Phloem: the long and the short of it

Trends Plant Sci. 2006 Jan;11(1):26-32. doi: 10.1016/j.tplants.2005.11.009. Epub 2005 Dec 13.


The structure of phloem sieve-element-companion-cell complexes reflects a duality of function: to conduct photoassimilates throughout the plant, and to exchange solutes between the phloem and surrounding tissues. The conceptual integration of these long- and short-distance functions requires the abandonment of a long-cherished concept in phloem physicochemistry, that source-sink turgor differentials control flow. The manifest inability of decentralized organisms such as plants to control phloem translocation centrally disqualifies such differentials as control variables; besides, the phloem is maximally efficient if the pressure differentials are small. Testing this hypothesis and whether turgor differentials are small will require a significant recommitment to studying the quantitative anatomy of phloem.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Biological Transport
  • Plant Stems / anatomy & histology
  • Plant Stems / metabolism
  • Plants / anatomy & histology*
  • Plants / metabolism*
  • Pressure