Pressure Flow Hypothesis-
Water in the xylem moves out laterally, by osmosis, to surrounding vascular, ground and epidermal tissues. The phloem, due to solute concentrations, draws water strongest near where the sucrose is transported into the vesicle?s sieve cells.
A sugar concentration gradient is the main means of producing the flow of sap in the phloem. The sucrose concentration is greatest at the source and least at the point of storage or the point of use by the plant. Thus the greatest sucrose concentration will pull the most water in by osmosis generating a pressure in the direction towards the sucrose sink where the carbohydrates are stored or used so this is the point of lowest sucrose concentration.
The pressure-flow hypothesis- sap moves down the concentration gradient pushed by the water drawn in by osmotic pressure at the top of the gradient.
The phloem draws water where the sucrose concentration is high but where the sucrose is at a low concentration the phloem's water moves back to the xylem. This model implies some water actually circulates in the plant.
http://en.wikipedia.org/wiki/Pressure_flow_hypothesishttp://www.biologie.uni-hamburg.de/b-online/e28/28d.htmhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Phloem.html
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