arbor vs understory

arbor

noun
  • A grove of trees. 

  • A shady sitting place or pergola usually in a park or garden, surrounded by climbing shrubs, vines or other vegetation. 

  • A spindle of a wheel. 

  • A bar for supporting cutting tools. 

  • An axis or shaft supporting a rotating part on a lathe. 

understory

noun
  • The layer of plants that grow in the shade of the canopy of a forest. 

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