"Consider wood is a bunch of fibers going, generally, in the same direction. While properly prepared edges may appear to be smooth, microscopically there is a lot of "hills and valleys" with the fibers. It is this fibrous consistency of the wood that allows a strong glue joint to form a bond in the fibers, not on the fibers. That also explains why a glue joint involving end grain is weak without a mechanical fastener, ie, biscuit, dowel, m/t. The glue only had the very tip of the wood fiber to attach to the mating surface."
This is the stuff that amazes me. I NEVER, before I started reading, have imagined or considered such minutia...detail...(for me, science) goes into all this work. I'll never forget asking what the difference is between laquer thinner, mineral spirits and turpentine. Nestor gave me a six-page paper on the chemical composition and distillation!! I'll never ask that question again! Even though I didn't understand most of it