Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.

We come now in this series to a tale that Steven Vogel in his book The Life of a Leaf calls mirabile dictu (wonderful to relate): the way water is raised to the top of a tall tree.¹ Clearly, unless water can be drawn several meters up the conduits in their tree trunks, large woody trees would be impossible. Many trees are 30 meters tall and some are even 100 meters. It turns out that this is only possible because of another ensemble of fitness in nature, which arises out of the so-called colligate properties of fluids, particularly water: primarily the remarkable and counterintuitive tensile strength of liquids working together with the fantastically great surface tension of a fluid confined in a narrow tube. 

Simple capillarity caused by surface tension (a generic property of all fluids) can easily raise water up to 100 meters if the tube is small enough. In tubes one hundredth of a micrometer (10 nanometers), the surface tension is so strong that it can support a column of water of 3 kilometers, or two miles high.² But because of viscosity (a measurement of internal friction), water’s resistance to flowing through such tiny conduits would be prohibitively high.³ In fact, the conduits in trees are 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 comments: “Thirty micrometers sends water only about 1.5 meters (5 feet) upward, and 300 micrometers is ten times worse: 15 centimeters, or 6 inches.”⁴

So How Do Trees Do It? 

How do trees manage to exploit capillarity to hold a column of water 100 meters high (which necessitates tiny tubes) while at the same time overcoming the viscous drag that such tiny tubes entail? As Holbrook and Zwienieki explain, plants solve the problem of the viscous or frictional cost of moving water through small tubes “by connecting the small capillaries in leaves [small enough to generate capillary forces powerful enough to hold a column 100 meters high] to larger conduits that provide a much wider transport channel that runs from the veins in the leaf down through the stem and into the roots.”⁵

The key point is that the critical capillary forces are not generated in the major conduits. As Holland and Zwienieki point out: 

The relevant capillary dimensions are not those of the large conduits that you would see if you cut down a tree and looked inside [with diameters of 0.03-.3 mm]… Rather, the appropriate dimensions are determined by the air-water interfaces in the cell walls of the leaves, where the matrix of cellulose microfibrils is highly wettable and the spacing between them results in effective pore diameters [which function as tiny capillaries] of something like 5 to 10 nm.⁶

This is the crucial point: The diameter of the pores is so small that the surface tension generated (as mentioned earlier) is able to support a water column three kilometers high, much higher than the highest tree. 

In other words, as the authors continue: “Trees and other plants overcome [the problem]… by generating capillary forces in small-diameter pores [at the interfaces in the leaves between the sap and the air] but transporting water between soil and leaves through larger  diameter conduits. That strategy allows them to achieve greater heights than with a straight-walled microcapillary.”⁷

But while capillarity — given the tiny diameter of the tubes at the interface — will suffice to hold up the 100-meter column, what pulls the sap upwards from the roots through the conduits to the stems and leaves at the top of the tree?

How the Suck Is Caused

The answer is that the evaporation or transpiration from the air-water interfaces in the leaf cell causes the suck by inducing a negative pressure in the fluid under the tiny menisci, which is transmitted to the whole system of conduits. It is a basic law of hydraulics that pressure in one part of an enclosed hydraulic system is transmitted to all other parts. As water molecules are lost from the leaves at the top of the tree, others must enter in the roots to take their place. The continual loss of water molecules lowers what is termed the water potential in the regions below the interfaces. This lowering of potential, transmitted to the whole hydraulic network, pulls the water up the conduits to the interfaces where it is lost by evaporation to the atmosphere. 

Tomorrow, “A Remarkable Mechanism for the Existence of Large Trees.”

Notes

1. Stephen Vogel, The Life of a Leaf (Chicago: Chicago University Press, 2010), Chapter Six.
2. N. Michele Holbrook, Maciej A. Zwieniecki, “Transporting Water to the Tops of Trees,” Physics Today 61 (2008): 76–77; Vogel, Chapter Seven.
3. Holbrook, Zwieniecki.
4. Vogel, 93. 
5. Holbrook, Zwienieck.
6. Ibid.
7. Ibid.