Pavel Novotny

The sunflower (Helianthus annuus) is an annual plant in the family Asteraceae and native to the Americas, with a large flowering head (inflorescence). The stem can grow as high as 3 meters (10 ft), and the flower head can reach 30 cm (11.8 in) 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.

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, 137.5°, 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 one direction and 55 in the other; on a very large sunflower there could be 89 in one direction and 144 in the other.