向性(Tropism)是一種生物學現象,指生物(多數為植物)受環境因素刺激而向某特定方向生長的現象。向性有正向性(朝向刺激而生長)和負向性(反向刺激而生長)的分別。這些現象都與植物生長素auxin有關,倘若auxin量多的一側生長速度快,少的一側生長速度慢,就會造成莖的彎曲,讓植物的生長呈現蔓性/向觸性,正向的向光性(即朝陽光刺激的方向生長),還有負向的向地性(即與地心引力相反的方向生長)。
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