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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 term "sunflower" is also used to refer to all plants of the genus Helianthus, many of which are perennial plants.
Description
What is usually 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 what are normally called "sunflower seeds", 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 and on sunny days track the sun on its journey along the sky from east to west, while at night or at dawn they return 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.
Cultivation and uses
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".
To grow well, sunflowers need full sun. They grow best in fertile, moist, well-drained soil with a lot of mulch. In commercial planting, seeds are planted 45 cm (1.5') apart and 2.5 cm (1") deep.
Sunflower "whole seed" (fruit) is sold as snacks, especially in China, the United States and Europe. It is also sold as food for birds and can be used directly in cooking and salads.
Sunflower oil, extracted from the seeds, is used for cooking (but is less cardiohealthy than olive oil), as a carrier oil and 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.
For farmers not intending to grow it, the sunflower is considered a noxious weed. The wild variety will grow unwanted in corn and soy bean fields which can have a negative impact on yields.
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.
Trivia
- 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).
Flower formation
Gallery
See also
References
- Shosteck, Robt. 1974. Flowers and Plants. An International Lexicon with Biographical Notes. Quadrangle/The New York Times Book Co. 329 pp.
- Wood, Marcia. June 2002. "Sunflower Rubber?" Agricultural Research. USDA. *
External links
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"Sunflower"