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Figure 11.14 A. A longitudinal section through the pulvi-nus of a sensitive plant (Mimosa pudica). B. The leaflets of the leaf toward the bottom of this picture have folded upward in response to being bumped. The other leaves of this sensitive plant have remained fully expanded.

Growth 211

Figure 11.15 A bush monkey flower. The white, two-lobed structure in the center is the stigma. A. The stigma as it appears before pollination. B. The stigma 2 seconds after being touched by a pollinator; the lobes have rapidly folded together.

Growth 211

Structure Mimosa Pudica

Figure 11.15 A bush monkey flower. The white, two-lobed structure in the center is the stigma. A. The stigma as it appears before pollination. B. The stigma 2 seconds after being touched by a pollinator; the lobes have rapidly folded together.

If a part of the stem of a sensitive plant (Mimosa pudica) is cut off and then immediately reattached with a water-filled piece of rubber tubing, it can be shown that something is transmitted through the water from above the cut. Within a few minutes after a leaf above is stimulated to fold, a leaf or two below the cut will also fold. Although potassium ions have been shown to leave cells in pulvini that are losing turgor, it is not known if the reaction is transmitted across water in the rubber tubing by these ions, by electrical charges, or by something else. A normal response to a stimulus by a sensitive plant (Mimosa pudica) takes from a few seconds to less than 2 minutes.

The redwood sorrel (Oxalis oregana), whose leaflets also have pulvini, flourishes on the floor of redwood forests where the light may be only 1/200th that of full sunlight. Occasionally, light of sufficient intensity to damage delicate leaves may temporarily penetrate the overhead canopy. When this occurs, the leaflets begin to fold downward within 10 seconds and are completely folded in about 6 minutes, unfolding once again when the brighter light is gone.

Turgor contact movements are not confined to leaves. Many flowers exhibit movements of stamens and other parts, most such movements facilitating pollination. The pollen-receptive stigmas found in flowers of the African sausage tree have two lobes. These lobes fold inward, enclosing pollen grains that have landed on them. The stigmas of monkey flowers (Fig. 11.15) and Asian cone flowers exhibit similar turgor movements upon contact, while the pollen-bearing stamens of barberry and moss rose flowers snap inward suddenly upon contact, dusting visiting insects with pollen (Fig. 11.16).

Orchid flowers exhibit some of the most spectacular of all turgor contact movements, including those by which little sacs of pollen are forcibly attached to the bodies of visiting insects. In a few instances, the benefactors even receive dunkings in small "pans" of fluid from which they escape via a trap door. Brief discussions and illustrations of a few examples are given in Figure 23.18.

Figure 11.16 Contact movements of the stamens of a barberry flower. A. Position of the stamens before contact. B. Position of the stamens after they have jerked inward in response to contact.

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