Thursday, March 6, 2008
Sunflowers in the bud stage show signs of heliotropism. At break of day, the faces of most sunflowers are turned towards the east. Over the course of the day, they move to track the sun from east to west, while at night they return to an eastward orientation. This movement is performed by motor cells in the pulvinus, a bendable 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, normally in an eastward orientation. The stem and leaves lose their green color.The wild sunflower naturally does not turn toward the sun; its flowering heads may face many directions when mature. However, the leaves typically exhibit some heliotropism.