On the knots... Look for how wood is sawn.  A knot is the base of a limb (or the dormant bud of one), with a conical cross-section, therefore they have a specific relationship to the grain pattern.  Which means they will show differently in wood that is flat sawn versus quarter-sawn.  But as a quick-and-dirty, a wider spacing between the grains means that part of the board is closer to flat to the normal of the tree, thus knots will be circular; a tighter spacing means you are moving towards perpendicular, and encountering the knots longitudinally instead of in smallest cross-section.

Anyhow...starting point for research there....