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. The sunflower is notable for turning to face the Sun, a behavior known as heliotropism.
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, and gold images of the "flower" as well as seeds were taken back to Europe early in the 16th century. Helianthus is from the Greek for "sunflower". In French, they are called "tournesol", and in Italian, "girasole".
In Greek myth, 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 the marigold.
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. 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.
Most flowerheads on a field of blooming sunflowers are turned towards the east, where the sun rises each morning (hence the sunflower's Spanish name girasol, and its French name tournesol). Immature sunflowers in the bud stage exhibit heliotropism; on sunny days the bud 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.
Cultivation and Uses
To grow well, sunflowers need full sun. They grow best in fertile, moist, well-drained soil with a lot of mulch. Seeds should be 45 cm (1.5') apart and planted 2.5 cm (1") deep.
Sunflower "whole seeds" (fruit) are sold as snacks, especially in the United States and Europe, and as food for birds. Sunflower oil, extracted from the seeds, is used for cooking (but is less cardiohealthy than olive oil), as a carrier oil and is used to produce biodiesel, for which it is less expensive than the olive product. The cake remaining after the seeds have been processed for oil is used as a livestock feed. Some recently developed cultivars have drooping heads. These cultivars are less attractive to gardeners growing the flowers as ornamental plants, but appeal to farmers, because they reduce bird damage and losses from some plant diseases. There are also new breeds of sunflowers which are transgenic, so that they are resistant to some diseases. Sunflowers also produce latex and are the subject of experiments to improve their suitability as an alternative crop for producing hypoallergenic rubber.
The sunflower is the state flower of the U.S. state of Kansas, and one of the city flowers of Kitakyushu, Japan.
The Jerusalem artichoke (Helianthus tuberosa) is related to the sunflower. The Mexican sunflower is Tithonia rotundifolia. False sunflower refers to plants of the genus Heliopsis.
Scientific literature reports, from 1567, that a 12 m (40'), traditional, single-head, sunflower plant was grown in Padua. The same seed lot grew almost 8 m (24') at other times and places (e.g. Madrid). Much more recent feats (past score years) of over 8 m (25') have been achieved in both Netherlands and Canada (Ontario).
Sunflower heads contain seeds in spiral patterns. There will be one family of spirals that goes clockwise and another family of spirals that interlaces between these, going anti-clockwise. Counting the number of spirals in each family will reveal each to be a Fibonacci number, typically 34 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.
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