The apoplastic tracers Cellufluor and lanthanum nitrate were used at the light and electron microscope levels, respectively, to determine whether an apoplastic pathway exists for solutes from the water column into leaf veins of the seagrass Thalassodendron ciliatum (Forssk.) Den Hartog. Tracers penetrated walls of epidermal and mesophyll cells of leaves within 4 h of exposure, but entered the apoplast of veins only 12–24 h later, their path being impeded by barriers in the walls of bundle sheath cells around the veins. The passage of lanthanum was blocked by the compound middle lamella between contiguous sheath cells and impeded by suberin lamellae in all bundle sheath walls, except in pit fields. In these regions, suberin lamellae appear to be microporous. Cellufluor probably crossed bundle sheath walls via the same pathway. The results suggest that bundle sheath cells may slow down the entry of certain solutes similar to these tracers into the veins.