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

sieve tube element Sieve elements are specialized cells that are important for the function of phloem, which is a highly organized tissue that transports organic compounds made during photosynthesis. Sieve elements are the major conducting cells in phloem. Conductin ...

s of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, 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 m ...

; 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 A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required fo ...

, mineral elements and other nutrients. Transport of sap in xylem is characterized by movement from the roots toward the

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

. 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 Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...

gradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients. Xylem sap transport can be disrupted by

cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...

—an "abrupt phase change

f water F, or f, is the sixth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''ef'' (pronounced ), and the plural is ''efs''. Hist ...

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 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 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. 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 long ...

'',

Many insects of the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Hemiptera Hemiptera (; ) is an order of insects, commonly called true bugs, comprising over 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, assassin bugs, bed bugs, and shield bugs. They range in size from to aroun ...

(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 Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

in phloem sap for which these adapted Hemiptera insects contain

symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasi ...

microorganisms which can then provide them with essential amino acids; and also insect "tolerance of the very high sugar content and

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 Maple syrup is a syrup made from the sap of maple trees. In cold climates, these trees store starch in their trunks and roots before winter; the starch is then converted to sugar that rises in the sap in late winter and early spring. Maple tre ...

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 prima ...

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

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eight ...

) 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 30 ...

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. Certain palm tree sap can be used to make palm syrup. 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 da ...

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