 He boldly proposed that, in analogy to the sulfur bacteria's forming elemental sulfur from H2S (hydrogen sulfide), plants would form oxygen from H2O (water).In 1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO2.This discovery was made by Robin Hill, and subsequently the light-driven release of oxygen in the absence of CO2 was called the "Hill reaction".