Semple, J.C., S.B. Heard, and ChunSheng Xiang. 1996. The asters of Ontario (Compositae: Astereae): Diplactis Raf., Oclemena E.L. Greene, Doellingeria Nees and Aster L. (including Canadanthus Nesom, Symphyotrichum Nees, and Virgulus Raf.). Univ. of Waterloo Biology Series 38. Google Scholar