After cooling and killing by freezing in solid C02 and fixation at -2°C, the white outer ring (probably callose) was found to be absent in electron micrographs of plasmodesmata. Some plasmodesmata were partly or entirely filled with a dark plug, but others were open with a lining of protoplasm. The observation of open plasmodesmata accords with the experimental fluid flow through them described in a previous paper (1972). The plugging mechanism seems similar to that of sieve-plate pores in higher plants.