maiden's honesty vs virgin's bower

maiden's honesty

noun
  • A plant of the species Clematis vitalba, evergreen clematis, a vine, native to the chalk and limestone soils of Britain, often considered invasive and noxious elsewhere. 

virgin's bower

noun
  • Any of various plants of the genus Clematis, including Clematis virginiana and Clematis viticella. 

How often have the words maiden's honesty and virgin's bower occurred in a corpus of books? (source: Google Ngram Viewer )