Sunflower
The sunflower (Helianthus annuus) is an annual plant native to the Americas in the family Asteraceae, with a large flowering head (inflorescence). The stem of the flower can grow as high as 3 metres tall, with the flower head reaching up to 30 cm in diameter with the "large" seeds. 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 (formally composite flower) of numerous flowers (florets) crowded together. The outer flowers are the ray florets and can be yellow, maroon, orange, or other colors, and are sterile. The florets inside the circular head are called disc florets.  The disc florets mature into what are traditionally called "sunflower seeds", but are actually the fruit (an achene) of the plant.  The true seeds are encased in an inedible husk.
The florets within this cluster are arranged spirally.  Typically each floret is oriented toward the next by approximately the golden angle, producing a pattern of interconnecting spirals where the number of left spirals and the number of right spirals are successive Fibonacci numbers. Typically, there are 34 spirals in 1 direction and 55 in the other; on a very large sunflower you may see 89 in one direction and 144 in the other.

# Heliotropism
Sunflowers in the bud stage exhibit heliotropism. At sunrise, 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 motion is performed by motor cells in the pulvinus, a flexible 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 blooming stage are not heliotropic anymore.  The stem has frozen, typically in an eastward orientation.  The stem and leaves lose their green color.
The wild sunflower typically does not turn toward the sun; its flowering heads may face many directions when mature. However, the leaves typically exhibit some heliotropism.

# Cultivation and uses
Sunflowers are native to the Americas.  There is some debate about where the sunflower was first domesticated. The earliest known examples of a fully domesticated sunflower were found at the Hayes site in Tennessee and date back to around 2300 B.C.  There were also other remains found at the Olmec site of San Andres dating some time before 2100 B.C.  The Incas used 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.
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) are sold as a snack food after roasting within heated ovens with or without salt added.  Sunflowers can be processed into a peanut butter alternative, Sunbutter, especially in China, Russia, the United States, the Middle East 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, as a carrier oil and to produce biodiesel, for which it is less expensive than the olive product. A range of sunflower varieties exist with differing fatty acid compositions; some 'high oleic' types contain a higher level of healthy monounsaturated fats in their oil than even olive oil.
During the 18th Century, the use of sunflower oil became very popular in Europe, particularly with members of the Russian Orthodox Church because sunflower oil was one of the few oils that was not prohibited during Lent.
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.  Sunflowers also produce latex and are the subject of experiments to improve their suitability as an alternative crop for producing hypoallergenic rubber.
For farmers growing other crops, the sunflower is considered a weed.  The wild variety will grow unwanted in corn and soybean fields and can have a negative impact on yields.

# Mathematical model
A model for the pattern of florets in the head of a sunflower was proposed by H Vogel. This is expressed in polar coordinates
where  is the angle, r is the radius or distance from the center, and n is the index number of the floret and c is a constant scaling factor. It is a form of Fermat's spiral. The angle 137.5 is related to the golden ratio and gives a close packing of florets. This model has been used to produce computer graphics representations of sunflowers. 

# Size
Sunflowers most commonly grow to heights between 2.5 and 3.5 m (8 - 12'). 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 Ontario, Canada.

# Cultural usage
- The sunflower is the state flower of the U.S. state of Kansas, and one of the city flowers of Kitakyushu, Japan.
- The sunflower is often used as a symbol of green ideology, much as the red rose is a symbol of socialism or social democracy.  The sunflower is also the symbol of the Vegan Society.

# Other species
- 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.

# Flower formation
- 1. The first stage of the flower formation
1. The first stage of the flower formation
- 2. The flower is still covered, but faces the sun
2. The flower is still covered, but faces the sun
- 3. The flower is nearly completely exposed
3. The flower is nearly completely exposed
- 4. The flower is completely exposed
4. The flower is completely exposed

# Gallery
- Sunflowers in Wilkesboro, North Carolina
Sunflowers in Wilkesboro, North Carolina
- Sunflowers in Manila, Philippines
Sunflowers in Manila, Philippines
- Bumble bee sampling Sunflower nectar
Bumble bee sampling Sunflower nectar
- Sunflowers growing near Fargo, North Dakota
Sunflowers growing near Fargo, North Dakota
- Lone sunflower about 2 m (6 ft, 6') tall
Lone sunflower about 2 m (6 ft, 6') tall
- Sunflower seedlings, just three days after germination
Sunflower seedlings, just three days after germination
- Large Russian Sunflower
Large Russian Sunflower
- Fruiting head
Fruiting head
- Sunflower seeds in many variations and sizes.
Sunflower seeds in many variations and sizes.
- Red sunflowers.
Red sunflowers.
- Small Sunflower
Small Sunflower
- Sunflower Macro
Sunflower Macro
- Sunflower farm, near Behshahr, Iran
Sunflower farm, near Behshahr, Iran
- Extreme close-up of sunflower head in Istanbul, Turkey
Extreme close-up of sunflower head in Istanbul, Turkey