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Symmetry

10 11 sections And skeletons internal symmetries of plants and animals The great majority of plants express radial symmetry in one form or another. In fact the great divide between the kingdoms of plants and animals is reflected in their dominant symmetries. Because plants are usually fixed and nonmotile they tend to be radial, whereas the majority of animals move of their own volition and as a result are bilateral, or, more accurately, dorsiventral see pg. 18 . The trunks and branches of trees usually indicate a radial arrangement in transverse crosssection, and the same is true of roots and vertical stems in general 1. Most regular actinomorphic flowers have a radial symmetry, as do many inflorescences 2. Placentation, too, is invariably arranged on a symmetrical plan below. Mushrooms, mosses and the tubular leaves of rushes also adopt this symmetry. Sessile animals, i.e. those which are attached and unable to move under their own power, usually have a plantlike, radial symmetry. The predominant number of these are marine creatures, such as sea anemones and seaurchins 3. Starfish and star corals are likewise centrestructured. The jewellike skeletons of the marine Protozoa which include the Radiolaria and Foraminifera, which are found in such profusion in the seas that they account for up to 30 of ocean sediments, also tend to adopt radial symmetries in their body form 4. 1. The trunks, branches and roots of trees show radial symmetry in crosssection 22 2. 3.
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