Texas has the slippery, shifting feel of a dream. Things change and change consequently from bayous and forests to prairies to bare, windswept plains. One of the state's big cities-Austin, Dallas, Fort Worth, Houston, San Antonio could conceivably fill a vacation by itself. The countryside offers so much variety-the wildflowers alone are worth a drive and the huge blue sky is best appreciated from more open spaces. At some point, it will dawn on you that the trees have disappeared, but you won't be able to say exactly when. Driving will likely be part of your visit to Texas. Unless you're planning to confine yourself to one place, you'll be covering some territory possibly a lot of territory and you'll likely be covering it in a car.