Josh Heupel broke spring practice down into thirds for Tennessee freshman quarterback Jake Merklinger. Fifteen practices in total for the rookie to first get his feet wet, then adjust to Tennessee’s uptempo offense, then make progress before the Orange & White Game to end spring drills. Merklinger hit his marks along the way. “Just his growth,” Heupel said Saturday during his press conference the split-squad scrimmage at Neyland Stadium, “fundamentally understanding what we’re doing, defensive structures and being a good decision-maker, just continuing to grow throughout the course of spring b…