cohesion-tension hypothesis

cohesion-tension hypothesis

a hypothesis that explains the ascent of water from roots to leaves in a plant as due to a combination of upward pull created by TRANSPIRATION losses producing a tension on the xylem vessels and cohesion of water molecules to each other, aided by the adhesion of water molecules to the sides of the narrow vessels. Evidence for the hypothesis comes from:
  1. the long rise in mercury in a column attached to a leafy shoot as compared with mercury rise in a vacuum;
  2. the decrease in tree trunk diameter during the day (due possibly to tension) when transpiration is at its highest level. see DENDROMETER.
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