algae vs duckweed

algae

noun
  • A particular kind of algae. 

  • Algal organisms viewed collectively or as a mass; algal growth. 

duckweed

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

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