Tropisms

Plants grow in response to their environment. When the direction they grow is determined by the location of the stimulus, it is called tropism.

Heliotropism in sunflowers

Types of tropism

​Tropisms can be positive or negative. Positive tropisms attract the plant toward the trigger, while negative tropisms push the plant away. Sunlight is generally a positive tropism, causing plants to lean, turn, and pivot to follow its rays. In some cases, particularly darkened jungle floors, sunlight is a negative tropism, initially, driving vines to seek trees which they will later climb. ​

Thigmotropism of pumpkin

Tropisms are generally named for the stimuli:

• gravitropism - roots grow downward (positive tropism) and stems grow upward (negative tropism) in response to gravity; also known as geotropism
• heliotropism - pivoting movement that allows plants to follow the sun’s movement across the sky
• hydrotropism - the root cap senses water in the soil and grows toward it
• phototropism - stems grow upward, toward sunlight
• skototropism - vines move away from sunlight in order to find trees to climb
• thigmotropism - tendrils twine around supports

Plants also respond to chemicals (chemotropism), electric fields (electrotropism), magnetic fields (magnetotropism), oxygen levels (aerotropism), and temperatures (thermotropism)

The opposite of tropism

When a plant responds in a direction that is not determined by the position of the stimulus, it is called a nastic movement. Common nastic movement triggers include being touched or the way a flower opens and closes each day (nyctinasty). When being touched causes flowers or leaves to suddenly close themselves up in a specific and regular pattern, rather than the thigmotropism seen in climbing tendrils, it is a nastic movement. The location of the finger has nothing to do with the way the flower closes. In the latter case, the sun going down may trigger the flower to close, but the direction of that closing is not related to the sun’s position.

Plants may not be able to get up and walk around, but they have a unique arsenal of movement-related responses they can use to help themselves grow and thrive.

Which tropisms do you see in your garden?