BOSTON, MA - Elisabeth Blair MacDougall, 78, an art historian who directed landscape architecture studies at the Dumbarton Oaks research center from 1972 to 1988 and helped to transform the study of gardens into an academic discipline, died of pneumonia Oct. 12 at Massachusetts General Hospital in Boston. She lived in Cambridge, Mass. Dr. MacDougall, who had taught at Boston and Harvard universities, analyzed the use of color and design in 16th- and 17th-century French and Italian gardens in much the same way art historians study other artistic productions of the period, colleagues said. John Dixon Hunt, who succeeded her as director of landscape architecture studies at Dumbarton Oaks, said, ?EUR??,,????'??She put garden history on the map. . . . Garden history has become popular because it requires one to be familiar with a variety of subjects, including art history, architectural history, literary history, geography, material culture and the history of ideas." Her publications include "The Villa Mattei and the Development of the Roman Garden Style," "The French Formal Garden" and "Fountains, Statues, and Flowers: Studies in Italian gardens of the Sixteenth and Seventeenth Centuries."