A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the species. The word tropism comes from the Greek trope ("to turn" or "to change"). Tropisms are usually named for the stimulus involved and may be either positive (towards the stimulus) or negative (away from the stimulus).
Phototropism
Phototropism is the growth response of a plant in response to light direction. Different parts of a plant exhibit different reactions to light. Stems exhibit positive phototropism while most roots exhibit negative phototropism.
Geotropism is the growth response of a plant in response to gravity. Roots exhibit positive geotropism while stems and leaves exhibit negative geotropism.
Thigmotropism is the growth response of a plant to physical contact (touch). Plants that cling to physical structures such as walls exhibit positive thigmotropism.
Hydrotropism is the growth response of a plant to water. Roots exhibit positive hydrotropism.
Chemotropism is the growth response of a plant to a particular chemical. Roots grow toward useful minerals in the soil but away from acids.
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