The Yew tree is a unique critter. The outer or "back" sapwood, is approximately flat, follows the natural growth rings. The inner side or "belly" of consists of heartwood. The heartwood resists compression and the outer sapwood performs better in tension. This combination in a single piece of wood (a self bow) forms a natural "laminate", somewhat similar in effect to the construction of a laminated longbow.
Most bowyers today grind away the sapwood leaving only the heartwood to make their glass laminated bows. Thus the glass performs the duties of the sapwood. The glue or epoxy adds strength to the heartwood. The very thin, straight grain of the yew makes it the ideal wood for a longbow. A true English longbow, with properly cured wood can take up to 4 years to produce. But it is well worth the wait.