One of my favorite botanical artists is Ernst Haeckel:
One type of botanical art that I feel really drawn to is of a more fantastical variety:

And these are from an amazing book: Codex Seraphinius, which seems to have been inspired by the Voynich Manuscript:





The Codex Seraphinius is basically an encyclopedia of an imaginary world written in an imaginary language that nobody has deciphered, (similar to the Voynich Manuscript, except the Codex Seraphinius might just be nonsense)
I also really like the imagnary botanical art of the creator of the imaginary world of Ilion:








