En plein air (French for 'outdoors') is the act of painting outside, directly from nature, rather than finishing work in a studio from sketches or memory. Its theory is credited to Pierre-Henri de Valenciennes, who argued in an 1800 treatise that landscape should be painted on the spot so that weather and light are caught as they change. Portable easels and the collapsible paint tube introduced by John G. Rand in 1841 made the practice practical, and the Barbizon painters of the 1830s proved its results. Monet, Renoir, Sisley and Bazille turned it into a radical program in the 1860s; parallel schools - the Macchiaioli in Tuscany, the Newlyn School in England, the Heidelberg School in Australia, the Group of Seven in Canada - adopted it worldwide. Painting outdoors from observation remains popular into the 21st century.