In contrast to linear polymers, linear-low-density polyethylenes (with more than similar to 10 branches per 1000 C atoms) crystallizing from linear nuclei to form row structures do not crystallize uniformly. Instead they pass through a series of morphological changes, accompanied by a continuously declining growth rate, because of the accumulation of more branched molecular sequences segregated at the growth front and the consequent progressive decrease in isothermal superfooling. Different concentrations of segregants, reflecting the different interfacial geometries, give spherulites-with their more open construction-a faster growth rate than rows and render the rows more susceptible to morphological instability. Nevertheless, cellulation does occur in spherulites of a metallocene polymer with 28 ethyl branches per 1000 C atoms. Systematic differences in morphology and relative growth rate between metallocene and Ziegler-Natta-catalyzed polymers are attributed to less mobile segregants for the former. There is no evidence for segregation in row structures of commercial linear polyethylenes except for tiny proportions elf possibly shorter molecules in pockets of subsidiary lamellae.