Sunflowers in the bud stage explain signs of heliotropism. At break of day, the faces of most sunflowers are turned towards the east. Over the path of the day, they move to track the sun from east to west, while at night they go again to an eastward orientation. This movement is performed by motor cells in the pulvinus, a bendy segment of the stem just below the bud. As the bud stage ends, the stem stiffens and the blooming stage is reached. Sunflowers in the blossoming stage are not heliotropic anymore. The stem has frozen, generally in an eastward orientation. The stem and leaves lose their green color. The wild sunflower obviously does not turn toward the sun; its flowering heads may face many directions when mature. On the other hand, the leaves normally exhibit some heliotropism.
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