Tropism occurs when an organism’s growth is acted upon by an external stimulus. Tropism can either be negative (away from the stimulus) or positive (towards the stimulus). The stimulus does not necessarily need to make direct contact with the organism (take for example the sun and sunflowers). Sunflowers are an excellent example of tropism as they “follow” the sun through the sky by positioning their “heads” as close to the sun as possible. This is an example of positive tropism. Several different types of tropism exist including hydrotropism (water), thermotropism (heat), thigmotropism (touch), chemotropism (chemicals), geotropism (gravity), heliotropism (sun), sonotropism (sound), etc. In the example above, we can see a vine demonstrating thigmotropism by growing towards the contact of the building.


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