frond vs rachis

frond

noun
  • The leaf of a fern, especially a compound leaf. 

  • Any fern-like leaf or other object resembling a fern leaf. 

rachis

noun
  • The main shaft of either a compound leaf, head of grain, or fern frond. 

  • An anatomical shaft or axis in a marine invertebrate. 

  • The central shaft of a feather. 

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