Last week the shop was filled with their bright yellow faces. We used them for two separate occasions. As you probably know, in Italy they are called Girasole - turn to the sun. If you've ever passed by a field of them growing, like in Tuscany or northern California or your field next door, it's clear that their faces follow their namesake as it moves across the sky. Wanting to know a little more about why they do this I came upon this explanation at Ask Mr. Smarty Plants
Heliotropism is the term used to describe the tracking of the sun by the flower buds of sunflowers. Other plants also have flowers and/or leaves that track the sun. First, let's discuss how the plants are able to track the sun. There is a collection of specialized cells at the base of the flower bud or leaf called a pulvinus that carry the "motor" cells that enable the plant leaf or flower to track the sun. These cells enlarge or shrink according the turgor pressure from the water inside them. In response to blue wavelength light, potassium ion concentration increases in the "motor" cells on the shadow side of the pulvinus. With the increase of potassium ions the osmotic potential in the cells becomes more negative and the cells absorb more water and elongate, turning the face of the flower to the sun.
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Keep your face to the sunshine and you cannot see the shadow. It's what sunflowers do. ~ Helen Keller