thumb|upright|Sap droplets of [[Dracaena trifasciata]]
Sap is a fluid transported in the xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.
Sap is distinct from latex, resin, or cell sap; it is a separate substance, separately produced, and with different components and functions.
Insect honeydew is called sap, particularly when it falls from trees, but is only the remains of eaten sap and other plant parts.
Types of sap
Saps may be broadly divided into two types: xylem sap and phloem sap.
Xylem sap
Xylem sap (pronounced ) 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"
Phloem sap
Phloem sap (pronounced ) consists primarily of sugars (mainly sucrose), hormones, and mineral elements dissolved in water. It flows from where carbohydrates are produced or stored (sugar source) to where they are used (sugar sinks). The pressure flow hypothesis proposes a mechanism for phloem sap transport, although other hypotheses have been proposed.
thumb|right|[[Leafhoppers feeding on sap, attended by ants]]
Many insects of the order Hemiptera (the half-wings), feed directly on phloem sap, and make it the primary component of their diet. Phloem sap is "nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, [yet] it is consumed as the dominant or sole diet by a very restricted range of animals".
A much larger set of animals do however consume phloem sap by proxy, either "through feeding on the honeydew of phloem-feeding hemipterans. Honeydew is physiologically less extreme than phloem sap, with a higher essential/non-essential amino acid ratio and lower osmotic pressure," The sap often is harvested from the sugar maple, Acer saccharum.
In some countries (e.g., Lithuania, Latvia, Estonia, Finland, Belarus, Russia) 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.
Certain palm tree sap can be used to make palm syrup. In the Canary Islands they use the Canary Island date palm while in Chile they use the Chilean wine palm to make their syrup called miel de palma.
See also
- Guttation
- Latex
- Resin