This member of the Asteraceae, or Compositae, family of flowering plants has a composite head that is supported by a series of modified leaves called the involucre. The head on the most common species of sunflower, Helianthus annuus, generally grows up to 35 centimeters in diameter and is comprised of two types of florets. Upon close examination, the large, dark disk at the center of the sunflower head is not a single appendage, but is an area comprised of many tiny, tightly-packed tubular flowers collectively called the disk flower. This type of floret takes the form of petals set in actinomorphic arrangement, being radially symmetric and generally containing pistils and stamens. The second type of floret — surrounding the disk flower — is called a ray flower. These yellow, petal-like flowers exhibit an elongated petal shape, are characterized by bilateral symmetry, and are generally sterile. Along the sunflower's tall and hairy stem are oval, coarsely-toothed leaves that are arranged in spirals.