This article summarizes the self-organising behaviour of in vitro microtubule preparations and the manner that it is triggered and affected by weak external factors, in particular, gravity. In these preparations, self-organisation also leads to the development of other, higher level, phenomena such as the collective transport and positioning of any colloidal or sub-cellular particles present. Self-organisation results not from static interactions but occurs by way of the chemical reactions involved in the formation and maintenance of microtubules from tubulin and guanosine triphosphate (GTP). An essential feature of these experiments is that the system is extremely simple; being initially comprised of only two reacting species, purified tubulin and GTP. No other biological agents, such as molecular motors, nucleating centres or associated proteins, are present. Both experiments and numerical simulations indicate that self-organisation arises from the reactive growth and shrinking of microtubules. We postulate that individual microtubules are strongly coupled to their neighbours via the chemical trails they produce by their reactive growing and shrinking and which causes the whole microtubule population to behave as a

bio-complexity, swarm intelligence, reaction-diffusion, microtubules, weightlessness