Tubular structures like blood vessels or lungs have such a geometrical complexity that they cannot be genetically coded at the level of individual branches. It is obvious that simple rules must exist that lead to the spontaneous generation of very complex structures by the simplelaws of physics. Recent progres in this field sheds light on the way blood vessels and lungs self-organize. Especially, the slow visco-plastic flow of tissue plays an important role in local rearrangements which allow the entire tree to gently span the organ to be perfused (or the lung to branch). However, physics laws have to play with genetic inuductions and expressions, and vice-versa. Recent studies show that in fact, genetic expressions may be completely slaved to stress fields. As a consequence, one can write a constitutive equation for tissue growth which is mechanically self-consistent, and contains only about 2 parameters. This shows that actually, nothing is otimized as such, however, the tissue follows a slow creeping flow which leads to morphogenesis, the final structure being just "the way it is". The morphogenesis is anyway arrested when oxygen or nutrients supplies are insufficient, so that the final animal, if feasible at all, is somewhat functional anyway.
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