Symmetry (biology)
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of
symmetry, either radial symmetry or bilateral symmetry. A small minority exhibit no symmetry .
In
nature and
biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.
Encyclopedia
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of
symmetry, either
radial symmetry or
bilateral symmetry. A small minority exhibit no symmetry .
In
nature and
biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.
Radial symmetry
Organisms with radial symmetry have body parts arranged in a regular, repeating pattern around a central axis or are completely symmetrical about a central axis .
These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom only.
Animals
Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for sessile animals such as the
sea anemone, floating animals such as
jellyfish, and slow moving organisms such as
sea stars . Animals in the phyla
cnidaria and
echinodermata exhibit radial symmetry.
Plants
Many
flowers, such as
dandelions and
daffodils, are radially symmetric. Roughly identical
petals,
sepals, and
stamen form at regular intervals around the center of the flower.
Special forms of radial symmetry
Tetramerism
Many jellyfish have four radial canals and thus exhibit tetramerous radial symmetry
Pentamerism
This variant of radial symmetry arranges roughly equal parts around a central axis at orientations of 72° apart.
Members of the phyla
echinodermata arrange parts around the axis of the mouth in five equal sectors. The
radiolarians demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma .
Flowering plants demonstrate symmetry of five more frequently than any other form.
Around 1510–1516 A.D.,
Leonardo da Vinci determined that in many plants a sixth leaf stands above the first. This arrangement later became known as 2/5
phyllotaxy, a system where repetitions of five leaves occur in two turns of the axis. This is the most common of all patterns of leaf arrangement.
Bilateral symmetry
In bilateral symmetry , only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves . Thus there is approximate
reflection symmetry. Often the two halves can meaningfully be referred to as the right and left halves, e.g. in the case of an animal with a main direction of motion in the plane of symmetry.
Animals
Most animals are bilaterally symmetric, including humans , and belong to the group
Bilateria. The oldest known bilateral animal is the
Vernanimalcula.
Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to
cephalization, and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates.
Plants
Flowers such as
orchids and sweet peas are bilaterally symmetrical. The leaves of most plants are also bilaterally symmetrical.
Asymmetry
The notable exception among animals are the
Porifera which have no symmetry.
See also
References
- Heads, Michael. "Principia Botanica: Croizat's Contribution to Botany." Tuatara 27.1 : 26-48.
- a website by the Monaco educational service
External links
- article called "Symmetry in Nature: Fundamental Fact or Human Bias?" By Ker Than
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