Figure 5A.1 Schoute's model for arborescent growth of corals. a: A Schoute tree results when branching occurs at the tips of a growing branch. b: A representative Schoute tree: growth of the coral Lobophyllia corymbosa. [After Dauget (1991)]
of trees, each named after the botanist who formulated it, and these seem to explain a considerable part of the variation of growth among corals.
The simplest is known as Corner's model and is characterized by simple linear growth at the tip. In a coral, the growing tip is the surface of the corallite, and the resulting structure is simply a column. Variation of shape among corals growing by Corner's model occurs when polyps detach from one corallite and set up another growth axis somewhere else. Corals growing by Corner's model typically exhibit little in the way of translocation of materials from one polyp to another.
Two of the growth models involve different rules for branching of a corallite as it grows. One, Schoute's model, relies on simple bifurcation of a growing tip (Fig. 5A.1). If a single polyp, after growing a corallite for some time, buds to produce two polyps, each of which starts to grow its own corallite, a bifurcated branch is formed. The result of repeated cycles of growth and budding is a ramifying structure. Schoute's model also applies to groups of polyps that form a sheet of growth. Fission of these groups into multiple zones of growth also results in ramifying structures, whose shape depends upon the ratio of the rates of longitudinal growth and budding (Fig. 5A.1).
The other branching model, known as Attim's model, initiates branching not at the tips but at the sides (Fig. 5A.2). This common mode of growth is made possible by the fact that coral polyps frequently form a sheet of living tissue that covers a columnar corallite. At some point, a lateral polyp might initiate budding, at which point a branch of growth will begin to extend laterally from the column. Repeated cycles of longitudinal growth and lateral budding will also produce, as in Schoute's model, an arborescent structure, but the interesting difference is in the kind of control needed to achieve the same end. Attim's model, because it relies on manipulating the growth and reproduction of the lateral polyps, involves some sort of mechanism that selectively represses or stimulates the growth of certain polyps. The same may occur in Schoute's model, but control of this type is not necessary, since a Schoute tree can result from simple growth and budding at the tip. As noted in the text, coordi-
Figure 5A.2 Attim's model for arborescent growth of corals. a: An Attim tree results when branching occurs along the length of a growing branch. b: A representative Attim tree: growth of thecoral Acropora formosa. [After Dauget (1991)]
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