The morphology of row structures in the alpha form of isotactic polypropylene has been investigated, with lamellar resolution, in a homopolymer and three of its fractions, by transmission electron microscopy following permanganic etching. In longitudinal view, dominant radial lamellae were not always perpendicular to the strain-induced central nucleus with which they shared a common c axis; lamellae parallel to the core increased in number at greater radial distances. The underlying reason is related to novel phenomena observed in transverse sections showing that lattice continuity is not maintained if its loss allows crystallization to occur in otherwise inaccessible regions. If there were lattice continuity, the lathlike habit of alpha-polypropylene lamellae would lead to gaps developing between neighbouring laths with increasing radial distance. In fact, such gaps are generally filled, frequently by lamellae which are edge-on, i.e. parallel to the rows, and rapidly, implying that a loss of continuity is not a significant barrier to growth. Space-filling nucleation increased as the crystallization temperature decreased, leading to finer structures at lower temperatures, but was severely inhibited by a large drop in tacticity in which case the gaps remained largely unfilled. Different mechanisms to effect this infilling have been identified with and without loss of continuity but principally incoherent nucleation; the implications for the properties of melt crystallized polymers in general are discussed.