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Tropisms are the response of a plant to something in its natural environment. Thigmotropism, in particular, is a general term used to describe a plant's response to physical contact. It is a natural occurrence for some plants to shy away at the touch of a hand, or coil at the feel of a wall or trellis. Roots themselves contain a certain degree of sensitivity to help them ease their way through soil. There are two kinds of thigmotropism: positive thigmotropism, or the respose of a plant's stem up toward the touching object, and negative thigmotropism, or a root's response away from a touching object into the soil.
The history of this response is quite varied. Native Africans pointed it out in jungles as early as the seventeenth century. Darwin performed experiments which contrasted geotropism, the response of a plant to gravity, and thigmotrapism. He found that thigmotropism is a stronger plant response than geotropism. His experiment contrasted the response of a vertical bean root to gravity and to touch. He found that the plant defied it's gravitational response in order to fulfill its thigmotropic response. Later experiments proved that the thigmotropic response is possibly the strongest tropic response in plants. This is for good reason. Thigmotropic responses can take the form of everything from a slight shiver to a big chomp. Some plants use this response to eat insects alive. Others practically conquer buildings with their curling tendrils. Thigmotropism is the closest a plant comes to imitating a typical mammalian response to physical stimuli.
Not much is known of the cause of thigmotropic responses. Tendrils curve by a process called "differential growth." Differential growth refers to the reaction that takes place in the side of a plant stem opposite the touching object. In other words, when an object touches a sensitive plant, the touching side will contract and the farther side will expand, curving the plant foward and over the touching body. Coiling can be slow, as is the case with most differential growth, or rapid. A phenomenon called "rapid contact coiling" allows certain plants to curve extremely fast by changing turgor pressure in their cells
Little is known about the transmittal of touch stimuli to plant cells. It does not appear to occur in the same way with every plant. Small hairs called "papillae" exist on the stem of a plant, which causes the initial message of touching. It is believed that a system of ionic impermeability in cells can transmit an action signal through the plant's whole body at once. It is also believed that the contracting and lenghtening response of the stem sides is a simultaneous process.
To create you own thigmotropism, you will need:
Two Venus Flytraps
A fly
A notebook
One plant will act as the control, and one will act as variable. Place the plants at a little distance away from each other and release the fly into one, without touching the plant. What is the plant's response? Write it down. You've recorded your first thigmotropism.
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