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From The Collaborative International Dictionary of English v.0.48:
gibberellin \gibberellin\ n. (Chem.), Any of a number plant growth hormones, the first of which was isolated in 1938 from the fungus Gibberella fujikuroi; more than 60 related gibberelins are known. The most important is gibberellin A3, also called gibberellic acid. They are used in agriculture for promoting plant growth. [WordNet 1.5 +PJC]