HELIOTROPISM
\hˈɛlɪˌɒtɹə͡ʊpˌɪzəm], \hˈɛlɪˌɒtɹəʊpˌɪzəm], \h_ˈɛ_l_ɪ__ˌɒ_t_ɹ_əʊ_p_ˌɪ_z_ə_m]\
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The directional growth of organisms in response to gravity. In plants, the main root is positively gravitropic (growing downwards) and a main stem is negatively gravitropic (growing upwards), irrespective of the positions in which they are placed. Plant gravitropism is thought to be controlled by auxin (AUXINS), a plant growth substance. (From Concise Dictionary of Biology, 1990)
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