"he gave them a few leaves from a little fern-plant growing on the wall which he knew they were very fond of"

   Asplenium ruta muraria is a species of fern native to Europe where it commonly grows on aging masonry, such as the wall in Delboeuf’s dream. 

Aspelnium ruta-muraria, Choceň, Czech Republic, (2007)
Creative Commons Attribution Share AlikeAspelnium ruta-muraria, Choceň, Czech Republic, (2007) - Credit: Petr Filippov