Nutation in plants

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Nutation, in plants, is the bending movements executed by organs of plants, such as stems, leaves, roots, etc., by which the part is inclined successively in various directions.[1]Nutations are due to the unequal rate of growth of different sides of the organ, an inequality which, so far as is known at present (c. 1915), is dependent upon internal (unknown) causes and is not called forth by the action of external stimuli. The word is often used in a broad sense in the phrase nutational movement, to include all the movements in plants caused by growth in contrast to variation movements or movements produced by reversible turgor changes.[2]

Simple nutation occurs in dorsiventral organs, such as flat leaves, both foliage and floral. The movements are only in one plane, depending upon the unequal growth of the opposite sides. When young the growth of the foliage leaves is most rapid upon their outer (dorsal) face, in consequence of which the lear applies itself to the axis, arches over the apex, and with its neighbirs forms a compact bud. Later growth becomes more rapid on the inner (ventral) face, the bud opens, and the leaves straighten out. Similar inequality of growth, but more sharply localized, leads to the folding and rolling of the leaf in the bud. Like movements of radial organs, such as stems, cylindrical leaves, and roots, have been termed circumnutation, or revolving nutation, to distinguish them from the simple nutation of dorsiventral organs. When any plant is in vigorous growth the axis rarely grows in length uniformly on all sides. The side on which growth is most rapid will push the apex over towards the side on which growth is less rapid. If the region of more rapid growth changes, shifting around the axis, the tip will be inclined successively to all points of the compass and with its simultaneous upward growth will describe a spiral, but, since the rate of growth is not uniform at successive intervals, the path described will be a very irregular spiral.[3]

Movements quite similar to those above described are called forth in plant organs by external stimuli. Thus twining plants exhibit both true nutation and nutation due to geotropic sensitiveness. Changes in temperature cause flowers to open or close by movements which resemble the simple nutation of dorsiventral organs. Thus, the tulip, crocus, and other vernal flowers are very sensitive to changes in temperature. The crocus can perceive a change of 0.5°C (0.9°F), and will respond to a change of 20°C (36°F) in two minutes.[4]