We use a novel dye-tracing technique to measure in vivo phloem transport velocity in cucumber (Cucumis sativus) plants during early seedling development. We focus on seedlings because of their importance in plant establishment and because they provide a simple source and sink model of phloem transport. The dye-tracing method uses a photodiode to track the movement of a bleach front of fluorescent dye traveling in the phloem from the cotyledons (source) to the roots (sink). During early seedling development, phloem transport velocity in this direction can change 2-fold depending on vascular connectivity and the number of actively growing sinks. Prior to leaf expansion, vascular bundles attached to the first developing leaf demonstrate a decline in basipetal phloem transport that can be alleviated by the leaf's removal. At this stage, seedlings appear carbon limited and phloem transport velocity is correlated with cotyledon area, a pattern that is apparent both during cotyledon expansion and after source area manipulation. When the first leaf transitions to a carbon source, seedling growth rate increases and basipetal phloem transport velocity becomes more stable. Because bundles appear to operate autonomously, transport velocity can differ among vascular bundles. Together, these results demonstrate the dynamic and heterogeneous nature of phloem transport and underline the need for a better understanding of how changes in phloem physiology impact growth and allocation at this critical stage of development.