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