My AP Biology ThoughtsUnit 8 Episode #21Welcome 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 environmentsLike 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 soilInstead, 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 themThey can have responses to both internal and external stimuli, however in this episode we will mainly be focusing on their external responses to stimuliPlant 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 environmentThe 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 auxinAuxin 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 turningGravitropism is the growth of a plant’s organ or change in the direction of its growth in response to gravitySince 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 itAuxin is also stimulated either at the root caps or the shoots to allow cell elongationFinally, 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 organsThigmotropism can be in the form of...