The plant has an erect rough-hairy stem, reaching typical heights of 3 metres. The tallest sunflower on record achieved 9.17 metres. Sunflower leaves are broad, coarsely toothed, rough and mostly alternate. What is often called the `flower` of the sunflower is actually a `flower head` or pseudanthium of numerous small individual five-petaled flowers `florets`. The outer flowers, which resemble petals, are called ray flowers. Each `petal` consists of a ligule composed of fused petals of an asymmetrical ray flower. They are sexually sterile and may be yellow, red, orange, or other colors. The flowers in the center of the head are called disk flowers. These mature into fruit sunflower `seeds`. The disk flowers are arranged spirally. Generally, each floret is oriented toward the next by approximately the golden angle, 137.5ð, 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; however, in a very large sunflower head there could be 89 in one direction and 144 in the other. This pattern produces the most efficient packing of seeds mathematically possible within the flower head.
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