Freitag, 31. August 2012

Plant of the Day (August 31st, 2012) - Helianthus annuus L.

After I've presented Chicory; today's Asteraceae is from the Sub-family of the Asteroideae. This “Plant of the Day” is also one of the most popular Asteraceae in the World. I'm speaking of Helianthus annuus L.: the sunflower (“Sonnenblume” in German).

H. annuus - habitus

The sunflower is a large, annual plant, which can reach heights between 1 and 2,5 metres (3.3 to 8.2 feet). The stalk grows upright and is covered with many bristles; giving the stalk a rough surface. The leaves are arranged in an alternate pattern. They are heart-shaped and have a long petiole.

The name “sunflower” comes from the ability of the species to move its leaves and young buds after the sun from east to west. This ability is called Heliotropism and is made possible by special cells: the so called motor cells.

H. annuus - inflorescence

The inflorescence is a large, terminal heads. Unlike C. intybus, this inflorescence consists of two different types of flowers: tubular disc flowers in the centre and ray flowers in the periphery. Only the disc flowers are fertile. But they are very small and inconspicuous with brown, scale-like petals. The ray flowers can have many colours but the most common is the typical golden yellow of the sunflowers.

H. annuus - leaves & stalk

The ripe fruits are small, pyramidal nuts; the so called Archenes. They are used to make sunflower oil or as grain. They contain unsaturated fatty acids and many vitamins like Vitamin E, Vitamin B or Vitamin A.

The wild sunflower is native to North and Middle America and was domesticated over 2500 years by the Incas and the indigenous people of America. In the 16th century, the sunflower was brought to Europe by the Spanish Conquistador Pizarro as ornamental plant. The species was also very popular in Old Russia. The domesticated sunflower needs fresh, nutrient-rich soils. Sometimes, some individuals starts to grow wild.

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