<P> The metabolic functioning of sieve - tube members depends on a close association with the companion cells, a specialized form of parenchyma cell . All of the cellular functions of a sieve - tube element are carried out by the (much smaller) companion cell, a typical nucleate plant cell except the companion cell usually has a larger number of ribosomes and mitochondria . The dense cytoplasm of a companion cell is connected to the sieve - tube element by plasmodesmata . The common sidewall shared by a sieve tube element and a companion cell has large numbers of plasmodesmata . </P> <P> There are two types of companion cells . </P> <Ol> <Li> Ordinary companion cells, which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube . </Li> <Li> Transfer cells, which have much - folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer . They are specialized in scavenging solutes from those in the cell walls that are actively pumped requiring energy . </Li> </Ol> <Li> Ordinary companion cells, which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube . </Li>

The process in which xylem and phloem tubes are used is called