F.F. Blackman, a British plant physiologist who discovered that photosynthesis is a two-step process (1905), only one of which uses light directly. He cultivated plants under different but controlled carbon dioxide concentrations, different light intensities and different temperatures and noted the effects of these variables on the rate of photosynthesis. Under low light intensity, photosynthesis is enhanced by increasing light, but is unaffected by increases in temperature and carbon dioxide. When light intensity is high, increases in both temperature and carbon dioxide accelerate photosynthesis. Black concluded that the initial "light" reactions are independent of temperature, while the second "dark" reactions are independent of light yet are limited by carbon dioxide and controlled by enzymes.