Figure 5A.3 Wood-Jones model for laminar growth of corals. The coral Acropora hyacinthus forms a flat, circular plate in the mature form. It grows by lateral addition of material to the plate, which eventually closes in on itself and forms a circular platform. [After Dauget (1991)]

nation of growth and reproduction between polyps is not necessary.

The last model is known to botanists as Aubreville's model, but coral biologists prefer to call it the Wood-Jones model. The three models discussed so far deal with growth in only one dimension, along the axis of a corallite. The Wood-Jones model considers growth in two axes, lateral and axial (Fig. 5A.3). In botany, Aubreville's model is frequently used to model the growth of leaves rather than stems and branches—a leaf can be thought of as a stem with a much higher rate of lateral growth than axial growth. The Wood-Jones model applies to corals that grow as massive flat plates, whose main extension is along the edges of the plate rather than along the vertical axis of a corallite column.

example, the ripples on a river bed or on a wind-blown sand dune are so regularly spaced (or for that matter why sand dunes themselves are regularly spaced). If a surface is growing by diffusion-limited accretion, the concentration gradients that feed accretion will be steepened the most at this characteristic distance, and growth at this location will be favored. Again, energy flow through the positive feedback loop driving accretion growth is modulated by the physical properties of the environment.

So I hope you are convinced that the process that generates bioconvection cells is fundamentally similar to that which drives accretive growth of sponges and corals. Both are modulated positive feedback systems. There are differences, of course, in what flows through the loops—in one it is gravitational energy driving convective flow, and in the other it is metabolic energy driving mineral transport and deposition. But at root, they are the same process.

If the processes that generate bioconvection cells and the growth forms of sponges and corals are similar, do the two structures that result similarly do physiological work? Again, I think the case is largely made. In the case of bioconvection cells, the work done driving the currents clearly does physiological work, transporting oxygen and carbon dioxide vertically through the culture at rates far faster than diffusion could carry them. Physiological work is also evident—admit-tedly, in a different way—in the growing branches of sponges and corals. Growth requires energy, and part of the physiological work that organisms must do to power growth is to capture energy. Most animals do this by moving their bodies, a la Willie Sutton, to where the food is. The process entails a host of physiological functions: locomotion, sensing where the food is, and taking appropriate actions to get there. Sessile animals like sponges and corals cannot move from one place to another, but if they cannot go to where the food is they at least can grow to where the food is. Thus, growth must be directed in a way that mimics locomotion. In the case of corals and sponges, growth toward food is ensured by the interaction between the

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