Uniaxially oriented polypropylene (PP) is molten in the synchrotron beam and crystallized from the quiescent melt, keeping its orientation in order to study the mechanisms of its crystallization. We document the different nanostructures observed as a function of melt-annealing temperature, undercooling, and time. In order to obtain a melt that crystallizes with high preferential orientation again, a melt-annealing temperature between 170 and 176 degrees C is chosen. Isothermal crystallization at 155 degrees C results in slow formation of (primary) lamellae placed at random. As the crystallization temperature is decreased (150, 145, and 140 degrees C), more and more secondary crystallites are observed which develop from a block mesostructure according to Strobl's mechanism. During the isothermal phase the blocks are fusing more or less to form imperfect lamellae. The structure evolution observed in the time-resolved small-angle X-ray scattering (SAXS) data during crystallization and remelting facilitates discrimination between the block structure and another frequently discussed morphology, Keller's cross-hatched structure. While after all our quiescent crystallization experiments most of the crystallites are blocks or incompletely fused lamellae, the hard-elastic precursor material which has been made under extreme gradients of temperature and pressure exhibits the melting of homogeneous and extended lamellae. As we apply a steep temperature gradient (-100 degrees C/min) to our melt in a nonisothermal crystallization experiment, we initially observe the formation of homogeneous and extended lamellae as well.