Turgor Movements

Turgor movements result from changes in internal water pressures and are often, but not always, initiated by contact with objects outside of the plant. The cells concerned may be in normal parenchyma tissue of the cortex, or they may be in special swellings called pulvini located at the bases of leaves or leaflets. Some turgor movements may be quite dramatic, taking place in a fraction of a second. Others may require up to 45 minutes to become visible.

The sudden movements of bladderworts (discussed in Chapter 7) involve turgor changes apparently triggered by electric charges released upon contact or as a result of variations in light or temperature. The springing of the trap of the Venus's flytrap (also discussed in Chapter 7) was thought to be brought about in similar fashion, but recent research has shown that the trap closes when its outer epidermal cells expand rapidly, and it reopens when the inner epidermal cells expand in the same way. About one-third of the ATP available in the cells is used in each movement, so that repeated stimulation of the trap by touching the trigger hairs readily fatigues the trap if sufficient time for ATP replenishment is not given between stimulations.

The sensitive plant (Mimosa pudica) has well-developed pulvini at the bases of its many leaflets and a large pulvinus at the base of each leaf petiole. When the leaf is stimulated by touch, heat, or wind, there is a type of chain reaction in which potassium ions migrate from one half of each pulvinus to the other half. This is followed by a rapid shuttling of water from half of the pulvinar parenchyma cells to those of the other half. The loss of turgor results in the folding of both the leaflets and the leaf as a whole (Fig. 11.14).

Secondary Pulvinus Mimosa

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