It’s not only their appearance that gives sunflowers their common name. In their infancy, sunflowers actually “follow the sun” during part of the day because of a process called heliotropism.

Although some sunflower species are perennials, it’s the common (annual) sunflower that exhibits heliotropism. Instead of sunflowers facing each other or oriented in random directions, there’s directional uniformity among plants of similar age. Before their flowers form and are pollinated, young sunflower plants face the sun at dawn and follow the sun’s movement as it moves across the sky. But the internal clock of their circadian rhythm reverses their movement after the sun goes down, and they once again face the eastern sky until the sun rises the following morning.

The intricate inner workings of plants such as sunflowers, which exhibit heliotropism, involve more than one mechanism that allows them to move. Specialized cells in a structure called the "pulvinus" are located at the base of leaves or flower buds. These cells contain motor cells that facilitate a plant’s movement as it tracks the sun. It’s the water within the pulvinus that creates "turgor pressure", which enlarges or shrinks pulvinus cells and triggers movement that includes bending of the stems.

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