This 168 piece Biogeochemical Cycle bundle features:● Carbon : CO2 cycle, and carbon molecule● Oxygen : O2 cycle, basic photosynthesis, oxygen molecule● Nitrogen : N2 cycle (terrestrial and fish tank), nitrogen fixing bacteria in legume root nodes, nitrogen molecule● Water : H2O cycle, evaporation, watershed, water moleculeYou save 20% when purchasing the bundle instead of each set individually.The bundle includes 85 PNG color illustrations, and 79 PNG blackline versions of the same illustra