"Given the techniques (once) available it was extraordinarily difficult to measure either the light absorbed or the oxygen evolved. It should also be borne in mind that the highest yields only occur in low light, i.e., when the changes to be measured were at their smallest. Moreover, photosynthetic organisms respire like any others. How to make an appropriate allowance for dark respiration, relatively large in relation to photosynthesis in these circumstances, could pose problems. In the long-run everything gradually happened together. Techniques improved, the Z-scheme was proposed and relentlessly authenticated, four photons (per carbon dioxide fixed) became untenable. These days the actual quantum requirement for leaves, following work by Demmig and Bjorkman, is put at about 9 and many species, provided that they are in good conditions and not stressed give values close to this figure. Why 9 and not 8? A quantum requirement of 8 (i.e. a quantum yield of 1 / 8 = 0.125 molecules of carbon dioxide or oxygen per photon) is probably the real underlying value because that is what the Z-scheme demands and the Z-scheme has become increasingly unassailable. The rest is down to measurement again. These are still very difficult to get exactly right and are biased to more (rather than to less) because it is not yet possible, in a leaf, to measure photosynthesis per se without inputs from other associated processes (e.g. sucrose synthesis). Also many sorts of stress will increase the apparent quantum requirement a little. Real stress such as photoinhibition can increase it a lot. In short, hydrogen transport from water to NADP cannot be easily measured in leaves without inadvertently measuring a few energy requiring reactions which push up the apparent quantum requirement. The consistency of the requirement for about 9 photons is also very striking. It is as if a wide range of species have gone as far as thermodynamics will permit. If this is so, there is little or no possibility of increasing quantum efficiency (the reciprocal of quantum requirement). In this regard, at least, green plants may be as efficient as it is possible."