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Water and Nutrient Transport (Xylem, Phloem)

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Figures (10)

Xylem (blue) transports water and minerals from the roots upwards.
Diagrammatic structure of xylem cells
A diagram showing the setup of a pressure bomb
Patterns of xylem development: xylem in brown; arrows show direction of development from protoxylem to metaxylem.
Cross-section of a flax plant stem: PithProtoxylemMetaxylemPhloemSclerenchyma (bast fibre)CortexEpidermis
Cross section of some phloem cells
Simplified phloem and companion cells: XylemPhloemCambiumPithCompanion cells
The process of translocation within the phloem
Photos showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips
Stripping the inner bark from a pine branch

Key Points

  • Xylem elements, except for the parenchyma, are rich in lignin and are main components of wood.
  • Phloem is also used as a popular site for oviposition and breeding of insects belonging to the order Diptera, including the fruit fly Drosophila montana.
  • When transpiration removes water at the top, the flow is needed to return to the equilibrium.
  • The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants.
  • Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes—water and minerals.
  • In most cases, the lower epidermis contains more stomata than the upper epidermis because the bottom of the leaf is cooler and transpiration there is safer.

Terms

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