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Zygomorphic ("yoke shaped", "bilateral" – from the Greek ζυγόν, zygon, yoke, and μορφή, morphe, shape) flowers can be divided by only a single plane into two mirror-image halves, much like a yoke or a person's face. Examples are orchids and the flowers of most members of the Lamiales (e.g., Scrophulariaceae and Gesneriaceae).
Radial symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as starfish; whereas bilateral symmetry favours locomotion by generating a streamlined body. Many flowers are also radially symmetric, or "actinomorphic".
The flowers called actinomorphics, radiate or polysymmetric have radial symmetry, as is the case of Tulipa gesneriana or Linum usitatissimum . In contrast, monosymmetrica l, dorsiventral or zygomorphic flowers have bilateral symmetry and the evolution of their shape is related to the need to attract and guide pollinators to them, as for example ...
A normal (left) and a peloric Streptocarpus showing the bilateral to radial symmetry alteration.. Pelorism has been of interest since a five-spurred variety of the common toad-flax (Linaria vulgaris L.) was first discovered in 1742 by a young Uppsala botanist on an island in the Stockholm archipelago and then in 1744 described by Carl Linnaeus.
Animals mainly have bilateral or mirror symmetry, as do the leaves of plants and some flowers such as orchids. [30] Plants often have radial or rotational symmetry, as do many flowers and some groups of animals such as sea anemones. Fivefold symmetry is found in the echinoderms, the group that includes starfish, sea urchins, and sea lilies. [31]
This is an example of radial symmetry. When flowers are bisected and produce only one line that produces symmetrical halves, the flower is said to be irregular or zygomorphic. If, in rare cases, they have no symmetry at all they are called asymmetric. [23] [24]
Flowering plants underwent a major diversification after this period. Darwin saw this as an "abominable mystery" in a letter to Joseph Hooker in 1879. The earliest flowers were principally actinomorphic or having radial symmetry with multiple axes of symmetry. These evolved flowers have bilateral symmetry or zygomorphy.
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. In many plants of the aster family such as the sunflower, Helianthus annuus, the circumference of the flower head ...