In The "Flat-Iron", the provocative correspondence Stieglitz recorded between the foreground tree’s forking branches and the building’s distinctive triangular shape sets up a contrast between natural and man-made, but the tree—unlike the Flatiron Building—reaches up to extend beyond the picture frame. That, along with its darker tone, makes it as least as assertive as the building, with its angularity and energy. In his identification with the tree, Stieglitz made photographic compositions in which trees hold their own and even prevail over their urban settings. By photographing strong vital trees at a time when those along Manhattan streets were deemed either expendable or in need of preservation, Stieglitz identified with nature as a bulwark against the disruptions urbanism. 
In The "Flat-Iron", Stieglitz takes full advantage of photogravure’s capacity to convey atmosphere. Here the bow of a ship is ominously cutting through a foggy haze pushing industry and urbanization forward while taking out any forms of nature in its path.
Beaton, Cecil, and Gail Buckland. The Magic Image: The Genius of Photography. London: Pavilion, 1989. p. 100
Greenough, Sarah, and Alfred Stieglitz. Alfred Stieglitz: The Key Set : the Alfred Stieglitz Collection of Photographs. Washington, D.C: National Gallery of Art, 2002. Pl 287
Homer, William I. Alfred Stieglitz and the American Avant-Guarde. Boston: New York Graphic Society, 1979. no. 9.
Johnston, Patricia A. Seeing High & Low: Representing Social Conflict in American Visual Culture. Berkeley: University of California Press, 2006. fig 11.8
Mauro, Alessandra. Photoshow: Landmark Exhibitions That Defined the History of Photography. , 2014. p. 115
Stieglitz, Alfred, Richard Whelan, and Sarah Greenough. Stieglitz on Photography: His Selected Essays and Notes. New York, NY: Aperture Foundation, 2000. p. 112
Stieglitz, Alfred, and Dorothy Norman. Alfred Stieglitz. New York: Aperture, 1976. p. 29.
Thornton, Gene. Masters of the Camera: Stieglitz, Steichen & Their Successors. New York: Ridge Press, 1976 p. 49
Whelan, Richard. Alfred Stieglitz: A Biography. , 1997. pp. 310-311
Witkin, , London, and Shestack. The Photograph Collector’s Guide. London: Secker & Warburg, 1979. p. 104
 Lukitsh, Joanne. "Alone on the Sidewalks of New York: Alfred Stieglitz’s Photography, 1892 – 1913." Seeing High Et Low / Ed. by Patricia Johnston. (2006): 210-227