The sunflower (Helianthus annuus) is an annual plant in the Family Asteraceae, with a large flower head (inflorescence). The stem of the flower can grow up to 3 metres tall with the flower head reaching 30cm in diameter.
History
Sunflowers are native to the Americas, and were domesticated around 1000 B.C. Francisco Pizarro found the Inca subjects venerating the sunflower as an image of their sun god. Gold images of the flower, as well as seeds, were taken back to Europe early in the 16th century. Helianthus is the Greek word for "sunflower".
Greek myth
In Greek mythology, a girl named Clytie fell in love with the sun god Apollo, and would do nothing but watch his chariot move across the sky. After nine days, she was transformed into a sunflower. However, the word "sunflower" and its cognates existed long before Helianthus annuus was brought to Europe, and it is thought that the myth (which is mentioned in Ovid's poem Metamorphoses) actually refers to heliotrope or marigold.
Description
The term "sunflower" is also used to refer to all plants of the genus Helianthus, many of which are perennial plants.
What is called the flower is actually a head (formerly composite flower) of numerous flowers crowded together. The outer flowers are the ray florets and can be yellow, maroon, orange, or other colors. These flowers are sterile. The flowers that fill the circular head inside the ray flowers are called disc florets.
The arrangement of florets within this cluster is typically such that each is separated from the next by approximately the golden angle, producing a pattern of spirals where the number of left spirals and the number of right spirals are successive Fibonacci numbers. Typically there are 34 spirals in one direction and 55 in the other; on a very large sunflower you may see 89 in one direction and 144 in the other.
The disc florets mature into "seeds". However, what we commonly call the seeds are actually the fruit (an achene) of the plant, with the true seeds encased in an inedible husk.
Heliotropism
Most flowerheads on a field of blooming sunflowers are turned towards the east, where the sun rises each morning. Immature sunflowers in the bud stage exhibit heliotropism; on sunny days tracks the sun on its journey along the sky from east to west, while at night or at dawn it returns to its eastward orientation. The motion is performed by motor cells in the pulvinus, a flexible segment of the stem just below the bud. The stem stiffens at the end of the bud stage, and when the blooming stage is reached the stem freezes in its eastward direction. Thus, blooming sunflowers are not heliotropic anymore, even though most flowerheads are facing the direction where the sun rises.
The inflorescence of the wild sunflower seen on roadsides does not turn toward the sun. In this sunflower, the flowering heads face many directions when mature. But the leaves exhibit some heliotropism.