PHYSICS OF $Q^2$-DEPENDENCE IN THE NUCLEON'S $G_1(x,Q^2)$ STRUCTURE FUNCTION SUM RULE

I discuss in this talk the physics of the $Q^2$ dependence of the $G_1(x,Q^2)$ structure function sum rule. For $Q^2>3$ GeV$^2$, the $Q^2$ variation is controlled by pure QCD radiative corrections. For $0.5<Q^2<3$ GeV$^2$, the twist-four contribution becomes significant, but stays perturbative. For $Q^2$ below $\sim 0.05$, the sum rule is determined by low-energy theorems. The rapid change of the sum rule between 0.05 and 0.5 GeV$^2$ signals the transition between parton and hadron degrees of freedom.