Plant movement involves a change in the position or orientation of a part of the plant, such as a stem or a leaf, relative to an external stimulus. This is distinct from simple growth, which is an irreversible increase in mass and size. These subtle shifts in position are fundamental adaptations that allow stationary organisms to optimize resource acquisition and respond to immediate threats in their fixed location.
Movement Directed by Growth (Tropisms)
Plants execute directional movements over time by adjusting their growth patterns, a phenomenon known as tropism. These movements are permanent because they involve differential cell elongation, causing a permanent curve. Phototropism, the response to light, is a classic example where a shoot bends toward a light source (positive response). The stem bends because the growth hormone auxin accumulates on the shaded side, promoting cell elongation there and pushing the stem toward the light.
Gravitropism, the response to gravity, ensures that the plant establishes proper orientation. Shoots exhibit negative gravitropism by growing upward, while roots display positive gravitropism by penetrating downward into the soil. Specialized starch-heavy organelles called statoliths, located within root cap cells, settle to the lowest point of the cell, signaling the direction of gravity and directing the flow of auxin to control growth.
Thigmotropism is a directional response to touch or contact with a solid object. This is commonly observed in climbing plants, whose tendrils coil tightly around a support structure. The tendril cells touching the support grow less than the cells on the opposite side, causing the tendril to wrap around the object and provide mechanical stability. Because these movements rely on the slow process of cell growth, the resulting changes in position are gradual and irreversible.
Rapid Movements Based on Internal Pressure (Nastic Responses)
In contrast to growth-driven tropisms, nastic movements are rapid, reversible changes in position independent of the stimulus’s direction. These movements rely on swift changes in internal water pressure, known as turgor pressure, rather than cell growth. The movement is triggered in specialized joint-like structures called pulvini, composed of motor cells that rapidly inflate or deflate.
A common example is seismonasty, the touch-induced movement seen in the sensitive plant, Mimosa pudica. Leaflets fold inward and the entire leaf collapses within seconds of being disturbed due to the rapid efflux of water from the motor cells in the pulvini. Another nastic movement is nyctinasty, or sleep movements, where leaves or flower petals open during the day and fold up at night in response to changes in light and temperature.
Carnivorous plants, such as the Venus flytrap, use a specialized form of rapid movement to capture prey. When an insect brushes against the sensory trigger hairs, it initiates a rapid change in water pressure in the cells lining the trap’s lobes. This sudden change causes the lobes to snap shut quickly, ensuring the capture of the prey.
How Plants Sense and Signal Movement
The mechanical movements of plants are governed by sophisticated internal signaling systems. For growth-driven tropisms, the primary signal is the plant hormone auxin, synthesized in the shoot tip and transported down the stem. When a stimulus like light or gravity is detected, specialized PIN proteins redistribute auxin to create a concentration gradient across the organ.
In stems responding to light, a higher concentration of auxin on the shaded side triggers an “acid-growth” response. This process involves the hormone activating proton pumps, which lowers the pH and allows cells to elongate faster. Conversely, in roots, higher auxin concentrations tend to inhibit cell elongation, allowing the root to curve in the appropriate direction.
For the rapid nastic movements, the signal is primarily electrical and chemical, resembling an action potential in animal nerve cells. When a trigger hair is touched, it generates an electrical signal that travels quickly to the motor cells in the pulvinus. This electrical wave initiates the rapid movement of ions, particularly potassium and chloride, out of the extensor motor cells. Water quickly follows the ions via osmosis, leading to a sudden loss of turgor pressure and the collapse of the structure.
Survival Benefits of Plant Mobility
The various forms of plant movement are evolutionary responses that increase survival and reproduction. Directional movements like phototropism ensure leaves are optimally positioned to intercept sunlight, maximizing photosynthesis efficiency. Gravitropism anchors the plant securely by guiding roots deep into the soil for stability and access to water and nutrients.
Rapid movements provide immediate protection against environmental threats and aid in nutrient acquisition. The collapse of leaves in Mimosa pudica upon touch may deter herbivores or shake off smaller pests. The snap-trap mechanism of carnivorous plants allows them to capture insects, supplementing their diet in nutrient-poor soils. The coiling movement of tendrils allows climbing plants to reach higher light levels without expending energy on building a thick trunk.