Positive and Negative Tropism

Plant Tropism Study Guide

by Katie Hinkle

What is a Tropism?

• ​Tropism is a plants response to external stimulus

• ​Even though plants are immobile they can still respond to things going on around them

​Phototropism

• ​Phototropism is a plant's response to its light source.

• ​Plants will bend and grow towards the light source in order to keep the process of photosynthesis going.

• ​If plants do not have light photosynthesis (the process by which they make food) cannot occur

​Hydrotropism

• ​Hydrotropism is a plant's response to water

• ​Roots will grow towards their source of water

• ​Just like plants need sunlight to make food they also need water for photosynthesis to occur

​Gravitropism

• ​Gravitropism is a plant's response to gravity

• ​Trees grow towards the sun because of phototropism, but the gravity acting upon them cause the roots to grow downwards.

​Thigmatropism

• ​Thigmatropism is a plant's response to touch

• ​Some plants will shrivel up when touched in order to stop bugs from feeding on them

• Grape vines and ivy will wrap around structures

• ​Carnivorous plants like Venus Fly Traps will close up in order to trap food to provide nutrients for the plant

​Auxin

• ​Auxin is the hormone in plants that promotes plant growth

• ​Auxin is located in the tips of shoots and roots

• ​Auxin will be located in parts of the plant that are not near the sun. This is because parts of the plants that are not directly in the sun still need to grow.

​Positive and Negative Tropisms

• When a plant is growing towards the stimulus we refer to that as a positive tropism

• When a plant is away from a stimulus we refer to that as a negative tropism

• It is possible for different parts of plants to have both negative and positive tropisms at the same time.

• For example the plant on the right is growing towards the sun so it has positive phototropism, but the roots are growing away from the sun so they have negative phototropism

​Use these videos from earlier in the lesson for review