Tribulus terestris vs caltrop

Tribulus terestris

caltrop

noun
  • A small, metal object with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to pedestrians, horses, and vehicles. 

  • The starthistle, Centaurea calcitrapa, a plant with sharp thorns. 

  • The same object represented as a heraldic charge. 

  • Any of a number of flowering plants in the family Zygophyllaceae, including several members of the genus Kallstroemia and the species Tribulus terrestris, native to warm temperate and tropical regions. 

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