The sporangiophore of the fungus Phycomyces is able to avoid obstacles placed a few millimeters from its growing zone. The work described in this thesis presents evidence that the avoidance response is mediated by gases. Avoidance occurs in still air in the diffusion limit, even at relative humidities close to 100%. It is shown that the effect of the surfaces of the obstacles cannot only be to reflect these gases: the surfaces must play a more active role. Models in which the surfaces adsorb growth-inhibitors or adsorb an inert precursor that is re-emitted as a growth-promoter that decays are in agreement with our experimental observations.
