Abstract
Reduction-diffusion theories can account for both morphogenesis and the sensitivity of biological systems to weak fields. They predict that gravity can cause the symmetry breaking that is necessary for pattern formation. Microtubules play an important role in organizing the cell, and recent studies hae shown that they can form in vitro dissipative structures. We have found that these structures show patterns of microtubular orientation that are gravity dependent and that the gravitational field causes symmetry breaking. This behavior, which cannot be explained by convection, is in accordance with the theory of dissipative structures. These results suggest that microtubular dissipative structures may play an important role both in morphogenesis and in accounting for the sensitivity of biological systems to weak fields. They aso provide another explanation for biological gravitropism.