nutrients. An example of negative thigmatropism is seen in a plant found in Central and South America. The touch-me-not immediately folds its leaves after being touched. It is moving away from the stimulus of touch.
Regardless of the perception that plants are not the most interesting and animated things out there, they actually have several unique ways to respond to their environments. Geotropism allows plants to correctly orient themselves for growth. The roots exhibit positive geotropism and grow towards the earth. The shoot shows negative geotropism by growing away from the earth. Phototropism occurs when plants move towards or away from sunlight. Lastly, we looked at thigmatropism. This response to touch can be positive, like when vines grow around a tree or wall that they touch. However, it can also be negative, like the folding of leaves seen in the touch-me-not. All of these responses to stimuli allow plants to thrive in their environments.
Phototropism in Action
In this experiment, students will be observing phototropism in plants and recording their result. To do this experiment, you'll need a houseplant (such as a spider plant), a succulent, or an herb (like basil) and a bright light source. This experiment will ask students to take before and after pictures of their plants. However, if technology is not available, students can make detailed observations.
Now that you're familiar with the different tropisms that plants exhibit, we're going to watch phototropism in action. For this experiment, you'll need access to a house plant (such as a spider plant), a succulent, or an herb as well as a strong light source and a camera. If you don't have access to a camera, you can make detailed observations about the location of the leaves and stems of the plant. Follow the instructions below, then answer the questions.
- Position your houseplant in front of the light source, with the light source directly to the right of the plant. Take a photo or carefully record observations about the location of the stem and leaves. Consider where they are in relationship to the lamp or other stationary objects.
- For one week, turn the light on during the day and off at night. Record observations or take a photo each day to record the location of the stem and leaves of the plant.
- After a week, take another photo of the final position of the plant.
- What happened to the position of the plant over time?
- How did this experiment represent a tropism? How did you know?
- How do you think we could test for other tropisms?
Students should observe that the plant changes the direction of growth towards the lamp. If this doesn't occur, it could be because other light sources were competing with the lamp. Try to allow the lamp to be the only light source. This experiment should demonstrate a tropism because the plant's position should change over time in response to the light. To test for other tropisms, students can try to position the plants differently, but still observe root growth with gravity and stem growth against gravity.