Some sources for history of Botany
CALL #       QK15 .M85 1981.
AUTHOR       Morton, A. G. (Alan G.)
TITLE        History of botanical science : an account of the development of 
               botany from ancient times to the present day / A.G. Morton.
IMPRINT      London ; New York : Academic Press, 1981.

CALL #       QK21.N6 R48 1992.
AUTHOR       Reveal, James L.
TITLE        Gentle conquest : the botanical discovery of North America with 
               illustrations from the Library of Congress / James L. Reveal.
IMPRINT      Washington, D.C. : Starwood Pub., c1992.