Heliotropism

Based on whether the plant part moves towards or away from the sun, heliotropism can be classified into two types:

1. Positive heliotropism – Movement towards the sun’s direction.
2. Negative heliotropism – Movement away from the sun’s direction.

In Leaves

According to the sun’s position, some plant orient their leaves either perpendicularly (diaheliotropism) or parallelly (paraheliotropism).
As the sun rises over the horizon, the leaves turn towards the sun in the east. Following the sun’s path, the leaves also turn from east to west as the day progresses. The next day, before dawn, the leaves reorient and face east to restart the solar tracking cycle. This phenomenon only happens on clear sunny days.

This rapid readjustment of leaves throughout the day is controlled by a specialized organ called the pulvinus. It is a fluid-controlled joint found in the petiole, the base of a leaf.

Advantage

The leaves orient horizontally in the morning and afternoon to capture the maximum sunlight. On the contrary, when the sun is most intense in midday, the leaves orient parallelly to avoid overheating and dehydration.

In Flowers

Some flowers slowly track the sun’s path across the sky from dawn to dusk. The sunflower is considered the most common example of a heliotropic flower. However, it only tracks the sun in its early developmental stage or the bud stage.

Many tropical flowers exhibit a modified form of heliotropism where they move opposite to the sun’s direction to reduce the chance of overheating (negative heliotropism).

Advantage

As the flowers face the sun, their temperature rises. This added warmth attracts more pollinators, enhancing fertilization and seed development.

In heliotropism, the plant movement is guided by the sun. While in phototropism, the directional movement occurs in the presence of any light source.