The model of multiple control points is also compatible with the fact that seeds vary in their degree of dormancy as a result of the environmental conditions after dispersal, cycling through dormancy and nondormancy. Secondary dormancy may be induced by a number of different metabolic alterations, such as the induction of ABA synthesis by drought or cold (although Bewley, 1997, discards the role of ABA in secondary dormancy), or the degree of primary dormancy may decrease, as after-ripening progresses, due to decreases in ABA concentration or increases in GA or cytokinin concentration