In hardwood, reaction wood is called tension wood, which has a higher content of cellulose than normal. Not only will it not take a finish like the rest, this is what makes some wide boards turn into pretzels when ripped. The sooner you can recognize this and cull it from your projects the better.
Most of the time in rough lumber, a zone of reaction wood will be wooly where the saw didn't get as smooth a cut. After being surfaced, there is usually tear out, and a duller appearance, along with a color change.