Blackman proposed the law of limiting factors in 1905. According to this law, when a process depends on a number of factors, its rate is limited by the pace of the slowest factor. Blackman's law of limiting factors determines the rate of the photosynthesis. Suppose a leaf is exposed to a certain light intensity which can utilize 5 mg. of CO2 per hour in photosynthesis. If only 1 mg. of CO2 enters the leaf in an hour, the rate of photosynthesis is limited due to CO2 factor. But as the concentration of the CO2 increases from 1 to 5 mg./hour the rate of photosynthesis is also increased along the line AB. In this case CO2 acts as a limiting factor.