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Mass flow, also known as mass transfer and bulk flow, is the movement of material matter. In physics, mass flow occurs in open systems and is often measured as occurring when moving across a certain boundary characterized by its cross-sectional area and a flow rate. In engineering and biology it may also be a flow of fluids in a tube or vessel of a certain diameter. A bulk transfer of particles of matter in a characterised type of flow is also known as bulk flow.
Examples include blood circulation and transport of water in xylem vessels and phloem tubes of plants. Transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via hydrogen bonding. If an air bubble forms the flow will be stopped as the column is broken and the pressure difference in the vessel cannot be transmitted; this is called an embolism. Once these embolisms are nucleated, the remaining water in the capillaries begins to turn to water vapor. Plants have physiological mechanisms to reestablish the capillary action within their cells. The "snapping" can be heard, and this sound can be used to measure the rate of cavitation within a plant.
In general, bulk flow in plant biology typically refers to the movement of water from the soil up through the plant to the leaf tissue. This happens in response to a pressure gradient, which allows water to flow from areas of higher pressure (such as the soil) to lower pressure (in the plant tissues). Another example of mass flow is the long distance transport of solutes, such as sucrose, in the phloem. This process is driven by a difference in hydrostatic pressure created from the unloading of solutes in the sink tissues. The solutes will move from high concentration(source) to low concentration(sink) along this pressure gradient.
- Pockman, W.T., Sperry, J.S., & O'Leary, J.W. 1995. Sustained and significant negative water pressure in xylem. 'Nature' 378: 715-716
- Taiz, Lincoln; Zeiger, Eduardo; Moller, Ian Max; Murphy, Angus (2015). Plant Physiology and Development. Sunderland, MA: Sinauer Associates, Inc. p. 66. ISBN 978-1605353531.
- Lambers, Hans (2008). Plant Physiological Ecology. 233 Spring Street, New York, NY: Springer Science+Business Media, LLC. p. 153. ISBN 978-0-387-78341-3.