We have studied melting of poly(butylene succinate), isothermally crystallized over a wide temperature range, employing a combination of the Hoffman Weeks plot and the Gibbs-Thomson crystallization line, determined by small-angle X-ray scattering measurements. A change in the slope alpha of the Hoffman-Weeks (H-W) line, accompanied by a change of the slope of the crystallization line, was observed for crystallization temperatures higher than 110 degrees C. a was reaching a value of 1, implying that no intersection point between the H-W line and the T-m = T-c line could be obtained. (T-m is the measured melting temperature and T-c is the temperature at which the sample was crystallized). This observation was corroborated by the crystallization line, which was found to be parallel to the melting line for T-c > 110 degrees C. We relate these changes in slope to different stabilization mechanisms of the secondary nuclei at the growth front of polymer lamellar crystals. For T-c> 110 degrees C, secondary nuclei are proposed to be stabilized by coalescence of neighboring nuclei, all having a small width. By contrast, for T-c > 110 degrees C, the number density of secondary nuclei is low and thus their coalescence is rare. Accordingly, nuclei are stabilized by growing in size, mainly increasing their width.