Sunflowers (Helianthus annuus) are annual plants native to the Americas, that possess a large inflorescence (flowering head).
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 seeds
The florets within the sunflower's cluster are arranged in a spiral 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. This pattern produces the most efficient packing of seeds within the flower head.
Source : http://en.wikipedia.org/wiki/Sunflower
See Also : Flowers, Florist, Florists