Difference Between Positive and Negative Tropism

The key difference between positive and negative tropism is that positive tropism is the movement of an organism or part of an organism towards a stimulus while negative tropism is the movement or growth of an organism or part of an organism away from a stimulus.

Living organisms respond to different stimuli in different ways. Plants respond in a different way than animals. Similarly, unicellular organisms react to stimuli differently. Tropism is the term used to describe these movements of organisms or parts of the organism towards or away from different stimuli. If an organism responds towards the direction of the stimulus, we call it positive tropism. In contrast, if an organism moves away from the stimulus, we call it negative tropism. If the stimulus is gravity, roots show positive geotropism (towards gravity) while shoots show negative geotropism (away from gravity).

CONTENTS

1. Overview and Key Difference
2. What is Positive Tropism
3. What is Negative Tropism
4. Similarities Between Positive and Negative Tropism
5. Side by Side Comparison – Positive vs Negative Tropism in Tabular Form
6. Summary

What is Positive Tropism?

Positive tropism is the movement or growth shown by organisms towards the direction of the stimulus. Therefore, organisms grow or move in the direction of the stimulus. For example, plant shoots grow upwards looking for sunlight. This is positive phototropism. Moreover, plant roots grow downwards in the soil according to gravity. This is also a positive geotropism.

Some single-celled organisms show positive chemotropism. They move towards certain chemical substances. Some organisms show positive thermotropism. They move towards specific temperatures. Twining plants and tendrils show thigmotropism. When they touch a hard surface, they grow or move towards the stimulus.

What is Negative Tropism?

Negative tropism is the movement or growth of an organism away from the stimulus. Therefore, organisms move or grow away in the direction of which the stimulus originates. Plant shoots grow away from the gravity. Hence, shoots show negative geotropism. Beetles show negative phototropism. They seek darkness for their safety. The growth of roots underneath the soil is a negative thigmotropism. When a growing root is in contact with an object such as rock, it grows away from it, showing negative thigmotropism. Some varieties of fish show negative chemotropism. They move away from chemicals which are hazardous to them.

What are the Similarities Between Positive and Negative Tropism?

• Positive and negative tropism are two types of responses shown by organisms based on the direction of the stimulus.
• They can be a movement or growth.

What is the Difference Between Positive and Negative Tropism?

Positive tropism is the movement or growth of the whole organism or a part of the organism towards a stimulus while negative tropism is the movement or growth of the whole organism or a part of the organism away from a stimulus. So, this is the key difference between positive and negative tropism. Moreover, the plant shoots show positive phototropism while the plant roots show negative phototropism. But, the plant roots show positive geotropism, while the plant shoots show negative geotropism. Thus, this is another difference between positive and negative tropism.

Below is a summary tabulation of the difference between positive and negative tropism.

Summary – Positive vs Negative Tropism

Tropism can be positive tropism or negative tropism based on the response for the direction of the stimulus. Positive tropism is the movement or growth towards the direction of the stimulus. In contrast, negative tropism is the movement or growth away in the direction of the stimulus. Thus, this is the key difference between positive and negative tropism.

Reference:

1. “Tropism.” Wikipedia, Wikimedia Foundation, 20 June 2020, Available here.
2. “Tropism.” Biology Dictionary, 29 Apr. 2017, Available here.

Image Courtesy:

1. “Phototrophic Response to Stimulus” By Mtomanelli15 – Own work (CC BY-SA 4.0) via Commons Wikimedia