gemma vs plumule

gemma

noun
  • An asexual reproductive structure, as found in animals such as hydra (genus Hydra) and plants such as liverworts (division Marchantiophyta), consisting of a cluster of cells from which new individuals can develop; a bud. 

plumule

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

  • One of the featherlike scales of certain male butterflies. 

  • The aftershaft of a feather. 

  • A down feather. 

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