Movements Resulting Primarily from Internal Stimuli

Nutations Charles Darwin once attached a tiny sliver of glass to the tip of a plant growing in a pot. Then he suspended a piece of paper blackened with carbon over the tip, and as the plant grew, he raised the paper just enough to allow the tip to touch the paper without hurting the plant. He found that the growing point traced a spiral pattern in the blackened paper. We know now that such nutations (also referred to as spiraling movements) are common to many plants (Fig. 11.7).

Nodding Movements Members of the Legume Family (Fabaceae), such as garden beans, whose ethylene production upon germinating causes the formation of a thickened crook in the hypocotyl, exhibit a slow, oscillating movement (i.e., the bent hypocotyl "nods" from side to side like an upside-down pendulum) as the seedling pushes up through the soil. This nodding movement apparently facilitates the progress of the growing plant tip through the soil.

Twining Movements Although twining movements are mostly stimulated internally, external forces, such as gravity and contact, may also play a role. These movements occur when cells in the stems of climbing plants, such as morning paper blackened with carbon paper blackened with carbon

Figure 11.7 Charles Darwin's demonstration of spiraling growth.

tip spirals as it grows, tracing its pattern on the paper

Figure 11.7 Charles Darwin's demonstration of spiraling growth.

glory, elongate to differing extents, causing visible spiraling in growth (in contrast with the spiraling movements previously mentioned, which are not visible to the eye). Tendril twining, which is initiated by contact, results from an elongation of cells on one side of the stem and a shrinkage of cells on the opposite side, followed by differences in growth rates (Fig. 11.8). Some tendrils are stimulated to coil by auxin, while others are stimulated by ethylene.

Contraction Movements In Chapter 6, we noted that the bulbs of a number of dicots and monocots have contractile roots, which pull them deeper into the ground. In lilies, for example, seeds germinating at the surface ultimately produce bulbs that end up 10 to 15 centimeters (4 to 6 inches) below ground level because of the activities of contractile roots. There is some evidence that temperature fluctuations at the surface determine how long the contracting will continue. When the bulb gets deep enough that the differences between daytime and nighttime temperatures are slight, the contractions cease. The aerial roots of some banyan trees straighten out by contraction after the roots have made contact with the ground. The "shrinking" of roots has been shown to take place at the rate of 2.2 millimeters (0.1 inch) a day in sorrel (Oxalis).

Nastic Movements When flattened plant organs, such as leaves or flower petals, first expand from buds, they characteristically alternate in bending down and then up as the cells in the upper and lower parts of the leaf alternate in enlarging faster than those in the opposite parts. Such non-directional movements (i.e., movements that do not result in an organ being oriented toward or away from the direction of a stimulus) are called nastic. Nastic movements may involve differential growth or turgor changes in special cells. Epinasty is the permanent downward bending of an organ, often the petiole of a leaf, in response to either an unequal flow of auxin through the petiole or to ethylene. Nastic movements that involve changes in turgor pressure include sleep movements and contact movements, which are discussed later.

Internal Stimuli
Figure 11.8 Typical twining of a tendril produced by a manroot plant. Note that the direction of coiling reverses near the midpoint.

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