Sap

Sap is a fluid transported in xylem cells (vessel elements) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.

Sap is not to be confused with latex, resin or cell sap; it is a separate substance, separately produced, and with different components and functions.

Saps may be broadly divided into two types: xylem sap and phloem sap.

Xylem sap (pronounced /ˈzaɪləm/) consists primarily of a watery solution of hormones, mineral elements and other nutrients. Transport of sap in xylem is characterized by movement from the roots toward the leaves.

Over the past century, there has been some controversy regarding the mechanism of xylem sap transport; today, most plant scientists agree that the cohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.

Xylem sap transport can be disrupted by cavitation—an "abrupt phase change [of water] from liquid to vapor"—resulting in air-filled xylem conduits. In addition to being a fundamental physical limit on tree height, two environmental stresses can disrupt xylem transport by cavitation: "increasingly negative xylem pressures associated with water stress, and freeze-thaw cycles in temperate climates.

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