MadSci Network: Botany
Query:

Re: Xylem vs. phloem transport directions

Date: Sun Jan 4 15:13:01 2004
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1073146280.Bt
Message:

The driving force for transpiration is the evaporation of water from the 
leaves or aboveground stems in leafless plants such as cacti. Therefore, water 
movement in xylem is nearly always one direction, from roots to shoots. Rarely 
it may be the reverse. If the soil is extremely dry water might move out of 
the roots into the soil. However, the leaf cuticle and closed stomata when 
plants are water-deficient make it virtually impossible for significant 
amounts of water to move into leaves from the air, even when it is raining.

The driving force in phloem movement is controlled by the plant via loading 
sugars into the phloem and creating a "pressure-flow." Loading the sugars 
causes osmosis of water into the phloem which creates pressure and pushes the 
phloem contents along. The plant can load sugars in leaves for transport to 
roots or developing shoots, flowers or fruits, which can be shoot-to-root or 
shoot-to-flower or shoot-to-fruit. The plant can also load sugars in 
underground storage roots or underground storage stems for transport to 
shoots. Thus, phloem movement can be in any direction.

For more details, search google.com for the pressure-flow hypothesis on phloem 
translocation or the cohesion-tension theory for water movement in the xylem.

References


Re: I need information on the circulatory (vascular) system of plants


Pressure-flow Hypothesis


Cohesion Tension Theory



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