Out of the three categories, coherent structures typically arise from instabilities in laminar or turbulent states. After an initial triggering, their growth is determined by evolutionary changes due to non-linear interactions with other coherent structures, or their decay onto incoherent turbulent structures. Observed rapid changes lead to the belief that there must be a regenerative cycle that takes place during decay. For example, after a structure decays, the result may be that the flow is now turbulent and becomes susceptible to a new instability determined by the new flow state, leading to a new coherent structure being formed. It is also possible that structures do not decay and instead distort by splitting into substructures or interacting with other coherent structures.