Flower Petals
Petals can differ dramatically in different species. The number of petals in a flower may hold clues to a plant's classification. For example, flowers oneudicots (the largest group of dicots) most frequently have four or five petals while flowers onmonocots have three or six petals, although there are many exceptions to this rule. The petal whorl or corolla may be either radially or bilaterally symmetrical (see Symmetry in biologyand Floral symmetry). If all of the petals are essentially identical in size and shape, the flower is said to be regular or actinomorphic (meaning "ray-formed"). Many flowers are symmetrical in only one plane (i.e., symmetry is bilateral) and are termed irregular or zygomorphic(meaning "yoke-" or "pair-formed"). In irregular flowers, other floral parts may be modified from the regular form, but the petals show the greatest deviation from radial symmetry. Examples ofzygomorphic flowers may be seen in orchids and members of the pea family. In many plants of the aster family such as the sunflower, Helianthus annuus, the circumference of the flower head is composed of ray florets. Each ray floret is anatomically an individual flower with a single large petal. Florets in the centre of the disc typically have no or very reduced petals.
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