This article addresses the physical chemical processes underlying biological self-organization by which a homogeneous solution of reacting chemicals spontaneously self-organizes. Theoreticians have predicted that self-organization can arise from a coupling of reactive processes with molecular diffusion. In addition, the presence of an external field such as gravity, at a critical moment early in the process may determine the morphology that subsequently develops. The formation, in vitro, of microtubules, a constituent of the cellular skeleton, shows this type of behavior. Preparations spontaneously self-organize by reaction−diffusion, and the morphology that develops depends on the presence of gravity at a critical bifurcation time early in the process. Numerical simulations of a population of microtubules involving only reactive and diffusive terms reproduce this behavior. Microtubules can grow from one end while shrinking from the other. The shrinking end leaves behind itself a chemical trail of high tu...