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Tropism Basics
Tropism, from the Greek word "tropo," or turning, refers to an organism's ability to move in response to stimuli. Tropism is generally thought of in relation to the aforementioned plants and their ability to grow toward sunlight or send roots toward sources of underground water. Plants are not unique in this respect, however, and sunlight and water are not the only external stimuli that organisms are capable of responding to and moving toward.
Lure of Chemicals
Chemotropism refers to an organism's ability to move toward a specific chemical, or set of chemicals. This is generally found in plants, when the pollen tube moves toward sugars in the style during reproduction, but it also can be found in certain types of bacteria and protists. There are varieties of bacteria that are oxygen-avid and thus will orient themselves to be on the edge of a water droplet when placed in a petri dish with water.
When Gravity Calls
Geotropism describes an organism's ability to move in response to the gravitational pull of the Earth. The most obvious example of this tropism is found in a plant's root system. A plant sends its fibrous roots, and taproot especially, in a direction that is determined by the plant's ability to sense the gravitational pull and, thus, ensure its roots grow in to deeper soil, even when planted on a slope or bank.
Temperature Effects
Thermotropism refers to an organism's ability to move in response to temperature. One example is the rhododendron, which can curl its leaves when handled, moving itself away from the source of heat. There are competing theories as to what the evolutionary advantage would be for plants to exhibit this behavior, but the prevailing wisdom is that they evolved this ability in order to better regulate their temperature and rate of dessication during periods of extreme temperature.