Asa Gray (1810-1888) is considered the most important American botanist of the 19th century. He was instrumental in unifying the taxonomic knowledge of the plants of North America. Of Gray's many works on botany, the most popular was his "Manual of the Botany of the Northern United States, from New England to Wisconsin and South to Ohio and Pennsylvania Inclusive". Known more simply as "Gray's Manual", it has gone through a number of editions with botanical illustrations by Isaac Sprague, and remains a standard in the field.