Plants produce different types of organs at different times in shoot development. Along with the major changes in organ morphology that take place during developmental transitions, more gradual patterns of variation occur. The identity of organs produced at a particular position on the shoot is determined by interactions between several independently regulated, temporally coordinated processes. Two of these processes are organ production and the specification of organ identity. Coordination of these processes is accomplished in part by a thermal clock and by signal transduction pathways that mediate the response of plants to light.