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Prometheus Unbound: The Fitness of Nature for Large Trees

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Editor’s note: Dr. Denton is a Senior Fellow with Discovery Institute’s Center for Science & Culture, whose work is featured in the new documetnary Privileged Species which premiered online this week. See it here. Denton holds an MD from Bristol University and a PhD in biochemistry from King’s College in London.

One of the critical requirements for sustainable fires, sufficiently hot to smelt metal ores, is wood. Thin twigs, dried grasses will burn, but such materials are unsuitable for making fires, which reach temperatures of several hundred degrees, sufficient to smelt metals from their ores. Wood or wood products such as coal or charcoal or coke are the only natural fuels available that will do. It was the mastery of fire that led to metallurgy and the energy stored in wood and coal (fossil wood) initiated the industrial revolution and the subsequently development of modern technology and science. And it was through technology and science that we learned of our special place in nature. Without nature’s gift of large trees there would be no wood, no charcoal, no coal, no sustainable fires for smelting metals. Prometheus would be well and truly bound.

Many conditions must be met if large woody trees are to be possible. To begin with, as with all plants their leaves must be able to loose heat when subjected to intense solar radiation. This is achieved by a combination of fundamental physical phenomena including evaporative cooling, convection, conduction, and radiation.1 In addition the trunks and branches of large trees must be made of a strong durable material providing tensile strength (resisting stretching) and compressive strength (resisting volume reduction) to sustain bending and compressive pressures. It is the Goldilocks combination of cellulose and lignin in plant cell walls that provides the necessary strength and is in all probability uniquely fit for this role. As the authors of a recent BIO-Complexity paper comment2:

[Wood is strong] enough to support tremendous loads on cantilevered branches, flexible enough to bend in wind and under snow load, durable enough to resist continual attack from pests and microbes for decades or centuries.

Moreover the slow breakdown of lignin promotes the production of soil, which again promote the existence of large trees.3 It is another example of the elegance and parsimony of nature’s fitness for life, where one substance or process satisfies two or more ends.

As well as having strong trunks and being able to keep cool in the sun, another crucial requirement for large trees is water. Water is essential for cellular physiology and in the case of plant cells for photosynthesis. Water is the source of the hydrogen atoms for sugar synthesis and the source of the oxygen atoms produced during the process — a vital substance for complex aerobes like ourselves. Water is also utilized by plants for evaporative cooling — one of the factors that attenuate the temperature of leaves in hot sun as mentioned above. So trees don’t just provide the combustible material (wood) for fire they provide like all green plants the essential reactant (oxygen) as well, which is also the provider of metabolic energy for aerobic organisms.

And this brings us to a tale that Steven Vogel in his The Life of a Leaf calls "mirabile dictu,"4 wonderful to relate: the way water is raised to the top of a tall tree. Clearly, unless water can be drawn several meters up the conduits in their trunks, large woody trees would be impossible. Many trees are thirty meters tall and some even one hundred meters. Recent work has revealed that this is only possible because of an ensemble of fitness in nature that arises out of the so-called "colligate properties of fluids." For a good review of the mechanisms involved see Vogel’s The Life of the Leaf, Chapters 6 and 7.

As is well known, surface tension, a generic property of all fluids, can easily raise water in small enough tubes to a hundred meters. Amazingly in tubes one hundredth of a micrometer (10 nm) in diameter, the surface tension is so "strong" that it can support a column of water of three kilometeres, or two miles high.5 But because of viscosity, water’s resistance to flowing through such tiny conduits would be prohibitively high.6 In fact, the actual size of the conduits in trees is between 0.03 and 0.3 millimeters in diameter, which is sufficiently wide to allow the sap to flow up through the tubes with minimal resistance. But as Vogel comments7: "Thirty micrometers sends water only about 1.5 meters (five feet) upward, and 300 micrometers is ten times worse: 15 centimeters, or six inches."

So how can surface tension hold up a column of water a hundred meters high? The answer is that the critical capillary forces are not generated in the major conduits. As Holland and Zwieniecki point out8:

The relevant capillary dimensions are not those of the large conduits which carry the water to the leaves … (diameters of 0.03-0.3 mm). Rather the appropriate dimensions are determined by the water air interfaces in the cell walls of leaves where the matrix of cellulose microfibrils is highly wettable and the spacing between them results in effective pore dimensions [which function as tiny capillaries] of something like 5-10 nm.

And this is the crucial point: their diameter is so small that the surface tension generated (as mentioned above) is able to support a water column three kilometers high, much higher than the highest tree.9 Moving water through such tiny tubes would face (as mentioned above) formidable viscous drag. However as Holland and Zwieniecki continue:

Plants solve the problem of the viscous or frictional cost [the moving water through small tubes] by connecting the small capillaries in leaves which do the job of holding up the water column by surface tension to larger conduits that provide a much wider transport channel that runs from the veins in the leaves through the stems and into the roots.

In other words, as they continue: "Trees and other plants overcome … [the problem] by generating capillary forces in small diameter pores but transporting water between soil and leaves through larger diameter conduits."

But while capillarity, given the tiny diameter of the tubes at the interface, will suffice to hold up the hundred-meter column, what pulls the sap up from the roots though the conduits to the stems and leaves at the top of the tree? What creates what Vogel calls the "supersuction"10 that pulls the water up that high from the roots to the leaves?

The answer is that the evaporation or transpiration from the air/water interfaces in the cell membranes of the cells causes "the suck." It does so by inducing a negative pressure in the fluid under the tiny menisci, which is transmitted to the whole system of conduits linking the leaves to the roots. Remarkably, the evaporation causes a negative pressure in the column of several atmospheres, and this is why as Vogel points out11: "When a tree is cut it does not bleed fluid like an injured person. Rather than exuding anything [it] sucks air in with an audible hiss detectable with a sensitive microphone."

As water molecules are lost from the top of the plant, others must enter in the roots to take their place. The continual loss of water molecules lessens what is termed the water potential in the regions below the interfaces and this "lowering of potential" is transmitted to the whole hydraulic network, drawing water up the conduits from the roots to the leaves. (It is a basic law of hydraulics that pressure in one part of an enclosed hydraulic system is transmitted to all other parts.12) That it is not capillary action that draws the water up is indicated by the fact that when transpiration stops, in very humid or cold conditions, while the hundred-meter water column is maintained by surface tension, the movement of water up the conduits ceases.13

But this leads to an intriguing question. Why does the column of water not break into pieces as it is tugged from above? The reason is because of the cohesiveness of a liquid; that is, the tendency of the molecules in a liquid to "stick together," a tendency more pronounced in water than in most other common fluids. Because of this cohesiveness, water columns have, although the notion is very counterintuitive, tensile strength.14 Tensile strength is the ability of a substance to resist being stretched. Just as you can pull a steel wire up a hundred meters without its breaking, because of the tensile strength of steel, it is the same with a water column.

Remarkably as Vogel points out15, experiments show "a rope of liquid water, a square centimeter in cross section" in an enclosed tube, has sufficient tensile strength that "one could hang from it a mass of nearly 300 kilograms." Steel is stronger but only ten times as strong! It is this very counterintuitive property of a fluid — especially water, because its colligative properties are so pronounced — of having "tensile strength" that allows the negative pressure caused by the evaporation in the leaves to pull sap from the roots up a hundred meters to the leaves without any break occurring in the column.

Consequently this remarkable mechanism so vital to the existence of large trees depends critically on two basic physical properties of water: its tensile strength, which prevents the "pull of evaporation" from breaking the water column, and the enormous surface tension generated by water in very narrow tubes or passages which holds up the column of water.

Vogel in his The Life of a Leaf waxes lyrical in contemplating the mechanism16:

The pumping system has no moving parts, costs the plant no metabolic energy, moves more water than all the circulatory systems of animals combined, does so against far higher resistance, and depends on a mechanism with no close analogy in human technology.

Holbrook and Zwieniecki comment in a similar vein in an article in Physics Today17:

Trees can be rightly called the masters of microfluidics. In the stem of a large tree, the number of interconnected water transport conduits can exceed hundreds of millions, and their total length can be greater than several hundred kilometres … On a sunny day, a tree can transport hundreds of gallons of water from the soil to its leaves, and apparently do it effortlessly, without making a sound and without using any moving parts. … The physics that underlies water transport through plants is not exotic; rather, the application of that physics in microfluidic wood matrix results in transport regimes operating far outside our day-to-day experience.

What is particularly striking about this mechanism is not only its stunning elegance but that it is the only mechanism that will work. There is no conceivable alternative way of drawing water up one hundred meters to the top of a tall tree. And this stunningly clever and unique mechanism depends in turn on the physical properties of water being exactly as they are.

Another remarkable aspect of this unique mechanism, which no researcher to my knowledge has highlighted, is the fact that the same vital fluid that is so essential to the basic physiological and biochemical functioning of the cells in the leaf and particularly for the process of photosynthesis is the very same fluid that possesses just the right physical properties to allow it to be raised to the leaves. So water not only provides one of the key chemicals in the process of photosynthesis and the ideal matrix for the physiological functioning of the cells in the leaf, but amazingly it provides through its own intrinsic properties a stunningly brilliant and unique means of "raising itself" from the roots to the leaves. We find, then, a further example of the breathtaking parsimony of nature’s magic: using the same substance or process to achieve completely diverse ends which work together to serve the purposes of life as it exists on earth

Without the ensemble of unique fitness that raises water in trees there would be no wood, fire, metallurgy, or modern technology. Nor would you be reading these paragraphs; nature would not be properly fit for mankind to utilize his cognitive powers to understand the world. It is wonderfully fitting that this unique and stunningly elegant mechanism is intimately related to our role as explorers and manipulators of the world, providing a further indicator supportive of the anthropocentric notion of a world order focused on our being.

References:

(1) Forbes, J. C. Plants in Agriculture. Cambridge [England]; New York, NY, USA: Cambridge University Press, 1992 see fig 4.18 p. 100 and section 4.9.1 "Thermal injury and its avoidance"; Lambers, H. Plant Physiological Ecology. 2nd ed. New York: Springer, 2008. p. 225-235.

(2) Leisola, M, Pastinen, O and Axe D D (2012) Lignin — Designed Randomness. BIO-Complexity, no. 3 (April 5). doi:10.5048/BIO-C.2012.3.

(3) Ibid.

(4) Vogel S (2010) The Life of a Leaf. University of Chicago Press, Chicago. Chapter 6.

(5) Holbrook ML and Zwieniecki MA (2008) Transporting water to the tops of trees, Phys.Today 61: 76-7; Vogel (2010) op cit., Chapter 7.

(6) Holbrook and Zwieniecki (2008) op cit.

(7) Vogel (2010) op cit., Chapter 6.

(8) Holland and Zwienieki (2008) op cit.

(9) Ibid.; Domec, J-C (2011) "Let’s Not Forget the Critical Role of Surface Tension in Xylem Water Relations." Tree Physiology 31, no. 4 (April 1): 359-60. doi:10.1093/treephys/tpr039; see also: http://en.wikipedia.org/wiki/Ascent_of_sap.

(10) Vogel (2010) op cit., Chapter 6. Surface tension creates negative pressure in the water being held up under the tiny menisci. See Hillel, D (2004) Introduction to Environmental Soil Physics. Amsterdam; Elsevier Academic Press, Boston, p 32.

(11) Vogel (2010) op cit., Chapter 6.

(12) Domec (2011) op cit.

(13) http://5e.plantphys.net/article.php?id=99.

(14) Tyree M T (1997) The tension cohesion theory of sap ascent: current controversies, Journal of Experimental Botany, 48, (No. 315) pp. 1753-1765.

(15) Vogel (2010) op cit.

(16) Ibid.

(17) Holbrook ML and Zwieniecki MA (2008) op cit.