Within source leaves, partition of assimilated carbon and export are regulated to both leaf and plant ontogeny and to environmental conditions. System goals, adaptive responses, control mechanisms, and information flow are developed from the viewpoint that a well-adapted plant is an integrated system. Regulation of partitioning and export involves both feedforward control and feedback homeostasis. Export is controlled by regulation of metabolism which supplies assimilated material to be translocated, by control of efflux into the free space of material destined for export, and by regulation of phloem loading. Control of export by the latter two processes depends on responsive metabolic processes that can supply sucrose and other transport molecules upon demand. An integrated model incorporating these features posits that control of export by mechanisms located in the source leaves is an important means of regulating translocation.