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The task of reviewing some of the ways development and differentiation may act as determinants of economic yield would be much simpler had knowledge advanced to a point where the basic principles concerned in the control of these processes in eukaryotic organisms were clear. Unfortunately, this stage has not yet been reached. There is no shortage of schemes and hypotheses to set beside a mountain of observational and experimental data, but the unifying thread which might allow us to pick out the significant and reject the irrelevant in any particular context is still lacking. What is incontestable is that development and differentiation are manifestations of gene function, so the fundamental problem can at least be defined: it is to understand how gene action is governed in ontogeny so as to give orderly expression to the potentialities attained during the evolutionary history of a species, producing an organism that is harmoniously coordinated both within itself and with the environment. I will begin by considering some general aspects of this problem as it applies to higher plants.