plumule vs rachis

plumule

noun
  • The aftershaft of a feather. 

  • One of the featherlike scales of certain male butterflies. 

  • The first bud, or growing point, of a plant embryo, situated above the cotyledons. 

  • A down feather. 

rachis

noun
  • The central shaft of a feather. 

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

  • An anatomical shaft or axis in a marine invertebrate. 

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