New developments in computer science, biology, mathematics and physics offer possibilities to obtain deeper understanding of growth and forms of organisms. It is now possible to carry out simulation experiments in which the growth process can be simulated in virtual computer objects. In this book, methods from fractal geometry are applied to model growth forms. As a case study, a type of growth process is used which can be found among various taxonomic classes of organisms such as sponges and corals. The growth of these organisms is simulated with 2D and 3D geometrical objects. The models presented in the book provide a rendering method for natural objects which is based on the actual growth process. The models can be used, for example, to understand the amazing variety of forms to be found in a coral reef. Models which mimic the growth of forms and the environmental influence on the growth process are also useful for ecologists. A combination of simulation models and the actual growth forms can be used to detect the effects of slow changes in the environment.

rowth and form of marine organisms inhabiting hard substrata, the G"marine sessile organisms," ischaracterized by anumber ofremarkable properties. One remarkable feature of these organisms is that many ofthem can be characterizedasmodularorganisms. Modularorganisms are typically built ofrepeated units, the modules, which might be a polyp in a coral colony or afrond in seaweeds. In most cases, the modulehas adistinctive form, while the growth form of the entire colony is frequently an indeterminate form. Indeterminategrowthindicatesthatthe same growthprocess mayresult in an infinite numberofdifferentrealizations ofthe growthform.This isincontrast to unitaryorganisms such asvertebrates and insects, in which a single-celled stage develops into a well-defined, determinate structure. In many cases the growth process in modular organisms leads to complex shapes, which are often quite difficult to describe in words. In most of the biological literature these forms are only described in qualitativeand rather vague terms, such as "thinlybranching,""tree-shaped" and "irregularlybranching." Anothermajor characteristic ofmarine sessile organisms is that there is frequentlyastrongimpactofthe physical environmenton the growthprocess, leading to a variety of growth forms. Growth by accumulation of modules allows the organism to fit its shape to its environment i.e., have plasticity. In many seaweeds, sponges, and corals, differences in exposure to water movement cause significant changes in morphology. Agood example of this plasticity is the Indo-Pacific stony coral Pocillopora damicornis(Veron and Pichon 1976) shown in Plg.r.i. In very sheltered environments, this species has a thin-branching growth form. The growth form gradually transforms to a more compact shape when the exposure to water movement increases.