<P> Sieve tube elements and companion cells are connected through plasmodesmata . Structurally, the walls of sieve tubes tend to be dispersed with plasmodesmata grouped together and it is these areas of the tube walls and plasmodesmata that develop into sieve plates over time . Sieve tubes tend to be found largely in angiosperms . They are very long and have horizontal end walls containing sieve plates . Sieve plates contain sieve pores which can regulate the size of the openings in the plates with changes in the surroundings of the plants . </P> <P> Formed through the vertical connections between multiple sieve tube members, sieve tubes are directly responsible for the transport through the minimum resistance surrounding their walls . By having the assistance of these pores that constitute a majority of the structure of sieve plates, the diameter of the sieve tubes can be regulated . This regulation is necessary for the sieve tubes to respond to changes in the environment and conditions within the organism . </P> <P> Sieve cells are long, conducting cells in the phloem that do not form sieve tubes . The major difference between sieve cells and sieve tube members is the lack of sieve plates in sieve cells . They have a very narrow diameter and tend to be longer in length than sieve tube elements as they are generally associated with albuminous cells . They have long, unspecialized areas with ends that overlap with those of other sieve cells . Sieve cells are also associated with gymnosperms because they lack the companion cell and sieve member complexes that angiosperms have . Sieve cells are very uniform and have an even distribution across of sieve areas . Their narrow pores are necessary in their function in most seedless vascular plants and gymnosperms which lack these sieve - tube members . </P> <P> Just as Sieve tube elements have companion cells which form complexes and function together, sieve cells have albuminous cells that aid them in functioning . These albuminous cells work between phloem and parenchyma . They connect parenchyma with mature sieve cells to help participate in transport of cells . There can be many of these albuminous cells that belong to one sieve cell, depending on the function of the tissue or organ . </P>

Difference between sieve tube element and xylem vessel