This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. Growth or movement of a sessile organism toward or away from a source of light.

n. the movement of a plant towards or away from light

n. The tendency of growing plant organs to move or curve under the influence of light. In ordinary use the term is practically synonymous with heliotropism.

n. exhibiting movement in a direction toward (positive phototropism) or away from (negative phototropism) a source of light.

n. The growth or the bending of organisms or of parts of organisms in relation to light; heliotropism.

n. Same as phototaxis, 2.

n. Same as phototropy, 2.

n. an orienting response to light

"Thus it was that the interpretations of J. Loeb (Die Tropismen, 1913) on the basis of experiments done with lower animals, estab - lished the neologist ideas of “phototropism” (orien - tation or displacement reaction in the direction of light), and of “thermotropism” (reaction directed to - wards a source of heat), to explain animal and perhaps also human behavior."

tropism   

heliotropism   

positive phototropism    negative phototropism