To test theories of the twisting or correlated orientation of crystallites along the radial arms of sperulites, we measured the radial growth rates (G) and the extinction spacings (s) of a series of molecular weight fractions of linear polyethylene. At equivalent degrees of supercooling the radial growth rate is faster for the lower molecular weight fractions. Comparison with earlier measurements shows that the critical free energy for the formation of a growth nucleus is essentially the same for a single crystal growing from dilute solution as for a spherulite crystallizing from the melt. The data are not consistent with the hypothesis thatsis directly proportional to the parameter &dgr;=D/G, but indicate an additional factor that has the temperature dependence of a nucleation process.