Plant Responses

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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 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.

Hydrotropism is the growth response of a plant to water. Roots exhibit positive hydrotropism.

Thigmotropism is the growth response of a plant to physical contact (touch). Plants that cling to physical structures such as walls exhibit positive thigmotropism.

Chemotropism is the growth response of a plant to a particular chemical. Roots grow toward useful minerals in the soil but away from acids.


Auxins are a class of plant hormones (or plant growth substances) with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development.

-Stimulates cell elongation
-Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture -Stimulates differentiation of phloem and xylem
-Stimulates root initiation on stem cuttings and lateral root development in tissue culture -Mediates the tropistic response of bending in response to gravity and light -The auxin supply from the apical bud suppresses growth of lateral buds -Delays leaf...
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