Tropism Definition

A tropism is an organism’s natural capacity to turn or move in reaction to a stimulus. Innate responses, unlike acquired abilities, are genetically hardwired. Tropism causes organisms to naturally gravitate toward a stimulus. Individual tropisms are generally termed by the stimulus that triggers the movement, which might be any indication of the environment.

In a positive tropism, the animal will go near the stimulus. In a negative tropism, the animal will avoid the tropism. Because some stimuli are always useful or always harmful to an organism, they become genetically entrenched. Taxis are motions that are generated by tropism.

Types of Tropism

Phototropism

Several photosynthetic creatures use sunlight to manufacture their food. As a result, many creatures have evolved to use sunshine as a stimulant. Many of these species want to get closer to the light. Organisms migrate toward the light due to positive phototropism. Positive phototaxis is seen in many algae, plankton, and tiny animals. This gets them to the same spot in the water, which has the greatest light by far. Even non-photosynthetic species have acquired this tropism, causing them to enter the same water stream as their prey.

Other species may have a negative response to light and attempt to avoid it. When you unearth a beetle from its hiding location, think of it as a beetle. Because darkness is normally associated with safety, the beetle will seek it out. This tropism has the same stimulus as plant tropism, but it works in the opposite way.

Heliotropism

In terrestrial plants, a similar phenomenon happens. Because of their roots, terrestrial plants have limited mobility. Plants, on the other hand, position their leaves facing the sun to absorb the maximum amount of sunlight. This tropism resembles phototropism in many ways, except that the organism’s location is fixed. Instead, to spin its leaves, the plant modifies the transpiration pressure, or aqueous pressure, in individual cells. Many crops, including sunflowers, maize, and even garden flowers, show this characteristic. Tracking the sun’s course ensures that plants get the maximum available light.

Chemotropism

Chemotropism, which is the tendency to rotate or migrate towards or far from a certain chemical molecule, is a common tropism in the animal world. Chemotropisms are used by several single-celled organisms for various functions. One molecule may signify a mate, while another may indicate a harmful or uncomfortable location. These primordial organisms will move toward or far from stimulus in the route that their ancestors determined to be the most advantageous for evolution.

Certain compounds still attract animals and higher creatures, but they may not always migrate towards them. To put it another way, despite having tropism, they do not usually depict taxis. Sharks, for example, demonstrate a positive blood chemotropism, which means they gravitate toward it. A shark, on the other hand, would often scrutinise or test  before swallowing it, demonstrating that other processes may overcome a tropism.

Other Tropisms

Because organisms can feel a wide range of stimuli, many different tropisms occur in nature. Other creatures may feel stimuli we may not even be aware of, whereas our senses are confined to a narrow visual, aural, and tactile range. Scientists have identified a variety of stimuli that seem to be linked to a particular tropism. Here are a few examples:

• Thermotropism is a tropism in which organisms migrate towards a certain temperature.
• Thigmotropism occurs when roots come into contact with a hard surface, such as rock. This tropism is triggered by the sensation of touch.
• Magnetotropism discusses how several organisms use magnetic forces as a directional cue and are attracted to certain poles.

Examples of Tropism

Fish

Fish have a range of tropisms and react to a number of stimuli. Negative thigmotaxis, or moving far from a stimulus that touches them, is common in fish. This allows them to dodge predators while still maintaining order in a huge group. Chemotropism is an additional tropism exhibited in fish. Fish are attracted to and repelled by a wide range of substances. When most fish are attacked or die, they produce a chemical that causes other fish to evacuate the area.

Salmon contain a unique chemotrophic capacity, allowing them to travel hundreds of kilometres across the ocean and back to the similar stream they were born in. Salmon are attracted towards this chemical stimulation due to a late-life tropism, and the streams have different chemical fingerprints. Salmon are born in the channel, mature in the ocean, then return to the stream to lay their eggs before dying.

Plants in a Lab

The concept of tropism originated in botany, when it was observed that plants moved in response to various stimuli. Scientists have discovered that no matter how a seed is planted, the roots always gravitate toward gravity, a phenomenon known as geotropism. The roots are naturally drawn to water and will migrate and turn in the direction of the most water. Hydrotropism is clearly advantageous to plants. Scientists discovered another tropism in plants while researching this phenomenon. When the water is depleted of oxygen, the roots will seek it out. The trigger is oxygen, which causes aerotropism. As these plant examples show, tropism has a long evolutionary history and serves an important function for the organism.

Related Biology Term

• Taxis:  An organism’s movement is produced by tropism.
• Nastic Movements: Movements of organisms in response to the presence, but not the direction, of a stimulus.
• Turgor Pressure: The pressure within plant cells’ vacuoles that can be regulated to move the leaves and stems of plants.

Question

1. Some animals accumulate in a certain spot because they reproduce asexually and do not move far from each other. This can be seen in coral colonies and bacteria. Is this an example of a tropism towards the spot of the original organism?

1. Yes
2. No
3. Only if the original organism was attracted there by a tropism

Answer

B is correct. If an animal reproduces asexually and cannot move once created, the animal never displays a taxis towards the stimulus. Although it may exist, it cannot be seen in any of the organism’s behaviors. These animals do not display a tropism toward the center of the colony, they just happen to reproduce in that form. Some larval forms of coral are attracted by tropisms to various spots on the sea floor, which they can settle and form a new colony. However, once the spot is picked, the same tropisms do not drive the actions of the resulting colony of individuals.

Question

2. A certain zooplankton species is a small, multi-celled organism resembling a shrimp that survives by feeding on algae. The zooplankton does not photosynthesize light itself, but has been shown to have a positive tropism toward sunlight. Why is this?

1. A. This tropism moves the plankton into the same region as its prey
2. The plankton needs sunlight to stay warm
3. The fish that eat the plankton like the dark

Answer

A is correct. Although the zooplankton does not need the light directly, the organisms it feeds on do. Throughout evolutionary time, the zooplankton has developed the same tropisms as its prey species, which allow it to be in the right place, at the right time. The fish that eat the zooplankton operate on similar tropisms, which cause them to congregate where the most zooplankton are. This continues up through the food-chain.

Question

3. If you’ve ever had a dog, you notice that nasty garbage odors will distract even the best-behaved of companions. Scientists believe that dogs evolved from wolves, feeding mainly off of human garbage. What would this attraction to garbage smell be called?

1. Garbotropism
2. Chemotropism
3. Phototropism

Answer

B is correct. The dogs are guided by their extraordinary noses to the smell. Odors are simple small molecules, distributed in the air. Dogs follow the concentration gradient of these molecules until they find the source, which has the highest concentration of the molecule. While it seems that dogs will investigate any gross smell, they are actually attracted to specific odors which signal that there may be something they might consider food. One man’s trash, another dog’s treasure!