bur vs burl

bur

noun
  • A rough, prickly husk around the seeds or fruit of some plants. 

  • Any of several plants having such husks. 

  • A rotary cutting implement having a selection of variously shaped heads. 

burl

noun
  • A tree growth in which the grain has grown in a deformed manner; a burr knot. 

  • Wood of a mottled veneer, usually cut from such a growth. 

  • A knot or lump in thread or cloth. 

verb
  • To remove the knots in cloth. 

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