Electron.microscope studies have been made of the cells of the phloem and pericyclic ground parenchyma of the stem of Cucurbita pepo (Duchesne). The mature sieve element contains no cytoplasm, either on the side walls or at the sieve plate, such as occurs in other cells. The walls are lined with a parietal layer which encloses an aqueous solution in which are dispersed fibrils of slime.