  We show that the $P_u(\om) \propto \om^{-7/3}$ shear velocity power spectrum
gives rise to a $P_\Theta (\om ) \propto \om^{-4/3}$ power spectrum for a
passively advected scalar, as measured in experiment [K. Sreenivasan, Proc. R.
Soc. London A {\bf 434}, 165 (1991)]. Applying our argument to high Rayleigh
number Rayleigh Benard flow, we can account for the measured scaling exponents
equally well as the Bolgiano Obukhov theory (BO59). Yet, of the two
explanations, only the shear approach might be able to explain why no classical
scaling range is seen in between the shear (or BO59) range and the viscous
subrange of the experimental temperature spectrum [I. Procaccia {\it et al.},
Phys. Rev. A {\bf 44}, 8091 (1991)].
