Smith, Gilbert M. (1937). A Textbook of General Botany (Third ed.). New York: The Macmillan Company. p. 80. Available on line in Biodiversity Heritage Library
Fuller, Harry J.; Carothers, Zane B. (1963). The Plant World (Fourth ed.). New York: Holt, Rinehart and Winston, Inc. p. 107. LCCN 63-10190.
loc.gov
lccn.loc.gov
Fuller, Harry J.; Carothers, Zane B. (1963). The Plant World (Fourth ed.). New York: Holt, Rinehart and Winston, Inc. p. 107. LCCN 63-10190.