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The Theory of the Rise of Sap in Trees: Some Historical and Conceptual Remarks

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Abstract

The ability of trees to suck water from roots to leaves, sometimes to heights of over a hundred meters, is remarkable given the absence of any mechanical pump. In this study I deal with a number of issues, of both a historical and conceptual nature, in the orthodox Cohesion-Tension (CT) theory of the ascent of sap in trees. The theory relies chiefly on the exceptional cohesive and adhesive properties of water, the structural properties of trees, and the role of evaporation (“transpiration”) from leaves. But it is not the whole story. Plant scientists have been aware since the inception of the theory in the late 19th century that further processes are at work in order to “prime” the trees, the main such process – growth itself – being so obvious to them that it is often omitted from the story. Other factors depend largely on the type of tree, and are not always fully understood. For physicists, in particular, it may be helpful to see the fuller picture, which is what I attempt to provide in nontechnical terms.

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Notes

  1. Martin Zimmermann, who died in 1984, was Charles Bullard Professor at Harvard University and Director of the Harvard Forest from 1970 until his death, and the author of an authoritative 1983 textbook on the ascent of sap. Ulrich Zimmermann is currently Senior Professor at the Biocenter of the University of Würzburg, Germany.

  2. Trees are allegedly even older than the bristlecone pine, but such claims seem to be rare in the literature. Note too that the bristlecone pine is a nonclonal tree; the oldest individual vegetatively cloned tree, which repeatedly sprouted from the same or newly cloned roots, was discovered in Sweden in 2008, a spruce aged over 9000 years. Clonal colonies (multiple trees connected by a common root system) can be much older.

  3. Xylem, a portion of the sapwood, consists of repeating hollow elements that come in two kinds: vessels (as in broad-leaved trees) and the evolutionarily more primitive tracheids (as in conifers). I shall not be concerned in the main with the differences between these kinds, and will generally refer to “conduits” or “elements” without discrimination. The hollow interior of the conduit is referred to as the lumen (plural lumina).

  4. Joly noted that “we do not know if hydrostatic tension has been detected anywhere else in nature”; see “Discussion” (ref. 2), p. 658. The remarkable mechanism of spore ejection from fern sporangia also testifies to the high tensile strength of water; see Tyree and Zimmermann, Xylem Structure ref. 22), p. 63, and Andrew M. Smith, “Negative Pressure Generated by Octopus Suckers: A Study of the Tensile Strength of Water in Nature,” Journal of Experimental Biology 157 (1991), 257-271.

  5. Negative pressure is defined as pressure less than atmospheric pressure, or the vapor pressure of water.

  6. The possibility of some evaporated water originating in storage compartments inside trees will be discussed at the end of this essay.

  7. It has been widely held that the principal reason for this rate of transpiration is that xylem sap in plants is very dilute–large amounts have to be transpired so that sufficient quantities of soil nutriments can be accumulated in leaf cells, a notion that goes back at least to Stephen Hales in 1727; see Hales, Vegetable Staticks (ref. 48), p. 6. However, since the 1990s, the idea that plant growth relies on transpiration has increasingly been called into question. It seems that the evolution of a leaf structure favoring high rates of photosynthesis – and hence intake of CO2 into the leaves through the stomata, the same apertures allowing for escape of water vapor – has in most habitats had greater survival value than one conserving water; see W. Tanner and H. Beevers, “Does transpiration have an essential function in long-distance ion transport in plants?” Plant, Cell and Environment 13 (1990), 745-750.

  8. The albedo of a surface is the degree to which that surface reflects solar energy.

  9. During the cooling periods in glacial cycles over the last two million years, loss of boreal forest is believed to have provided a positive feedback in relation to glaciation. Conversely, 6000 years ago the expansion of boreal forests following the shrinkage of ice sheets from the last ice age would have given rise to a positive feedback on warming; see George B. Bonan, “Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests,” Science 320 (2008), 1444-1449, on 1445.

  10. In assessing the long-term role of forests in climate change, the feedback of global warming on the forests must also be taken into consideration. On the one hand, for example, in a warmer world with less snow, the decrease of albedo linked to the masking of snow becomes less significant, and carbon storage in trees increases as a result of future CO2 fertilization. On the other hand, if droughts become more frequent, the evaporative cooling of forests diminishes; see G. Bala, K. Caldeira, M. Wickett, T.J. Phillips, D.B. Lobell, C. Delire, and A. Mirin, “Combined climate and carbon-cycle effects of large-scale deforestation,” Proceedings of the National Academy of Sciences 104 (2007), 6550-6555, and especially Bonan, “Forests and Climate Change” (above footnote), p.1447. More recent studies are by Vivek K. Arora and Alvaro Montenegro, “Small temperature benefits provided by realistic afforestation efforts,” Nature Geoscience 4 (2011), 514-518, and by Julia Pongratz and colleagues, J. Pongratz, C.H. Reick, T. Raddatz, K. Caldeira, and M. Claussen, “Past land use decisions have increased mitigation potential of reforestation,” Geophysical Research Letters 38 (2011), L15701, 5 pages. A drying trend in the Southern Hemisphere has led in the decade 2000-2009 to a global decrease in the amount of atmospheric carbon fixed by plants and accumulated as biomass; see Maosheng Zhao and Steven W. Running, “Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009,” Science 329 (2010), 940-943. Very recently it has also been discovered that the advance of forests into Arctic tundra caused by rising temperatures releases carbon into the atmosphere from the soil through a process called positive priming. Such release of carbon may outweigh the carbon captured in growth of the forests, another way boreal afforestation may accelerate climate change; see Iain P. Hartley, Mark H. Garnett, Martin Sommerkorn, David W. Hopkins, Benjamin J. Fletcher, Victoria L. Sloan, Gareth K. Phoenix, and Philip A. Wookey, “A potential loss of carbon associated with greater plant growth in the European Arctic,” Nature Climate Change 2 (2012), 875-879, published online June 17, 2012. Finally, higher temperatures and changing rainfall patterns also encourage damaging insect and fungal infestations; for example, high-elevation pine – including bristlecones – in the West of the United States and Canada are increasingly vulnerable to mountain-pine beetles and whitepine blister rust, as shown by Juliet Eilperin, “Protecting pine forests from warming threat, The Washington Post (May 14, 2012), A8, and Sophie Quinton, “As Politicians Debate Climate Change, Our Forests Wither,” The Atlantic (June 15, 2012), 13 pages, website <http://www.theatlantic.com/national/archive/2012/06/as-politicians-debate-climate-change-our-forests-wither/258549/>.

  11. It is noteworthy that in J.S. Rowlinson’s admirable semi-historical 2002 book on the physics of cohesion, which contains fascinating detail of the long struggle to understand the cohesive properties of water, there is but a single brief mention of the rise of sap in trees; see Rowlinson, Cohesion (ref. 3), p. 19.

  12. Transpiration takes place through the bark, branches and twigs of trees, but by far the greatest amount is through leaves when there are any. Of that amount the greatest part is through the stomata, which open and close depending on a number of factors both within the leaf and without. The CO2 absorbed from the atmosphere passes in the opposite direction through the open stomata; but for every CO2 molecule used for the production of sugars, several hundred water molecules are released to the atmosphere. Trees are thirsty; see Holbrook, Zwieniecki, and Melcher, “Dynamics” (ref. 22), pp. 492-493. As John Sperry puts it: “The transpiration stream represents a river of water flowing in exchange for a relative trickle of carbon”; see Sperry, “Hydraulics” (ref. 22, p. 304.

  13. The exact site of the evaporation surfaces inside the leaves is still the subject of debate; see, for example, Kramer and Boyer, Water Relations (ref. 19), p. 204.

  14. A useful brief account of the physics of capillarity is found in Pickard, “Ascent of Sap in Plants” (ref. 21), Section III, pp. 192-198; for more background, see Rowlinson, Cohesion (ref. 3), pp. 86-102.

  15. I shall return to this process in more detail in the section on Priority Issues below.

  16. There is, however, often a lag between absorption of water in the roots and the onset of transpiration in the morning; the reasons for this will be touched on later. The speeds of the transpiration flow in coniferous tracheids are in the range 20-40 centimeters per hour; in the vessels of broad-leaved trees, a common speed is 5 meters per hour, though 44 meters per hour has been recorded in an oak, as discussed by Canny, “Flow and Transport in Plants” (ref. 22), p. 28, and by Meinzer and colleagues, “Dynamics” (ref. 104), pp. 105-114.

  17. Direct solar radiation is only one factor; there is also radiation from the soil and surrounding objects, as well as heat being provided by the surrounding air; see Kramer and Boyer, Water Relations (ref. 19), p. 207, and Ehlers and Goss, Water Dynamics (ref. 22), p. 89.

  18. Böhm shares his name with the celebrated Austrian sculptor – anglicized as Joseph Boehm – whose 1885 statue of Charles Darwin is found in the Natural History Museum in London.

  19. “The most important common feature to both papers was the identification of the cell walls of [leaf] parenchyma cells, whether living or dead, as the sites where surface tensions develop due to the transpiration of water. Both papers emphasized that a moist cell wall is impermeable to air, so that even at negative pressures air cannot be sucked into conducting elements”; see Richter and Cruiziat, “Brief History” (ref. 21), p. 3 of 5. It is worth noting that this last point was clearly made in the 1894 abstract by Dixon and Joly (ref. 31).

  20. Copeland makes this droll remark: “Ten years ago Böhm alone imagined (publicly) that capillarity could play the leading role in the ascent of sap. It had been tried and found wanting. Then it was named cohesion and sprang at once into popular favor”; see Copeland, “Rise of the Transpiration Stream” (ref. 21), p. 191.

  21. An account of the process of elimination used by Dixon and Joly in the months before arriving at their final theory in 1894 is found in Dixon, Transpiration (ref. 21), Chapter IV, pp. 81-100. A series of existing theories were analysed and rejected, including gravitational and electrical ones; a combination of Quinke’s theory of thin films of water being drawn up the xylem walls with the Unger-Sachs imbibition theory was also entertained and discarded.

  22. I owe clarification on this point to John Sperry (private communication, 2013).

  23. Some of the following comments are based on the helpful analysis of these experiments found in Harré, Method (ref. 49), Chapter 8, pp. 77-97.

  24. A similar statement is found in Pickard: “the less than 100 nm [nanometers] interstices of the cell wall will support without difficulty the 100 m [meters] long water columns of the largest redwood tracheary elements”; see Pickard, “Ascent of Sap in Plants” (ref. 21), p. 195. In the same vein (no pun intended), Steudle writes: “Xylem walls consist of a porous net of wettable polymers (cellulose, lignin, hemicelluloses, etc.). Pores (interfibrillar spaces) are of an order of 10 nm [nanometers], which corresponds to 30 MPa [megapascals] of capillary pressure. Hence the porous hydrophilic matrix will be imbibed with water like a sponge”; see Steudle, “Cohesion-Tension Mechanism” (ref. 22), p. 854. In fact, Dixon already stresses the role of adhesion between water and xylem walls in sustaining hanging threads of water; he cites experiments performed with Joly (discussed below) as showing that such adhesion is stronger than that between water and glass. “This is quite to be expected, when we take into account the manner in which water permeates the substance of the walls of the tracheae when brought into contact with them”; see Dixon, Transpiration (ref. 21), p. 87.

  25. “[The] distinction between cavitation (normally, fracture at room temperature and negative pressure) and boiling (normally, fracture at elevated temperature and atmospheric pressure) is easily blurred since both are aspects of the single phenomenon of nucleus growth”; see Pickard, “Ascent of Sap in Plants” (ref. 21), p. 200.

  26. This is of course a wild understatement. Besides its hydraulic prowess, a tree of any height produces all the complex molecules needed for its physical structure and metabolism – including the hormones (auxins) that tell the stem to grow upwards and the roots to grow downwards – from only sunlight, carbon dioxide, water, nitrogen, and a few trace elements. All of this is far beyond human manufacturing capabilities; see Suzuki and Grady, Tree (ref. 12), Introduction, pp. 1-7.

  27. Hales himself seems to have had at least a rudimentary understanding of this point: “But whether it be by an horizontal or longitudinal shooting, we may observe that nature has taken great care to keep the parts between the bark and wood always very supple with slimy moisture, from which ductile matter the woody fibres, vesicles and buds are formed…. We see here too that the growth of shoots, leaves and fruit, consists in the extension of every part; for the effecting of which, nature has provided innumerable little vesicles, which being replete with dilating moisture, it does thereby powerfully extend, and draw out every ductile part”; see Hales, Vegetable Staticks (ref. 48), p. 194.

  28. Unlike the rise of xylem sap resulting from transpiration, the downward transport of sap in the phloem involves a positive-pressure osmotic-flow mechanism.

  29. There is a certain tension in what Dixon wrote. We read: “When allowance is made for the resistance opposed by the conducting tracts to the motion of water in them, we must conclude, that the supply of water raised by these forces [root pressure and atmospheric pressure] to a height of 10 metres above the roots, must be exceedingly small. It follows that the water in the tracheae above this level is at all times in tension, and, in times of vigorous transpiration, whenever the loss cannot be made good by the lifting pressure of the atmosphere, the water in the tracheae of leaves, at lower levels also, is in a tensile state.” (pp. 89-90) This picture of course stands in need of revision at least in the light of the annual drying out of tall vines. But following a discussion of the cavitation effects due to bubble formation in the xylem, Dixon concedes that “the periodic flooding of the tracheae with water forced upwards by root-pressure will bring the bubbles into solution and will re-establish the conditions for tension throughout the water-tracts.” (p. 95) Root pressure is thus not an insignificant factor after all. And in his final Summary, Dixon states that the “configuration, physical properties, and structure of the wood compel us to admit that the water in the conducting tracts, when not acted upon by a vis à tergo [root pressure], must pass into a state of tension. This state is necessitated by the physical properties of water when contained in a completely wetted, rigid and permeable substance which is divided into compartments.” (p. 210) The next sentence is that cited in the text. Dixon still does not explicitly explain how the xylem is wetted, but for heights greater than 10 meters, root pressure seems to be the only candidate in his 1914 account. And as noted earlier, in his account transpiration is not the only lifting mechanism in the leaves; a vitalist notion of secretory actions taking place in cell walls is also upheld, which dominates transpiration when the surrounding air is saturated.

  30. In one study of sugar maple saplings, 80% loss of conductivity in upper twigs, and 60% in trunk xylem was reported; see Tyree and Sperry, “Vulnerability” (ref. 72, p. 26).

  31. Most famously, R. D. Preston in 1952 investigated the effects of pairs of transverse cuts separated longitudinally but coming from opposite sides and overlapping. Little effect on transpiration takes place if the distance between such cuts is greater than a critical distance, which is roughly double the typical length of the xylem conduits in the given tree. Single transverse cuts severing up to 90% of the cross-sectional area of the xylem likewise have little effect on transpiration in some cases, which is consistent with the belief that the stem of a woody plant has a hydraulic resistance that is small compared to other parts of the water pathway in the plant. For further discussion, see J.F.G. Mackay and P.E. Weatherley, “The effects of transverse cuts through the stems of transpiring woody plants on water transport and stress in the leaves,” Journal of Experimental Botany 24 (1973), 15-28.

  32. The sucrose in the sap is produced in the leaves and descends through the phloem, but it is transported radially into the xylem. The sugar concentration of the xylem sap is one fortieth of commercial maple syrup, obtained by boiling the exuded sap.

  33. What prevents this seeding from taking place at typical negative xylem pressures is that the pores in the vessel walls are very small, of nanometer scale. Any curved air–water interfaces therein require, because of the large surface tension of water, a large input of energy to expand; see Pickard, “Ascent of Sap in Plants” (ref. 21), p. 223, who dismissed the possibility of air seeding, and Holbrook, Zwieniecki, and Melcher, “Dynamics” (ref. 22), pp. 493-494.

  34. It is worth noting that Canny, “Flow and Transport” (ref. 22), pp. 279-286, proposed a vitalistic mechanism based on tissue pressure (which originates from metabolic processes) that supports the ascent of sap in the xylem and plays an important role during the refilling of cavitated conduits. The theory is now widely rejected on the alleged grounds that it is inconsistent with the laws of thermodynamics; see, for example, Jonathan P. Comstock, “Why Canny’s theory doesn’t hold water,” American Journal of Botany 86 (1999), 1077-1081.

  35. For the meaning of these terms, and the nature of thermodynamics and special relativity as principle theories, see Brown, Physical Relativity (ref. 98), Chapter 5, pp. 69-90.

References

  1. Sir Isaac Newton, Opticks or A Treatise of the Reflexions, Refractions, Inflexions & Colours of Light. Based on the Fourth Edition London 1730. With a Foreword by Albert Einstein. An Introduction by Sir Edmund Whittaker. A Preface by I. Bernard Cohen (New York: Dover Publications, 1979), Query 31, pp. 375-406, on p. 394.

  2. Francis Darwin, “On the Ascent of Water in Trees,” Report of the Sixty-Sixth Meeting of the British Association for the Advancement of Science held at Liverpool in September 1896 (London: John Murray, 1896), 674-683, on 678; reprinted as “Report of a Discussion on the Ascent of Water in Trees,” Annals of Botany 10 (1896), 630-643, on 635; Vines, “Discussion,” 644-647; Joly, “Discussion,” 647-660; FitzGerald, “Discussion,” 660-661.

  3. J.S. Rowlinson, Cohesion: A Scientific History of Intermolecular Forces (Cambridge and New York: Cambridge University Press, 2002), p. 262.

  4. Guillermo Angeles, Barbara Bond, John S. Boyer, Tim Brodribb, J. Renée Brooks, Michael J. Burns, Jeannine Cavender-Bares, Mike Clearwater, Hervé Cochard, Jonathan Comstock, Stephen D. Davis, Jean-Christophe Domec, Lisa Donovan, Frank Ewers, Barbara Gartner, Uwe Hacke, Tom Hinckley, N. Michelle Holbrook, Hamlyn G. Jones, Kathleen Kavanagh, Bev Law, Jorge López-Portillo, Claudio Lovisolo, Tim Martin, Jordi Martínez-Vilalta, Stefan Mayr, Frederick C. Meinzer, Peter Melcher, Maurizio Mencuccini, Stephen Mulkey, Andrea Nardini, Howard S. Neufeld, John Passioura, William T. Pockman, R. Brandon Pratt, Serge Rambal, Hanno Richter, Lawren Sack, Sebastiano Salleo, Andrea Schubert, Paul Schulte, Jed P. Sparks, John Sperry, Robert Teskey, and Melvin Tyree, “The Cohesion-Tension Theory,” New Phytologist 163 (2004), 451-452.

  5. Ulrich Zimmermann, Heike Schneider, Lars H. Wegner, and Axel Haase, “Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?” New Phytologist 162 (2004), 575-615.

  6. Philippe Gerrienne, Patricia G. Gensel, Christine Strullu-Derrien, Hubert Lardeux, Philippe Steemans, and Cyrille Prestianni, “A Simple Type of Wood in Two Early Devonian Plants,” Science 333 (2011), 837.

  7. David B. Lindenmayer, William F. Laurance, and Jerry F. Franklin, “Global Decline in Large Old Trees,” Science 338 (2012), 1305-1306.

  8. George Koch, Stephen Sillett, Gregg Jennings, and Stephen Davis, “How Water Climbs to the Top of a 112 Meter-Tall Tree,” Essay 4.3 (2006), 5 pages, in Plant Physiology Online, Fifth Edition; website <http://5e.plantphys.net/article.php?ch=4&id=100>. They note that for California redwoods, summer fog is an important supply of water, and that direct absorption by the leaves may supplement the uptake from root capture of fog drip.

  9. Al Carder, Forest Giants of the World Past and Present (Markham, Ontario: Fitzhenry & Whiteside, 1995), pp. 1-18, 66-78. Richard McArdle, a former chief of the U.S. Forest Service, estimated a Douglas-fir in Washington to be 120 meters in 1924. The tallest Australian eucalypt today is 100 meters, and there is strong anecdotal evidence of taller ones in the past. The tallest known Sitka spruce (Picea sitchensis) in California has a height of nearly 97 meters.

  10. Joel Bourne, “Redwoods. The Super Trees,” National Geographic 216 (October 2009), 12 pages. A redwood tree’s annual rate of wood production increases with age for at least 1500 years.

  11. T.T. Kozlowski and S.G. Pallardy, Physiology of Woody Plants (Second Edition San Diego, London, Boston, New York, Sydney, Tokyo, Toronto: Academic Press, 1997), p. 299. It is noteworthy that the oldest trees can live on dry sites; as befits such conditions, bristlecone pines are small, thick trees with highly reduced growth rates.

  12. David Suzuki and Wayne Grady, Tree: A Life Story (Vancouver, Toronto, Berkeley: Greystone Books, 2004), p. 149.

  13. B.A. Meylan and B.G. Butterfield, Three-dimensional structure of wood (Hong Kong: Chapman and Hall, Ltd., 1972), give a spectacular collection of electron microscope photographs of woody structure.

  14. See, for example, Ernst Steudle, “Trees under tension,” Nature 378 (1995), 663-664, on 663.

  15. Rubin Shmulsky and P. David Jones, Forest Products and Wood Science: An Introduction. (Sixth Edition New York: John Wiley & Sons, 2011), p. 443, note that the weight of wood used today as a raw material in the United States is still greater than the weight of all metals and plastics combined.

  16. See, for example, Colin Tudge, The Secret Life of Trees: How They Live and Why They Matter (London: Penguin Press Science, 2006).

  17. Suzuki and Grady, Tree (ref. 12), p. 68. It is noteworthy that in this elegant and highly readable book on trees, built around the life cycle of a Douglas-fir, the authors present, also in chapter 2, a confused account of the mechanism of sap rise, ultimately regarding it as a “mystery.”

  18. David J Beerling, and Peter J. Franks, “The hidden cost of transpiration,” Nature 464 (2010), 495-496; Steudle, “Trees under tension” (ref. 14).

  19. Paul J. Kramer and John S. Boyer, Water Relations of Plants and Soils (San Diego, New York, Boston, London, Sydney, Tokyo, Toronto: Academic Press, 1995), p. 203.

  20. Ibid., p. 1447; see also Richard A. Betts, “Afforestation cools more or less,” Nature Geoscience 4 (2011), 504-505.

  21. Historical sketches of varying lengths can be found in Darwin, “Report” and Vines, Joly, and FitzGerald “Discussions” (ref. 2); Edwin Bingham Copeland, “The Rise of the Transpiration Stream: An Historical and Critical Discussion,” Botanical Gazette 34 (1902), 161-193, 260-283; Henry H. Dixon, Transpiration and the Ascent of Sap in Plants (London: Macmillan and Co., 1914), Chapter IV, pp. 81-100; Edwin C. Miller, Plant Physiology; With reference to the green plant (New York and London: McGraw-Hill Book Company, 1938), pp. 855-872; Kozlowski and Pallardy, Physiology of Woody Plants (ref. 11), pp. 259-260; K.N.H. Greenidge, “Ascent of sap,” Annual Review of Plant Physiology 8 (1957), 237-256; William F. Pickard, “The Ascent of Sap in Plants,” Progress in Biophysics and Molecular Biology 37 (1981), 181-229; and especially the online essay by Hanno Richter and Pierre Cruiziat, “A Brief History of the Study of Water Movement in the Xylem,” Essay 4.1 (2002), in Plant Physiology Online, Fifth Edition, 5 pages, website <http://5e.plantphys.net/article.php?ch=&id=98>.

  22. Succinct outlines of the CT theory are found in N. Michele Holbrook, Maclej A. Zwieniecki, and Peter J. Melcher, “The Dynamics of ‘Dead Wood’: Maintenance of Water Transport Through Plant Stems,” Integrative and Comparative Biology 42 (2002), 492-496, and Pierre Cruiziat and Hanno Richter, “The Cohesion-Tension Theory at Work,” Essay 4.2 (2006), in Plant Physiology Online, Fifth Edition, 4 pages, website <http://5e.plantphys.net/article.php?ch=4&id=99>. More detailed review papers are Greenidge, “Ascent of sap” (ref. 21); M.J. Canny, “Flow and Transport in Plants,” Annual RevXylemiew of Fluid Mechanics 9 (1977), 275-296; and Ernst Steudle, “The Cohesion-Tension Mechanism and the Acquisition of Water by Plant Roots,” Annual Review of Plant Physiology and Plant Molecular Biology 52 (2001), 847-875. Lengthier treatments are found in in the textbooks of Kramer and Boyer, Water Relations (ref. 19), pp. 234-251, Kozlowski and Pallardy, Physiology of Woody Plants (ref. 11), pp. 259-260; M.T. Tyree and M.H. Zimmermann, Xylem Structure and the Ascent of Sap (Berlin, Heidelberg, New York: Springer-Verlag, 2002), pp. 49-88, this being the second edition of the authoritative Martin H. Zimmermann, Xylem Structure and the Ascent of Sap (Berlin, Heidlberg, New York, Tokyo: Springer Verlag, 1983), pp. 37-65; Wilfried Ehlers and Michael Goss, Water Dynamics in Plant Production (Wallingford and Cambridge, Mass.: CABI Publishing, 2003), pp. 82-84; and Park S. Nobel, Physicochemical and Environmental Plant Physiology (Third Edition Amsterdam: Elsevier Academic Press, 2005), p. 459. For a physicist, the technical review paper by Pickard, “Ascent of Sap in Plants” (ref. 21), may be the most informative and satisfying, though it is somewhat out of date, particularly in relation to mechanisms for refilling embolized xylem conduits. An excellent, up-to-date, somewhat less technical review is John S. Sperry, “Hydraulics of Vascular Water Transport,” in Przemyslaw Wojtaszek, ed., Mechanical Integration of Plant Cells and Plants [Signaling and Communication in Plants, Vol. 9] (Berlin and Heidelberg: Springer-Verlag, 2011), pp. 303-327. The literature is huge; these publications represent a very small selection; a useful list of key papers is found in Angeles, et al., “Cohesion-Tension Theory” (ref. 4).

  23. Quoted in Ehlers and Goss, Water Dynamics (ref. 22), p. 82, who attribute this quotation to Josef Böhm, “Capillarität und Saftsteigen,” Bericht der Deutschen Botanischen Gesellschaft 11 (1893), 203-212, on 207.

  24. Eduard Strasburger, Ueber den Bau und die Verrichtungen der Leitungsbahnen in den Pflanzen (Jena: Gustav Fischer, 1891); idem, Ueber das Saftsteigen (Jena: Gustav Fischer, 1893).

  25. Darwin, “Report” (ref. 2), p. 640.

  26. Ibid., p. 631; Copeland, “Rise of the Transpiration Stream” (ref. 21), provides much detail on the range of ideas and experimental evidence being brought to bear on the issue in the 19th century – and his own skepticism about the validity of the CT theory.

  27. Herman Helmholtz, “Ueber galvanische Polarisation in gasfreien Flüssigkeiten” [1873], in Wissenschaftliche Abhandlungern. Erster Band (Leipzig: Johann Ambrosius Barth, 1882), pp. 821-834, on p. 830.

  28. Böhm, “Capillarität und Saftsteigen,” (ref. 23), p. 206. For a useful summary of Böhm’s energetic experimental work on the rise of sap, see Ehlers and Goss, Water Dynamics (ref. 22), pp. 82-84. The translation from the German of these passages is due to John Pannell (private communication).

  29. Patrick N. Wyse Jackson, “A Man of Invention: John Joly (1857-1933), Engineer, Physicist and Geologist,” in David Scott, ed., Treasures of the Mind. A Trinity College Dublin Quatercentenary Exhibition (London: Sotheby’s, 1992), pp. 89-96.

  30. Michael Jones, “‘A Name Writ in Water’: Henry Horatio Dixon 1869-1953,” in Scott, Treasures of the Mind (ref. 29), pp. 97-103; online at website <http://www.tcd.ie/Botany/tercentenary/300-years/chairs/henry-horatio-dixon.php>, 5 pages.

  31. Henry H. Dixon and J. Joly, “On the Ascent of Sap” (Abstract), Proceedings of the Royal Society of London 57 (1894), 3-5.

  32. Henry H. Dixon and John Joly, “On the Ascent of Sap,” Philosophical Transactions of the Royal Society of London B 186 (1895), 563-576.

  33. Ibid., p. 563.

  34. Copeland, “Rise of the Transpiration Stream” (ref. 21), p. 186. E. Askenasy, “Ueber das Saftsteigen,” Verhandlungen des Naturhistorisch-Medizinischen Vereins zu Heidelberg 5 (1896), 325-345; idem, “Beitrage zur Erklrung des Saftsteigens,” ibid., 429-448.

  35. Joly, “Discussion” (ref. 2), p. 647.

  36. For details, see Darwin, “Report” (ref. 2), pp. 635-642; Vines, “Discussion: (ref. 2), pp. 646-647; Joly, “Discussion” (ref. 2), pp. 647-654; FitzGerald, “Duscussion: (ref. 2), pp. 660-661; see also Pickard, “Ascent of Sap in Plants” (ref. 21), p. 220.

  37. Pickard, “Ascent of Sap in Plants” (ref. 21), p. 185.

  38. Joly, “Discussion” (ref. 2), p. 648.

  39. Richter and Cruiziat, “Brief History” (ref. 21), p. 2 of 5.

  40. Ibid.

  41. Darwin, “Report” (ref. 2), p. 635.

  42. Joly, “Discussion” (ref. 2), pp. 648-649.

  43. See, for example, Steudle, “Cohesion-Tension Mechanism” (ref. 22), p. 854.

  44. Joly, “Discussion” ref. 2), p. 652, where Joly refers to a model involving a porous pot connected to a supply of water, but the argument is supposed to illuminate what happens in trees.

  45. Pickard, “Ascent of Sap in Plants” (ref. 21), pp. 220-221.

  46. Joly, “Discussion” (ref. 2), p. 659.

  47. Dixon, Transpiration (ref. 21), p. 29. It is noteworthy that the five Böhm papers cited by Dixon do not include his 1893 paper.

  48. Stephen Hales, Vegetable Staticks: Or, An Account of some Statical Experiments on the Sap in VEGETABLES, etc. (London: W. and J. Innys and T. Woodward, 1727; reprinted London: Macdonald and New York: American Elsevier, 1969).

  49. R. Harré, The Method of Science. A Course in Understanding Science, based upon the De Magnete of William Gilbert, and the Vegetable Staticks of Stephen Hales (London and Winchester: Wykeham Publications and New York: Springer-Verlag, 1970), p. 65. He includes a short biography of Hales in Chapter 7, pp. 70-76; amongst Hales’s many contributions to science was the discovery of carbon dioxide in air.

  50. Franz Floto, “Stephen Hales and the cohesion theory,” Trends in Plant Science 4 (1999), 209.

  51. Hales, Vegetable Staticks (ref. 48), p. 43.

  52. Ibid., p. 56. It seems that Hales himself may not have been the first to connect capillarity with the rise of sap; see Dixon, Transpiration (ref. 21), p. 27, who refers to the 1723 views of Christian Wolff who “believed that the forces involved were the expansion of air and capillarity.”.

  53. Rowlinson, Cohesion (ref. 3), p. 264.

  54. Ibid., Section 3.2, pp. 86-102, for an account of their work, particularly related to capillarity.

  55. For details of the early work in the field, see George S. Kell, “Early observations of negative pressures in liquids,” American Journal of Physics 51 (1983), 1038-1041.

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  57. Hales, Vegetable Staticks (ref. 48), p. 9; Darwin, “Report” (ref. 2), p. 630.

  58. See, in particular. Steudle, “Trees under tension” (ref. 14), p. 663, who suggests that nonbotanists typically make the mistake of attributing sap rise to, if anything, capillary action in the xylem.

  59. Hales, Vegetable Staticks (ref. 48), pp. 77-78. It is hard to reconcile this passage with the claim by Beerling and Franks, “The hidden cost of transpiration” (ref. 18), p. 496, that Hales, despite being aware of the role of “perspiration” in the ascent of sap, did not discover that plants transpire water from leaves.

  60. P.F. Scholander, Warner E. Love, and John W. Kanwisher, “The Rise of Sap in Tall Grapevines,” Plant Physiology 30 (1955), 93-104. Studies include that of the common northern grapevine (Vitis labrusca), which can sometimes attain heights of 17 to 18 meters in wooded areas in Massachusetts.

  61. As suggested by William F. Pickard, “How might a Tracheary Element which is Embolized by Day be Healed by Night?” Journal of Theoretical Biology 141 (1989), 259-279; the positive-pressure mechanism in tall vines is based on temperature-associated osmotic water uptake from rehydrated cells of the bark. It is noteworthy that Hales, Vegetable Staticks (ref. 48), Chapter III, pp. 57-68, performed the first published experiments designed to measure root pressure, in grapevines (Vitis vinifera).

  62. Pickard, “How might a Tracheary Element” (ref. 61), p. 264, and Melvin T. Tyree; Sebastiano Salleo; Andrea Nardini; Maria Assunta Lo Gullo, and Roberto Mosca, “Refilling of Embolized Vessels in Young Stems of Laurel. Do We Need a New Paradigm?” Plant Physiology 120 (1999), 11-21.

  63. Nobel, Physicochemical and Environmental Plant Physiology (ref. 22), p. 53.

  64. Pickard, “How might a Tracheary Element” (ref. 61), p. 268.

  65. S.H. Vines, “The Suction-force of Transpiring Branches,” Annals of Botany 10 (1896), 429-444. The happily-named Vines was measuring significant tension within leafless branches”; see Vines, “Discussion: (ref. 2), pp. 644-647. This capillarity view has a certain amount in common with the “imbibition” theory of sap rise that was prominent prior to the 1880s and largely associated with the name of the great German botanist Julius von Sachs (1832-1897), though this theory held that the flow of sap takes place within the walls of xylem conduits; see Copeland, “Rise of the Transpiration Stream” (ref. 21), pp. 179-181, and Dixon Transpiration, (ref. 21), pp. 83-84.

  66. For remarks on the farsighted 1886 work of Nägeli, see Copeland, “Rise of the Transpiration Stream” (ref. 21), pp. 183-185.

  67. See in particular the survey of results in Greenidge “Ascent of sap” (ref. 21), pp. 238-249.

  68. Tyree and Zimmermann, Xylem Structure (ref. 22), p. 62.

  69. M.F. Donny, “On the Cohesion of Liquids and their Adhesion to Solid Bodies,” Philosophical Magazine 28 (1846), [291]-294. Hasok Chang, Inventing Temperature: Measurement and Scientific Progress (Oxford and New York: Oxford University Press, 2004), pp. 8-39, provides a fascinating treatment of the history of the theory of boiling.

  70. J.J. Oertli, “The stability of water under tension in the xylem,” Zeitschrift für Pflanzenphysiologie 65 (1971), 195-209, on 208.

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  72. Oertli, “The stability of water” (ref. 70), p. 207. The point seems to be widely accepted. “The conditions for heterogeneous nucleation at preexistent sites appear to be equally hard to fulfill: the tracheary element is presumably water filled from earliest differentiation with the result that its putatively hydrophilic boundaries should be thoroughly wet and unlikely to stabilize nuclei….”; see Pickard, “Ascent of Sap in Plants” (ref. 21), pp. 205, 195. “Xylem conduits are water filled from inception and contain no entrapped air bubbles that could nucleate cavitation”; see M.T. Tyree and J.S. Sperry, “Vulnerability of Xylem to Cavitation and Embolism,” Annual Review of Plant Physiology and Plant Molecular Biology 40 (1989), 19-38, on 20.

  73. Kramer and Boyer, Water Relations (ref. 19), p. 203. It should be noted that this remark occurs in the context of a discussion of how important transpiration is for the growth of a plant.

  74. Sperry, “Hydraulics” (ref. 22), p. 308.

  75. Steudle, “Cohesion-Tension Mechanism” (ref. 22), p. 854.

  76. For a review of the mechanism of cell growth in plants, and the role of osmosis therein, see J.S. Boyer, “Water Transport,” Annual Review of Plant Physiology 36 (1985), 473-516, on 492-500, and John S. Boyer, A.J. Cavalieri, and E.-D. Schulze, “Control of the rate of cell enlargement: Excision, wall relaxation, and growth-induced water potentials,” Planta 163 (1985), 527-543. John Sperry has pointed out to me (private communication) that in cell growth, as opposed to transpiration, the air-water meniscus is stressed from below, but in both cases the pressure drop is most proximally generated by the air-water meniscus by way of capillary action.

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  79. These results were reiterated in Dixon, Transpiration (ref. 21), p. 87.

  80. Dixon and Joly, “On the Ascent of sap” (ref. 32), p. 572.

  81. Tyree and Sperry, “Vulnerability of Xylem” (ref. 72), p. 26. Apart from vines, root pressure can act to recover embolized xylem conduits in herbs, shrubs, and small trees; it has never been recorded in gymnosperms or in most forest trees; see Tyree, Salleo, Nardini, Assunta Lo Gullo, and Mosca, “Refilling of Embolized Vessels” (ref. 62).

  82. Dixon, Transpiration (ref. 21), p. 210.

  83. Pickard, “Ascent of Sap in Plants” (ref. 21), pp. 205-208.

  84. Tyree and Sperry, “Vulnerability of Xylem” (ref. 72).

  85. For further details on the nature of cavitation, see Tyree and Zimmermann, Xylem Structure (ref. 22), section 4.1, pp. 89-94.

  86. Hales, Vegetable Staticks (ref. 48), Chapter IV, pp. 69-84.

  87. Pierre Cruiziat, Hervé Cochard, and Thierry Améglio, “Hydraulic architecture of trees: main concepts and results,” Annals of Forrest Science 59 (2002), 723-752, on 736.

  88. For a brief review of this mechanism, see James K. Wheeler and N. Michele Holbrook, “Cavitation and Refilling,” Essay 4.4 (2007), in Plant Physiology Online, Fifth Edition, 6 pages, website <http://5e.plantphys.net/article.php?ch=4&id=395>; for more detail see also Cruiziat, Cochard, and Améglio, “Hydraulic architecture of trees” (ref. 87), pp. 734-741, and especially Holbrook, Zwieniecki, and Melcher, “Dynamics” (ref. 22), pp. 493-495.

  89. Clark L. Stevens and Russell L. Eggert, “Observations on the Causes of the Fow of Sap in Red Maple,” Plant Physiology 20 (1945), 636-648, on 647-648.

  90. For a useful review, see Tyree and Zimmermann, Xylem Structure (ref. 22), Section 3.9, pp. 81-88.

  91. For details of the two main competing models, and recent anatomical evidence in favor of the 1995 osmotic model due to Tyree, see Damien Cirelli; Richard Jagels, and Melvin T. Tyree, “Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch,” Tree Physiology 28 (2008), 1145-1155.

  92. Pickard, “Ascent of Sap in Plants” (ref. 21), p. 223.

  93. John A. Milburn, “Sap Ascent in Vascular Plants: Challengers to the Cohesion Theory Ignore the Significance of Immature Xylem and the Recycling of Münch Water,” Annals of Botany 78 (1996), 399-407.

  94. U. Zimmermann, F.C Meinzer, R. Bankert, J.J. Zhu, H. Schneider, G. Goldstein, E. Kuchenbrod, and A. Haase, “Xylem water transport: is the available evidence consistent with the cohesion theory?” Plant, Cell and Environment 17 (1994), 1169-1181, and Ulrich Zimmermann, Frederick. Meinzer, and Freidrich-Wilhelm Bentrup, “How Does Water Ascend in Tall Trees and Other Vascular Plants?” Annals of Botany 76 (1995), 545-551.

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  96. William T. Pockman, John S. Sperry, and James W. O’Leary, “Sustained and significant negative water pressure in xylem,” Nature 378 (1995), 715-716.

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  98. See Harvey R. Brown, Physical Relativity: Space-time structure from a Dynamical Perspective (Oxford: Clarendon Press, 2005), pp. 86-87, for the similar episode in the history of the special theory of relativity involving Kauffmann’s experimental “refutation” of the Lorentz-Einstein theory in experiments performed between 1901 and 1905.

  99. T.H. van den Honert, “Water Transport in Plants as a Catenary Process,” Discussions of the Faraday Society 3 (1948), 146-153.

  100. A review of the hydraulics of leaves, which represent an important hydraulic bottleneck in trees, is found in Lawren Sack and N. Michele Holbrook, “Leaf Hydraulics,” Annual Review of Plant Biology 57 (2006), 361-381, and Athena D. McKown; Hervé Cochard, and Lawren Sack, “Decoding Leaf Hydraulics with a Spatially Explicit Model: Principles of Venation Architecture and Implications for Its Evolution,” The American Naturalist 175 (2010), 447-460. The latter is the first detailed examination of the hydraulic consequences and implications of key leaf venation traits for the economics, ecology, and evolution of plant transport capacity.

  101. Holbrook, Zwieniecki, and Melcher, “Dynamics” (ref. 22), p. 495, and further references therein.

  102. For reviews, see Melvin T. Tyree and Frank W. Ewers, “The hydraulic architecture of trees and other woody plants,” New Phytologist 119 (1991), 345-360, and Cruiziat, Cochard, and Améglio, “Hydraulic architecture of trees” (ref. 87).

  103. Frederick C. Meinzer; Shelley A. James, and Guillermo Goldstein, “Dynamics of transpiration, sap flow and use of stored water in tropical forest canopy trees,” Tree Physiology 24 (2004), 901-909.

  104. F.C. Meinzer, J.R. Brooks, J.-C. Domec, B.L. Gartner, J.M. Warren, D.R. Woodruff, K. Bible, and D.C. Shaw, “Dynamics of water transport and storage in conifers studied with deuterium and heat tracing techniques,” Plant, Cell and Environment 29 (2006), 105-114, on 105.

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Acknowledgments

Detailed comments on the first draft of my paper were provided by Pierre Cruiziat and by John Sperry and members of his research group: Duncan Smith, David Love, and Allison Thompson. Their comments, much appreciated, corrected a number of errors and provided helpful suggestions for improvements. Melvin Tyree also kindly provided helpful remarks. Bryn Harris kindly transcribed the original LaTeX pdf file into Word, and Patrick Wyse Jackson, curator of the Geological Museum, Trinity College Dublin, kindly authorized reproduction of the image of John Joly. John Pannell provided guidance on key points, and special help with understanding Böhm’s 1893 paper; without his constant encouragement and inspiration the project would never have been completed. I dedicate my paper to Rom Harré, a mentor and philosopher whose extraordinarily wide interests include plant science and its history. Finally, I thank Roger H. Stuewer for his meticulous and time-consuming editorial and technical assistance with this paper, and for his encouragement generally.

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    Harvey R. Brown

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Harvey R. Brown is Professor of Philosophy of Physics in the Faculty of Philosophy at the University of Oxford, and a Fellow of Wolfson College, Oxford.

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Brown, H.R. The Theory of the Rise of Sap in Trees: Some Historical and Conceptual Remarks. Phys. Perspect. 15, 320–358 (2013). https://doi.org/10.1007/s00016-013-0117-1

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