Pith rays are natural wood effects, they constitute 5–10 % of wood. Because of their cells, which are oriented vertically to the annual circles, nutrients can get from phloem to wood and pith. They also have a storage function – they store energetically rich substances like starch and fat. Their abundance in the grain guarantees that it is a solid wood, not an imitation or plywood. The widest ones, well visible with naked eye, emerge for example in beech and especially in oak. When the cut is made radially in an angle, which goes exactly through the longitudinal axis of the trunk, pith rays form typical glossy surfaces that are colloquially called little mirrors.