Most wood movement will be in the width of a solid board. You would ideally want to glue two boards together, so that the grain is in the same direction. With changes in moisture, both boards (the laminate) will expand and contract in a similar manner. If you were to glue boards together, where the grain is in opposing directions, the expanding and contraction of the wood will be in opposite directions, and that will lead to checking/cracking of the top and bottom surfaces.
Plywood gets its strength and stability by alternating layers of veneer and having the grain running perpendicular to each layer, and the glue between layers is a brute force method of keeping the material more stable.
I hope this answers your question.
-- Nicky
