Encouraged by John M. Macfarlane, the head of the botany department, Pennell wrote his doctoral thesis on the Scrophulariaceae (as then circumscribed), a group on which he was to become a world authority.
He also wrote many other works including The evolution and distribution of flowering plants (Apocynaceae, Asclepiadaceae), The evolution and distribution of fishes, Fishes the source of petroleum, and The quantity and sources of our petroleum supplies.
John F. Kennedy | Pope John Paul II | Elton John | John | John Lennon | John Wayne | John McCain | John Kerry | John Cage | Olivia Newton-John | John Williams | John Peel | John Adams | John Steinbeck | John Travolta | John Milton | John Zorn | John Marshall | John Howard | John Singer Sargent | John Ruskin | John Updike | John Maynard Keynes | John Coltrane | John Cleese | St. John's | John Waters | John Lee Hooker | John Huston | John Ford |