MEDULLARY RAYS Notice the horizontal white lines on this excellent piece of Englemann Spruce.  They are medullary rays and because they are perfectly horizontal they indicate that this ukulele soundboard is perfectly quartersoawn.  Medullary rays distribute water and minerals from the center of the tree to the outer bark.  In some woods they are quite prominent and add beauty and interest to the finished instrument.  This lovely Spruce was perfectly cut at Pacific Rim Tonewoods. [...]
http://www.griffinukuleles.com/blog/medullary-rays