Sunflower
2008/9 Schools Wikipedia Selection. Related subjects: Plants
| Sunflower | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 |
||||||||||||||
| Scientific classification | ||||||||||||||
|
||||||||||||||
| Binomial name | ||||||||||||||
| Helianthus annuus L. |
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 florets (small flowers) crowded together. The outer florets are the sterile ray florets and can be yellow, maroon, orange, or other colors. The florets inside the circular head are called disc florets, which mature into what are traditionally called " sunflower seeds," but are actually the fruit (an achene) of the plant. The inedible husk is the wall of the fruit and the true seed lies within the kernel.
The florets within the sunflower's cluster are arranged in a spiraling pattern. 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 follow 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 colour.
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
The sunflower is native to the Americas. Current research shows that it may have been domesticated twice, first in Mexico and later in the middle Mississippi Valley. Alternatively, it may have been introduced northward from Mexico at an early date as corn (maize) had been. The evidence thus far is that the sunflower was first domesticated in Mexico by at least 2600 BC. The earliest known examples of a fully domesticated sunflower north of Mexico have been found in Tennessee and date back to around 2300 B.C. Many indigenous American peoples used the sunflower as the symbol of the sun deity, including the Aztecs and the Otomi of Mexico and the Incas in South America. Gold images of the flower, as well as seeds, were taken back to Spain early in the 16th century. Some researchers argue that the Spaniards tried to suppress cultivation of the sunflower because of its association with solar religion and warfare.
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. In Germany, it is used together with rye flour to make Sonnenblumenkernbrot (literally: sunflower whole seed bread), which is quite popular in German-speaking 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 margarine as well as and 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.
Traditionally, several Native American groups planted sunflowers on the north edges of their gardens as a "fourth sister" to the better known three sisters combination of corn, beans, and squash. Annual species are often planted for their allelopathic properties.
However, for commercial farmers growing commodity crops, the sunflower, like any other unwanted plant, is often considered a weed. Especially in the midwestern USA, wild (perennial) species are often found 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 θ is the angle, r is the radius or distance from the centre, 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 (26') at other times and places (e.g. Madrid). Much more recent feats (past score years) of over 8 m 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.
- Subject of Van Gogh's most famous still life, Sunflowers (series of paintings)
Varieties
- American Giant Hybrid
- Arikara
- Autumn Beauty
- Aztec Sun
- Black Oil
- Dwarf Sunspot
- Evening Sun
- Giant Primrose
- Indian Blanket Hybrid
- Irish Eyes
- Italian White
- Kong Hybrid
- Large Grey Stripe
- Lemon Queen
- Mammoth Sunflower
- Mongolian Giant
- Orange Sun
- Red Sun
- Ring of Fire
- Rostov
- Soraya
- Sunny Hybrid
- Taiyo
- Tarahumara
- Teady Bear
- Titan
- Valentine
- Velvet Queen
Other species
- The Maximillian sunflower (Helianthus maximillianii) is one of 38 species of perennial sunflower native to North America. The Land Institute and other breeding programs are currently exploring the potential for these as a perennial seed crop
- The Jerusalem artichoke (Helianthus tuberosa) is related to the sunflower, another example of perennial sunflower.
- The Mexican sunflower is Tithonia rotundifolia. It is only very distantly related to North American sunflowers.
- False sunflower refers to plants of the genus Heliopsis.