<P> Photosynthetic CO assimilation (A) can be calculated from </P> <P> A = (C a − C i) g / 1.6 P (\ displaystyle A = (C_ (a) - C_ (i)) g / 1.6 P) </P> <P> where C and C are the atmospheric and sub-stomatal partial pressures of CO, respectively . The rate of evaporation from a leaf can be determined using a photosynthesis system . These scientific instruments measure the amount of water vapour leaving the leaf and the vapor pressure of the ambient air . Photosynthetic systems may calculate water use efficiency (A / E), g, intrinsic water use efficiency (A / g), and C. These scientific instruments are commonly used by plant physiologists to measure CO uptake and thus measure photosynthetic rate . </P> <P> There is little evidence of the evolution of stomata in the fossil record, but they had appeared in land plants by the middle of the Silurian period . They may have evolved by the modification of conceptacles from plants' alga - like ancestors . However, the evolution of stomata must have happened at the same time as the waxy cuticle was evolving--these two traits together constituted a major advantage for early terrestrial plants . </P>

Explain the process of opening and closing of stomata with figure