Tropism comes from the ancient Greek word tropos, meaning “a turn.” Tropism is a fundamental biological phenomenon defined by the growth or turning movement of an organism in a direction determined by an external stimulus. This mechanism is particularly important for sessile organisms, such as plants, which cannot relocate. Plants use tropisms to continually adjust their architecture, optimizing access to resources like light, water, and nutrients.
Defining Directional Growth
Tropism is primarily a growth response, meaning the directional change is permanent and involves differential cell expansion rather than temporary movement. The plant’s reaction is always described relative to the direction of the stimulus itself.
Growth toward the source of the stimulus is known as positive tropism. For instance, the downward growth of a root into the soil is positive tropism in response to gravity. Conversely, growth away from the stimulus is called negative tropism. A plant shoot growing upward, away from the gravitational pull of the earth, demonstrates negative tropism. This positive or negative designation describes the range of directional growth responses observed in plants.
Categorizing Responses by Stimulus
Plants exhibit a range of tropisms, named for the specific environmental trigger that elicits the response. Phototropism is the growth response to light. Shoots display positive phototropism, bending toward light to maximize photosynthesis, while roots typically show negative phototropism, growing away from light.
Geotropism, also called gravitropism, is the growth response to gravity. The downward growth of roots is a positive gravitropic response, ensuring anchoring and access to underground water. The upward growth of stems is a negative gravitropic response, positioning the leaves for optimal light capture. Roots also exhibit positive hydrotropism, which is directional growth toward a moisture gradient sensed by the root cap.
The response to physical touch is called thigmotropism, commonly observed in climbing plants. Tendrils show positive thigmotropism, coiling around a support structure upon contact. Conversely, roots demonstrate negative thigmotropism, adjusting their growth direction to move away from rigid obstacles like rocks, allowing for unimpeded penetration through the soil.
Chemotropism involves growth guided by a chemical gradient, crucial for plant reproduction. The primary example is the growth of the pollen tube after landing on a flower’s stigma. The tube demonstrates positive chemotropism by elongating directly toward the ovule, guided by specific chemical signals released to ensure fertilization.
The Role of Hormones in Direction
The underlying biological mechanism that executes these directional movements relies heavily on specialized plant hormones known as auxins. Auxins are synthesized in the growing tips of shoots and roots, and their redistribution across a plant organ causes the characteristic bend. This concept is central to the Cholodny-Went hypothesis, which explains how an external stimulus is translated into directional growth.
In phototropism, blue light is sensed by photoreceptor proteins, which trigger the lateral movement of auxin from the illuminated side of the stem to the shaded side. The resulting higher concentration of auxin on the shaded side causes those cells to elongate faster than the cells on the lit side, forcing the stem to curve toward the light source. This movement is facilitated by specialized protein channels embedded in the cell membranes, such as PIN proteins.
A similar, though more complex, mechanism governs gravitropism. Specialized cells in the root cap, called statocytes, contain dense starch granules known as statoliths. These statoliths sediment to the lowest point of the cell, sensing the direction of gravity. This signal causes auxin to accumulate on the lower side of both the root and the shoot. Critically, high concentrations of auxin stimulate elongation in shoots but inhibit elongation in roots. This means the lower side of the shoot grows faster (bending up), while the lower side of the root grows slower (bending down).
How Tropism Differs from Other Movements
The requirement for directional growth distinguishes tropisms from other types of plant movement. The most common alternative is Nastic Movement, a plant response that is entirely non-directional relative to the stimulus. While a stimulus is required, the direction of the movement is predetermined by the plant’s anatomy, not the location of the trigger.
A common example of nastic movement is the rapid closing of the leaves of the Mimosa pudica or the snapping shut of a Venus flytrap, both responding to touch. These actions are driven by rapid changes in turgor pressure within specialized motor cells and are not directional growth responses. Another distinct type of movement is Taxis, which is the directional movement of an entire motile organism, typically single-celled, toward or away from a stimulus.