brushwood vs fern

brushwood

noun
  • Small trees and shrubs. 

  • Branches and twigs fallen from trees and shrubs. 

fern

noun
  • Any of a group of some twenty thousand species of vascular plants classified in the division Pteridophyta that lack seeds and reproduce by shedding spores to initiate an alternation of generations. 

How often have the words brushwood and fern occurred in a corpus of books? (source: Google Ngram Viewer )