Plant Behavior

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Plant Behavior

My AP Biology Thoughts

Unit 8 Episode #21

Welcome to My AP Biology Thoughts podcast, my name is Shriya and I am your host for episode #21 called “Plant Behavior (phototropism, gravitropism, thigmotropism)”. Today we will be discussing the definitions of all of those concepts as well as a few examples to go along with them. Then, we will connect all of that to the overarching topic of evolution. Hope you enjoy!.

Segment 1: Introduction to Plant Behavior (phototropism, gravitropism, thigmotropism)

  • We will be discussing the topic of plant behavior which is how plants respond to changes in their environments
  • Like all organisms, they detect and respond to stimuli in their environments, but they cannot physically move to get away from danger since they are rooted to the soil
  • Instead, a plants primary means of response is to change how it is growing based on a number of factors 
  • Their responses are normally hormone based because they do not have nervous systems to control them
  • They can have responses to both internal and external stimuli, however in this episode we will mainly be focusing on their external responses to stimuli
  • Plant roots always grow downwards because they have specialized cells that respond to gravity which is an example of a tropism 
  • A tropism is turning towards or away from a stimulus in the environment
  • The first type of tropism is called phototropism which means the plant is growing towards a light source 
  • This response is controlled by a plant growth hormone called auxin
  • Auxin stimulates cells on the dark side of a plant to grow longer which provokes the plant to bend towards the light 
  • The next type of tropism is called gravitropism, or geotropism where “geo” means the Earth and tropism refers to turning
  • Gravitropism is the growth of a plant’s organ or change in the direction of its growth in response to gravity
  • Since plant cells are able to sense gravity, if a root is not growing towards the center of the earth, the cells become aware of that and change it
  • Auxin is also stimulated either at the root caps or the shoots to allow cell elongation
  • Finally, there is also thigmotropism which is the response to touch 
  • The stimulating factor is generally a hard surface that can change the direction of the plant’s growth or the growth of one of its organs
  • Thigmotropism can be in the form of opening or closing of parts of the plant such as the petals or leaves, the coiling of the plant around the surface, as well as other ways

Segment 2: Examples of Plant Behavior

  • The first example is a picture of a plant shoot exhibiting the response of phototropism
  • It is growing toward the light source which is the sun in this case, and its cells are being elongated due to the auxin being released on the dark side of the shoot
  • There are two types: positive phototropism is growth towards a light source; negative phototropism is growth away from light
  • Shoots generally display positive phototropism which is being demonstrated in this example since it is bending towards the light which helps the green parts of the plant get closer to a source of light energy, which can then be used for photosynthesis

  • This picture is demonstrating gravitropism in both a positive and negative way because the roots are growing down, and the shoots are growing up
  • Positive gravitropism occurs when roots grow into soil because they grow in the direction of gravity 
  • Amyloplasts, which are found in the root caps of plants, settle downward in response to gravity
  • Negative gravitropism occurs when shoots grow up toward sunlight in the opposite direction of gravity

  • The most common example of thigmotropism can be seen in the behaviors of a plant called a tendril
  • Tendrils are common on twining plants such as the morning glory, and prior to touching an object, tendrils often grow in a spiral like form
  • However, when the tendril touches an object, epidermal cells control the growth of the tendril which can result in the tendril completely circling the object within five to ten minutes
  • Thigmotropism by tendrils allows plants to “climb” objects and thereby increases their chances of intercepting light for photosynthesis.

Segment 3: Digging Deeper into Plant Behavior

How does this topic fit into the greater picture of ecology?

  • Overall, ecology is the branch of biology that deals with the relations of organisms to one another and to their physical surroundings
  • The behaviors exhibited by plants display their relation to the environment and how they need to respond to their surroundings in order to survive
  • Their physical surroundings are known as stimuli because they insinuate a response from the plants through their upward and downward growth 

Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit www.hvspn.com. See you next time!

Music Credits:

  • “Ice Flow” Kevin MacLeod (incompetech.com)
  • Licensed under Creative Commons: By Attribution 4.0 License
  •  http://creativecommons.org/licenses/by/4.0/

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