On the shore of White Rock Lake, the Dallas Arboretum is one of those city treasures that many residents still don't know about. Those who do often visit the arboretum only around Easter when the azaleas--with 2,400 varieties--are in glorious bloom, or in the fall for the magnificent mums. But on those days when the thermometer hits 95 degrees, take a picnic and head for the Palmer Fern Dell in the Jonsson Color Garden. The misters that keep the moisture-loving ferns and pond plants thriving also make the temperature plummet more than 20 degrees. Or maybe it just feels like it. Open to the public from 10 a.m. to 5 p.m. seven days a week (except Thanksgiving, Christmas Day and New Year's Day). Adults $7, seniors $6, children 3 to 12 $4. Parking $4.