Have you ever wondered how a tree supports itself as it reaches towards an opening in the canopy? Or how a long limb holds itself horizontal? The answer is reaction wood. Reaction wood is a special kind of wood that forms on the top or bottom of a limb or leaning trunk. In general conifers “push” and hardwoods “pull”. This means that in gymnosperms (conifers) this specialized wood forms on the underside of the limb or trunk and is called compression wood. In angiosperms (hardwoods) it is formed on the top of limbs and branches and is called tension wood. Reaction wood has different properties than normal growth, being more brittle, denser, prone to cracking, etc. depending on whether it’s a conifer or hardwood.   Source: Trees: Their Natural History. Thomas, Peter