A tropism is the innate ability of an organism to turn or move in response to a stimulus. As opposed to a learned ability, innate reactions are genetically programmed. Organisms with a tropism will naturally turn toward a stimulus. A stimulus can be any signal from the environment, and individual tropisms are often named after the stimulus that causes the movement. In a positive tropism the animal will move toward the stimulus. In a negative tropism, the animal will move away from the tropism. Certain stimuli become genetically engrained because they are always beneficial or always detrimental to an organism. A movements caused by a tropism is called a taxis.
Types of Tropism
Many photosynthetic animals produce their food from sunlight. Therefore, sunlight has evolved as a stimulus for many organisms. Many of these organisms move towards the light. This positive phototropism causes organisms to move toward the light. Many algae, plankton, and small invertebrates exhibit positive phototaxis. This brings them to the same area in the ocean, which contains by far the most light. Even organisms that are not photosynthetic have developed this tropism, which moves them to the same water column as their prey items. Other organisms may exhibit a negative reaction to light, and try to move away from it. Think of a beetle when you uncover it from its hiding place. The beetle will seek darkness, as darkness usually means safety. This tropism has the same stimulus but the opposite direction of the tropism in plants.
A similar phenomena occurs in land plants. Terrestrial plants are restricted in their movements, due to their roots. Instead, plants orient their leaves toward the sun, to collect the most sunlight. This tropism is similar to phototropism, but the position of organism cannot change. Instead, the plant turns its leave by reducing or increasing turgor pressure, or water pressure, in individual cells. This can be seen in many crops, such as sunflowers, corn, and even garden flowers. Following the trajectory of the sun ensures that the plants receive the maximum amount of light.
A widely spread tropism in the animal kingdom is chemotropism, or the tendency to turn or move towards or away from a specific chemical substance. Some single-celled organisms use different chemotropisms for different purposes. For example, one chemical might signal a mate, while another signals a dangerous or unpleasant area. These simple organisms will simply move towards or away from stimuli in the direction that has been most evolutionarily rewarding to their ancestors. In higher organisms, certain chemicals still attract animals, though they may not always move towards it. In other words, while they have the tropism, they do not always exhibit taxis. For instance, sharks have a positive chemotropism for blood, or they tend to move towards blood. However, a shark will often inspect a meal or test it before devouring the prey, showing that other mechanisms can override a tropism.
Many other tropisms exist in nature, due to the large variety of stimuli that organisms can sense. While our senses are limited to a specific visual, auditory, and tactile range, other organisms can sense stimuli we might not even know are present. Scientist have managed to document a number of stimuli that appear to be related to a specific tropism. Some are listed below:
- Thermotropism: A tropism that causes organisms to move towards a specific temperature.
- Thigmotropism: Often, roots may turn when they touch a hard surface, like rock. The stimulus of touch drives this tropism.
- Magnetotropism: Many animals may use the magnetic fields as a source of direction, and be drawn to certain poles.
Examples of Tropism
Fish exhibit many tropisms, and respond to a wide variety of stimuli. Fish usually display negative thigmotaxis, or move away from any stimulus touching them. This helps them both avoid predators and maintain their order in a large school. Another tropism seen in fish is chemotropism. Fish are both drawn to and avoid a variety of different chemicals. Most fish release a chemical when they are attacked or die which warns other fish to flee the area. Salmon have an extraordinary chemotrophic ability, in that they are able to travel thousands of miles in the ocean and still return to the same stream in which they were spawned. The streams have unique chemical signatures, and salmon are drawn to this chemical stimulus in a tropism that appears late in life. The salmon are born in the stream, migrate to the ocean to grown, and travel up the stream one last time to lay their eggs and die.
Plants in a Lab
The idea of a tropism was first established in botany, as it was seen that plants would move in response to different stimuli. Scientists notice that no matter how you plant a seed, the roots always establish themselves toward gravity, known as geotropism. The roots are also inherently attracted to water, and will move and turn toward the most water. This hydrotropism is obviously beneficial to plants. In studying this phenomena, scientist found that plants have another tropism. When the water is oxygen deprive, the roots will seek oxygen as well. This is known as aerotropism, and the stimulus is oxygen. As seen by these plant examples, a tropism often has a deep evolutionary base, and is filling an essential need of the organism.