The New Student's Reference Work/Nutation

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Nuta′tion, regular bending of plant parts, such as bud scales, flower leaves, stems etc., due to unequal growth on the sides.  The stimulus inciting it may be light, heat or gravity.  (See Irritability.)  Thus the flowers of the tulip and crocus are sensitive to temperature changes of a few degrees, opening with rising and closing with falling temperature, because the growth of the outer face is hastened and that of the inner face retarded, and vice versa.  When a cylindrical stem has its growth hastened on every side in regular succession, the tip describes a more or less regular circle, as in twining plants like the hop and morning glory.  Some nutations are apparently spontaneous.