[[File:Sap of Sansevieria trifasciata.jpg|thumb|left|150px|Sap droplets of ''[[Sansevieria trifasciata]]'']]
Saps may be broadly divided into two types: xylem sap and phloem sap.
===Xylem sap===
Xylem sap (pronounced {{IPAc-en|ˈ|z|aɪ|l|ə|m}}) consists primarily of a watery solution of [[plant hormone|hormones]], [[dietary mineral|mineral elements]] and other [[nutrient]]s. Transport of sap in xylem is characterized by movement from the [[root]]s toward the [[leaf|leaves]].<ref name=marschner1983>{{cite book|last=Marschner|first=H|chapter=General introduction to the mineral nutrition of plants|year=1983|doi=10.1007/978-3-642-68885-0_2|title=Inorganic Plant Nutrition|pages=5–60|isbn=978-3-642-68887-4|publisher=Springer|series=Encyclopedia of Plant Physiology|volume=15 A}}</ref>
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]] [[gradient]]s, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.<ref>{{cite journal|last=Zimmerman|first=Ulrich|title=What are the driving forces for water lifting in the xylem conduit?|journal=Physiologia Plantarum|year=2002|doi=10.1034/j.1399-3054.2002.1140301.x|pmid=12060254|volume=114|issue=3|pages=327–335}}</ref><ref name=tyree1997>{{cite journal|last=Tyree|first=Melvin T.|title=The cohesion-tension theory of sap ascent: current controversies|journal=Journal of Experimental Botany|year=1997|doi=10.1093/jxb/48.10.1753|url=http://jxb.oxfordjournals.org/content/48/10/1753.short|volume=48|issue=10|pages=1753–1765}}</ref>
Xylem sap transport can be disrupted by [[cavitation]]—an "abrupt phase change [of water] from liquid to vapor"<ref name=sperry1994/>—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 [[moisture stress|water stress]], and freeze-thaw cycles in temperate climates.<ref name=sperry1994>
{{cite journal |last=Sperry|first=John S. |author2=Nichols, Kirk L. |author3=Sullivan, June E |author4=Eastlack, Sondra E. |title=Xylem Embolism in ring-porous, diffuse-porous, and coniferous trees of Northern Utah and Interior Alaska |journal=Ecology |year=1994 |volume=75 |issue=6 |pages=1736–1752 |doi=10.2307/1939633|jstor=1939633 |url=http://bioweb.biology.utah.edu/sperry/publications/Sperry%20et%20al.%201994%20Ecology.pdf }}</ref>
