When Plants Move - Tropisms
Rockin' and Rollin'
When we place a plant in our garden or in a pot, we never think that it might some day pull up roots and walk away. Generally, we accept that plants will stay put where we plant them unless they propagate or someone steals them. While this is true, plants do exhibit various kinds of movement, and in certain plants this include movements that are quite noticeable without the aid of a time-lapse camera. Movements in plants in response to stimuli are known as tropisms. In this article I will share with you the most common tropisms that you are likely to see happening in your plants.
Touch me and I'll follow
Plants can move in response to contact with something. Normally, plants will contact other plants or miscellaneous items in their garden environment, such as dead twigs, flat surfaces, and chain link fencing. When plants respond to these encounters by moving in a manner differently than the way they moved before they made contact, the response is called thigmotropism. This is how the tendrils of vines, or the stems of twining vines, are able to wrap around objects as they grow. You can see this in action if you have a fast-growing vine climbing a wire or mesh. Find a stem that has not wrapped around anything yet and arrange it so that it is touching a wire or stalk that it can wrap around. Then come back to view it in a few hours. The vine should have begun wrapping around the object and you will see that it curved towards the area touched by the object.
Thigmotropic movement doesn't have to be slow, either. In the thumbnail picture above, you can see a leaf of the venus flytrap plant; this leaf will close quickly when the hairs on the red inner surface are contacted by an unwary fly or other small insect. The sensitive plant (Mimosa pudica) has fine pinnate leaves that will close quickly and droop when touched. The movement occurs at swollen junctions on the leaf, called pulvini. Still other plants, such as impatiens or balsam, will employ quick movements of parts of the seed pod to scatter ripe seeds far from the parent plant.
Following the Sun
One very common movement in plants is their lean towards light or towards the sunlight. This is especially noticeable with indoor plants, but plants growing in shady locations show it when those locations are surrounded by sunny ones. Movement in response to light is known as phototropism, not to be confused with the movement of an actress in an effort to stay in front of the camera! Movement in response to sunlight in particular is known as heliotropism. This can be movement towards the light source, as in the green parts of plants, or movement away from it, as in roots. Widespread phototropic response in plants is the reason why we have to turn our indoor plants from time to time to keep them from becoming permanently lopsided.
Right side up
Plants grow green side up and root side down in response to another stimulus, gravity. If a pot tips over, the stem and leaves will curve so they are upright again. This can be frustrating if you didn't know your favorite plant had tipped over because it will take much longer to straighten up than it did to bend! Movement in response to gravity is known as gravitropism. I've learned that when you have tall plants in pots outside, you need to check them frequently to be sure they don't tip over. To be safer, you need to secure them to avoid the deformed shape that results from the inevitable gravitropic response of a tipped-over plant.
Going where the warmth is
Plant movement in response to warmth or heat is called thermotropism. Think of it as similar to the movement of folks in northern climates to Florida in the wintertime! Several other tropisms are movement in response to water (hydrotropism) and movement in response to chemicals such as fertilizers or acids (chemotropism). Plant movements mentioned here are mediated by plant growth regulators known as auxins. See my article, Plant Growth Hormones, for more information about auxins.
Image credit: Wikimedia Commons
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