Phloem function is closely related to sieve tube structure. Since we encounter many problems to investigate sieve tubes without inducing artifacts, the in vivo structure of sieve tubes is still a matter of debate and so is the translocation mechanism. There is no doubt that translocation occurs as mass flow (click the link below to see a movie of phloem translocation), but the force that drives the flow is still a matter of debate. The currently most widely accepted hypothesis is the “pressure flow hypothesis” outlined by Ernst Munch (Mu[umlaut]nch) in his book “Die Stoffbewegung in der Pflanze” (1930 [in german]). The hypothesis includes the loading of assimilates generated by photosynthesis (mainly sugars) into the phloem in source tissues (tissues where more assimilates are generated than consumed, usually mature leaves). This loading process increases the number of osmotically active substances in the sieve tubes of sources. The osmotically active substances attract water which flows into the sieve tubes and increases the internal hydorstatic pressure. In sink tissues (tissues where assimilates are consumed such as roots, fruits etc.) the assimilates are taken out of the phloem which reduces the number of osmotically active substances. This results in a loss of water and hydrostatic pressure. The resulting hydrostatic pressure differential between source and sink, causes the sieve tube sap to flow through the tube system.
The “pressure flow hypothesis” is currently the most plausible explanation for phloem flow. However, it is imporatnt to know that major parameters, such as the pressure differential between source and sink have actually never been measured due to difficulties to access the phloem tissue with probes. For a detailed discussion on phloem translocation click here, or the link to : Knoblauch and Peters, Plant Cell and Environment (2010) in the literature list.