A petal is one member or part of the corolla of a flower. It is the inner part of the perianth that comprises the sterile parts of a flower and consists of inner and outer tepals. These tepals are usually differentiated into petals and sepals. The term "tepal" is usually applied when the petals and sepals are similar in shape and color. In a "typical" flower the petals are showy and colored and surround the reproductive parts. The number of petals in a flower is indicative of the plant's classification: dicots having typically four or five petals and monocots having three, or some multiple of three, petals.
Flower of the Primrose Willowherb (Ludwigia octovalvis) showing petals and sepals
There exists considerable variation in form of petals among the flowering plants. The petals can be united towards the base, forming a floral tube. In some flowers, the entire perianth forms a cup (called a calyx tube) surrounding the gynoecium, with the sepals, petals, and stamens attached to the rim of the cup.
The flowers of some species lack or have very much reduced petals. These are often referred to as apetalous. Examples of flowers with much reduced perianths are found among the grasses.
The petals are usually the most conspicuous parts of a flower, and the petal whorl or corolla may be either radially or bilaterally symmetrical. 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 of zygomorphic flowers may be seen in orchids and members of the pea family.
Apetalous Chickweed was first identified in Illinois in 1985, and it has since established itself in several NE counties.
Apetalous Chickweed is locally common in lawns along sidewalks and roadsides, sometimes forming sizable colonies.
Apetalous Chickweed is similar to Stellaria meadia (Common Chickweed) in having ovate leaves, stems with lines of hairs, and a variable number of stamens in each flower.
A new apetalous B. napus mutant was identified in a breeding nursery at AAFC-Saskatoon in 1996.
The apetalous characteristic was strongly expressed at the early and full bloom stages, but late developing flowers had petals that were reduced in size and number.
Seventeen completely apetalous plants were found that had normal plant vigour and developed normal sized leaves and self seed was produced on eight of these plants in the field.
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