Blackman's Law of Limiting Factors

Blackman's law of limiting factors states that when a process depends on the combined action of several separate factors, the rate of the process at any moment is governed by the pace of whichever one of those factors is the 'slowest' — meaning whichever factor is present in the lowest amount relative to what the others would allow. Taking light and CO2 as an example: suppose a plant is photosynthesising at a fixed light intensity that is only strong enough to make use of 10 mg of CO2 per hour. As CO2 concentration is increased from a low starting point, the photosynthetic rate rises along with it — but only up to the point where the fixed light intensity can no longer keep pace; beyond that point, further increases in CO2 produce no further increase in the rate, because light has now become the limiting factor, and only increasing the light intensity itself can push the rate higher again. Plotted as a graph, this produces a curve with a sharp break and a flat plateau exactly where control pas …