What do you do after you beat Whitney in Pokemon Crystal?

Make sure you went up to Route 36 and talked to Floria (the girl by the weird tree). Then go back to Goldenrod City and go into the building to the right of the Gym. You'll get a squirtbottle. Go squirt the tree, which is actually a Sudowoodo, and then continue up to the northern part of Route 36 and then to Ecruteak City.