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Sap is a fluid transported in xylem cells (tracheids or vessel elements) or phloem sieve tube elements of a plant. The xylem cells transport water and nutrients throughout the plant. Plant sap is not to be confused with latex or resin; it is a separate substance, separately produced, and with different components and functions.
Xylem sap 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.
Phloem sap consists primarily of water, with sugars, hormones, and mineral elements dissolved in it. It flows from where carbohydrates are produced or stored to where they are used. The pressure flow hypothesis proposes a mechanism for phloem sap transport.
In some countries (e.g., Russia, Latvia, Estonia or Finland) harvesting the early spring sap of birch trees (so called "birch juice") for human consumption is common practice; the sap can be used fresh or fermented and contains xylitol.
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