containing respectively 75% and 25% of the total H band light, and found $\rm C_{31}$ in the range 1-3 for pure disk objects, and $\rm C_{31}~>~3$ in galaxies with prominent bulges. Given this result, it is important to investigate whether or not the inverse correlation between SFR and mass, reported in the previous section, follows from the anti-correlation between $\rm H\alpha$ E.W. and $\rm C_{31}$. In other words, whether or not the $\rm H\alpha$ E.W. is strong in pure disk galaxies and weak in bulge dominated galaxies, as claimed by Devereux & Young (1991). However KTC94 showed that variations of $\rm H\alpha$ E.W. do not simply reflect different contributions of bulge light to the continuum, but real variations of the disk SFR. Points in Fig. 5 a and b (and in the following figures, unless otherwise specified) are coded according to $\rm C_{31}$: filled dots are pure disks, open symbols are bulge dominated systems and crosses indicate those objects for which no structural information is available. Pure disks dominate the low $\rm L_H$ - high $\rm H\alpha$ - late-type regime. Also in Fig. 6 (a, b, c), where we plot the $\rm H\alpha$ - $\rm L_H$ relation in separate morphological 