A type of plant cell occurring within the phloem. Sieve elements combine to form a series of tubes (sieve tubes) connecting the leaves, shoots, and roots in a fine network. Food materials are transported from one element to another via perforations termed sieve areas or sieve plates (see mass flow). Sieve elements contain little cytoplasm and no nucleus, but are supported in their metabolic activities by adjacent companion cells in angiosperms and by albuminous cells in gymnosperms. See also P-protein.