Types of sap
Saps may be broadly divided into two types: xylem sap and phloem sap.
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.
Phloem sap consists primarily of water, with sugars, hormones, and mineral elements dissolved in it. It flows from where carbohydrates are produced or stored (sugar source) to where they are used (sugar sink).
The pressure flow hypothesis proposes a mechanism for phloem sap transport. although other hypotheses have been proposed. Phloem sap is also thought to play a role in sending informational signals throughout vascular plants. "Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata."
A large number of animals, all of the single insect order Hemiptera, 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" 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 microorganisms 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." 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.
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 and contains xylitol.
Preparations made from the sap of Aloe vera are widely used for their purported soothing, moisturizing, and healing properties. Aloe vera gel is also used as an ingredient in commercially available lotions, yogurt, beverages, and some desserts. Note, however, that so-called aloe products such as "aloes", "aloe gel" etc. are not generally true sap. They are largely distinct substances, different from each other and different from the true phloem and cellular saps. They also are produced in different cellular structures, much as other distinct materials such as latex and resin are produced in special vessels in various other species of plants.
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