Synthesized answer
The provided passages do not contain information about what questions remain unanswered in "Essai sur la géographie des plantes" [1]. The passages only state the title of the work, its authors, and its categories and page count [1].
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Essai sur la géographie des plantes by Alexander von Humboldt, Aimé Bonpland Categories: Botany Pages: 168