1) phototropism—This refers to how plants tend to response to "light" as a stimulus; for instance, plants may grow in a "bending" pattern TOWARD light.
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2) gravitotropism—This refers to how plants respond to gravity. The "stems" of plants exhibit "negative gravitotropism"; that is, the stems of plants tend to "pull up" (away from the force of gravity). The roots of plants exhibit "positive gravitotropism"; that is, they tend to grow "downward" toward the Earth.
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3) thigmotropism—This refers to how plants responds when "exposed to touch." For instance, when plants grow around a fence or trellis.
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