Photoassimilates translocate from regions of carbohydrate synthensis(source) to regions of carbohydrate utilization or storage(sink). In the source, assimilate loads into the phloem for long-distance transport. Current evidence suggests that there are twig loading mechanisms : one involves assimilate transfer via the apoplasm and then load into the phloem by carrier-mediated proton-sucrose cotransport, while the other involves movement through the continuous symplastic connections between the mesophyll cells and the phloem. Inspite of problems associated with the interpretation of experiments, the evidence for apoplastic loading remains convincing because the apoplastic loading systems explains well the observed accumulation capacity arid the selectivity of assimilate uptake by tile phloem.