Passage II

Urushiols are the oils in poison ivy that cause allergic reactions in humans. The higher the U: S ratio－the ratio of unsaturated (U) urushiols to saturated (S) urushiols—the more severe the reaction.

From 1999 to 2004, poison ivy plants (PIPs) were grown in 2 identical outdoor plots under identical conditions except for the atmospheric CO₂ concentration.

Figure 1 shows, for each plot, the yearly dry biomass per plant. Figure 2 shows the yearly percent of original PIPs surviving in each plot. Figure 3 shows, for each plot in 2004, the percent of U urushiols per plant, the percent of S urushiols per plant, and the U: S ratio per plant.

Figures adapted from "Biomass and Toxicity Responses of Poison Ivy (Toxicodendron radicans) to Elevated Atmospheric CO₂." @2006 by the National Academy of Sciences.