duckweed vs spume

duckweed

noun
  • Any of several reduced floating aquatic plants in the subfamily Lemnoideae of the family Araceae. 

spume

noun
  • Foam or froth of liquid, particularly that of seawater. 

verb
  • To froth. 

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