Artist Andrew Wyeth, who portrayed the hidden melancholy of the people and landscapes of Pennsylvania's Brandywine Valley and coastal Maine in works such as "Christina's World," died early Friday. He was 91. Wyeth died in his sleep at his home in the Philadelphia suburb of Chadds Ford, according to Hillary Holland, a spokeswoman for the Brandywine River Museum. The son of famed painter and book illustrator N.C. Wyeth, Andrew Wyeth gained wealth, acclaim and tremendous popularity. But he chafed under criticism from some experts who regarded him as a facile realist, not an artist but merely an illustrator. "The world has lost one of the greatest artists of all time," George A. Weymouth, a friend of Wyeth's who is chairman of the board of the Brandywine Conservancy, said in a statement.