Sap is a fluid transported in

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

cells (vessel elements or tracheids) or

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...

sieve tube elements of a

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

. These cells transport water and nutrients throughout the plant. Sap is distinct from

latex Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latexes are found in nature, but synthetic latexes are common as well. In nature, latex is found as a milky fluid found in 10% of all flowering plants (angiosperms ...

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on n ...

, or

cell sap A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic mo ...

; 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

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...

s toward the

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

. 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 Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure

gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...

s, 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 f waterfrom 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 Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. There are two types of water scarcity: physical or economic water scarcity. Physical water scarcity is where ...

, and freeze-thaw cycles in temperate climates.

Phloem sap

Phloem sap (pronounced ) consists primarily of sugars, hormones, and mineral elements dissolved in water. It flows from where

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...

s are produced or stored (sugar source) to where they are used (sugar sinks). The

pressure flow hypothesis The pressure flow hypothesis, also known as the mass flow hypothesis, is the best-supported theory to explain the movement of sap through the phloem. It was proposed by Ernst Münch, a German plant physiologist in 1930. A high concentration of or ...

proposes a mechanism for phloem sap transport, although other hypotheses have been proposed. Phloem sap is thought to play a role in sending informational signals throughout vascular plants. According to ''

Annual Review of Plant Biology ''Annual Review of Plant Biology'' is a peer-reviewed scientific journal published by Annual Reviews. It was first published in 1950 as the ''Annual Review of Plant Physiology''. Sabeeha Merchant has been the editor since 2005, making her the lo ...

'',

Many insects of the order

Hemiptera Hemiptera (; ) is an order (biology), order of insects, commonly called true bugs, comprising over 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, Reduviidae, assassin bugs, Cimex, bed bugs, and shield bugs. ...

(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, etit is consumed as the dominant or sole diet by a very restricted range of animals". This apparent paradox is explained by the fact that phloem sap is physiologically extreme in terms of animal digestion, and it is hypothesized that few animals take direct advantage of this because they lack two adaptations that are necessary to enable direct use by animals. These include the existence of a very high ratio of non-essential/ essential amino acids in phloem sap for which these adapted Hemiptera insects contain symbiotic

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s which can then provide them with essential amino acids; and also insect "tolerance of the very high sugar content and osmotic pressure of phloem sap is promoted by their possession in the gut of sucrase- transglucosidase activity, which transforms excess ingested sugar into long-chain

oligosaccharides An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a saccharide polymer containing a small number (typically two to ten) of monosaccharides (simple suga ...

." 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," or by feeding on the biomass of insects that have grown on more direct ingestion of phloem sap.

Human uses

Maple syrup is made from reduced

sugar maple ''Acer saccharum'', the sugar maple, is a species of flowering plant in the soapberry and lychee family Sapindaceae. It is native to the hardwood forests of eastern Canada and eastern United States. Sugar maple is best known for being the prim ...

xylem sap. The sap often is harvested from the sugar maple, ''Acer saccharum''. In some countries (e.g., Lithuania, Latvia,

Estonia Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, a ...

Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of B ...

Belarus Belarus,, , ; alternatively and formerly known as Byelorussia (from Russian ). officially the Republic of Belarus,; rus, Республика Беларусь, Respublika Belarus. is a landlocked country in Eastern Europe. It is bordered by ...

Russia Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...

) harvesting the early spring sap of

birch A birch is a thin-leaved deciduous hardwood tree of the genus ''Betula'' (), in the family Betulaceae, which also includes alders, hazels, and hornbeams. It is closely related to the beech- oak family Fagaceae. The genus ''Betula'' contains ...

trees (so called " birch juice") for human consumption is common practice; the sap can be used fresh or

fermented Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...

and contains

xylitol Xylitol is a chemical compound with the formula , or HO(CH2)(CHOH)3(CH2)OH; specifically, one particular stereoisomer with that structural formula. It is a colorless or white crystalline solid that is freely soluble in water. It can be classifi ...

. Certain palm tree sap can be used to make

palm syrup Miel de palma (lit. palm honey, also palm syrup) is an edible sweet syrup produced from the sap of a number of palms. It is produced in the Canary Islands and coastal regions of South America. Species In the Canary Islands, palm syrup (''miel de ...

. In the Canary Islands they use the

Canary Island date palm ''Phoenix canariensis'', the Canary Island date palm or pineapple palm, is a species of flowering plant in the palm family Arecaceae, native to the Canary Islands off the coast of Morocco. It is a relative of ''Phoenix dactylifera'', the true dat ...

while in Chile they use the

Chilean wine palm ''Jubaea'' is a genus of palms with one species, ''Jubaea chilensis'' or ''Jubaea spectabilis'', commonly known in English as the Chilean wine palm or Chile cocopalm, and palma chilena in Spanish. It is native to southwestern South America and i ...

to make their syrup called ''miel de palma''.

References

External links