plumule vs seedling

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. 

seedling

noun
  • A young plant grown from seed. 

  • a tree smaller than a sapling. 

  • one grown in a nursery for transplanting. 

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