Hadron wave functions and pion decay constant

Low-$Q^2$ form factors of hadrons are described in terms of their constituents (valons). Their behaviors at low $Q^2$ are contrasted with those at high $Q^2$, and the inappropriateness of extrapolating from one region to the other is emphasized. On the basis of the valon model the wave functions of hadrons are determined from the data on form factors. The result is used to calculate the ratio $\frac{F_{\pi }\left(Q^2\right)}{F_p\left(Q^2\right)}$ at intermediate values of $Q^2$, and agreement with data is found for $Q^2<10\mathrm{}$ Ge${\mathrm{V}}^2$. The pion decay constant is then calculated with the help of the pion wave function so determined; the result $f_{\pi }=102\mathrm{}$ MeV agrees with data to within 10%.