Symbolism, Imagery, Allegory
Freud's Dream of the Botanical Monograph is a short and sweet little ditty that goes a little something like this:
I had written a monograph on a certain plant. The book lay before me and I was at the moment turning over a folded coloured plate. Bound up in each copy there was a dried specimen of the plant, as though it had been taken from a herbarium. (5.2.7)
Freud's interpretation of this dream is complex, and he returns to it multiple times throughout the book. The most important symbolic significance that he teases out of it relates to the meaning of the "certain plant" that he studies in the dream.
Because Freud "really had written something in the nature of a monograph on a plant" (5.2.10), the monograph in the dream reminds him of his work on the coca-plant. That's cocaine, ladies and gents. So, the "certain plant" in the dream becomes a symbol of Freud's work on the medicinal properties of cocaine—as well as a symbol of his mixed feelings about that work.
Freud viewed his work on the coca-plant with both positive and negative associations: positive, because he prided himself on having made important contributions to anesthesiology; and negative, because his recommended use of cocaine as a painkiller led to the death of his friend and colleague Ernst Fleischl von Marxow. With this in mind, the symbolic significance of the "certain plant" in the dream doesn't just relate to the coca-plant itself, but to a whole slew of Freud's professional ambitions and anxieties as well.