Reduce oven temperature to 325°F. In a medium glass bowl, beat the egg whites and cream of tartar with electric mixer on high speed until soft peaks form. Slowly beat in remaining ¼ cups sugar, 1 tablespoon at a time, until stiff peaks form and mixture is glossy. Spoon egg white mixture onto pie filling. Spread over filling, carefully sealing meringue to edge of crust to prevent shrinking.