Ratio of the average of hydrostatic α-elements O and Mg to the average of explosive α-elements Si, Ca, and Ti (HEx ratio) for our sample of GCs. In the lower panel, the ratio is computed using only stars of the unpolluted P component in each GC (see text). The blue and red symbols show accreted and in situ GCs, respectively, and the empty grey circles show field stars from Gratton et al. (2003). The green square shows field stars in the core of Sgr dSph. A typical error is shown in both panels (average from all GCs).