MadSci Network: Botany |
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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