harebell vs mare's-tail

harebell

noun
  • A perennial flowering plant, Campanula rotundifolia, native to the Northern Hemisphere, with blue, bell-like flowers. 

mare's-tail

noun
  • Erigeron canadensis (syn. Conyza canadensis), an annual weed 

  • especially, common mare's tail (Hippuris vulgaris) 

  • Equisetum, a fern ally 

  • A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. 

  • Any aquatic plant of the genus Hippuris, having narrow leaves in whorls. 

How often have the words harebell and mare's-tail occurred in a corpus of books? (source: Google Ngram Viewer )