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How do trees transport water such long distances? Part 2: the mechanism remains a mystery (to me)

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and I still haven’t found what I’m looking for…

So scientists have learned a lot, though not everything, about water’s travels from soil to leaf in a plant or tree. It’s a fascinating story, and I’m keen to learn more. But the real mystery for me is about energy. As the excellent Nature article, upon which I’m mostly relying, points out, animals have a pump-based circulatory system to distribute nutrients, oxygen and so forth, but plants are another matter, or another form of organised matter.

I actually posed two questions in my last post. How do plants – and I think I should specify trees here, because the massive distance between the soil and their top leaves makes the problem more dramatic – move water such large distances, and how do they know they have to transport that water and how much water to transport?

So let’s look at the Nature Education explanation:

The bulk of water absorbed and transported through plants is moved by negative pressure generated by the evaporation of water from the leaves (i.e., transpiration) — this process is commonly referred to as the Cohesion-Tension (C-T) mechanism. This system is able to function because water is “cohesive” — it sticks to itself through forces generated by hydrogen bonding. These hydrogen bonds allow water columns in the plant to sustain substantial tension (up to 30 MPa when water is contained in the minute capillaries found in plants), and helps explain how water can be transported to tree canopies 100 m above the soil surface.

Notice how we’re again returning to the explanations questioned by Wohlleben – transpiration and capillary action. But we’re introduced to something new – the C-T mechanism. The thesis is that water’s cohesiveness through hydrogen bonding creates a tension (the tension that makes for capillary action) that enables water to be shifted up to 100 metres – all because of the minuteness of capillaries found in plants. And trees? Somehow, I just can’t see it. Perhaps the key is in the phrase ‘helps explain’.  There must surely be more to this. The thesis also mentions ‘negative pressure’ generated by transpiration. This is the signalling I wrote about before. Somehow the plant’s chemistry recognises that there’s an imbalance, and of course this happens in all living things, regardless whether they have a complex nervous system. So maybe there’s no need to worry about ‘knowing’. All living organisms respond to their ever-changing environment by altering their internal chemistry, by opening or closing barriers, by selectively adding or subtracting nutrients, and there are unknowns everywhere about precisely how they do that. It’s a kind of organised chemistry that seems like everyday magic from the outside, whether we’re focusing on a beech tree or our own intestines.

The C-T mechanism is only new to me I should add. It can actually be traced back to 1727 and a book by Stephen Hales, in which he pointed out that without what he called perspiration the water in a plant would stagnate, and that it was also required to allow for the capillary movement of water, because ‘the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters’. But this ‘reciprocal proportion’, according to Wohlleben, as quoted in the last post, can only account for a maximum of 3 feet of upward force in ‘even the narrowest of vessels’.

The water transport system, referred to in the last post as the water potential difference or gradient, also has another name, the Soil Plant Atmosphere Continuum (SPAC). I also mentioned something about an ‘apoplastic pathway’. Water enters the tree by the roots, which are divided and subdivided much like branches and twigs above-ground, with the thinnest examples being the fine root hairs. Water enters through the semi-permeable cell walls by osmosis. Cell-to-cell osmosis carries the water deeper into the root system, and thence into an apoplastic pathway. According to this video, this pathway provides an uninterrupted flow of water (no cell wall barriers) which allows a mass flow ‘due to the adhesive and cohesive properties of water’. This is the cohesion-tension theory again. Apparently, due to evaporation, a tension is created in the apoplast’s continuous stream, leading to this ‘mass flow’.

This makes absolutely no sense to me. What I’m so far discovering is that it’s pretty hard to start from scratch as an amateur/dilettante and get my head around all this stuff, and in my reading and video-watching I’ve yet to find a straightforward answer to the how of long distance, fast transport of water in plants/trees – there probably isn’t one.

I’ll try again after a diet of videos – so far I’ve found a large number of videos in Indian English, and their accents defeat me, I’m sad to say. No transcripts available. Meanwhile, I’ve compiled a little glossary (from various sources) to help myself…

apoplast – within plants, the space outside the plasma membrane within which material can diffuse freely. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.

Casparian stripa band of cell wall material deposited in the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall – the cell wall being made of lignin and without suberin – whereas the Casparian strip is made of suberin and sometimes lignin.

cortical cells – in plants, cells of the cortex, the outer layer of the stem or root of a plant, bounded on either side by the epidermis (outer) and the endodermis (inner).

exudation – An exudate is a fluid emitted by an organism through pores or a wound, a process known as exuding.

guttation – water loss, when water or sap collects (at times of low evaporation, dawn & dusk), at tips of grass, herbs (not to be confused with dew, caused by condensation).

hydrostatic pressure – the pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. This increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above.

lignin – a class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily.

osmosis – the movement of water from an area of high to low concentration through a semi-permeable membrane. ‘Pumps’ in the cell membrane transport the specific ions into the cell which means water moves in by osmosis thus maintaining hydrostatic pressure.

phloem – the living tissue that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to parts of the plant where needed. This transport process is called translocation.

plasmodesmata – narrow threads of cytoplasm that pass through the cell walls of adjacent plant cells and allow communication between them.

root pressure – the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves. Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day

sap – a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.

suberin – an inert impermeable waxy substance present in the cell walls of corky tissues. Its main function is as a barrier to movement of water and solutes.

symplast – the network of cytoplasm of all cells interconnected by plasmodesmata. The movement of water occurs from one cell to another through plasmodesmata

tracheid – a type of water-conducting cell in the xylem which lacks perforations in the cell wall.

vascular (plants) – also known as tracheophytes and also higher plants, form a large group of plants (over 300,000 accepted known species) that are defined as those land plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant.

xylem – one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports some nutrients.


On the Trump’s downfall. What a memo. One wonders if the DoJ is running out of patience with the wannabe dictator and his imbecilities, which may bring things to a head sooner rather than later. But those in the know say that Mueller is always thorough and unlikely to be distracted, so I shouldn’t project my own impatience onto him. Dog give me strength to suffer the horrorshow for a while longer.



Written by stewart henderson

February 5, 2018 at 3:48 pm

Posted in biology, botany

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How do plants transport water? Part 1: xylem, transpiration and a mysterious water potential difference

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roots, xylem, upward flow, transpiration – but how does it work? Find out in the next thrilling episode, maybe.
Stolen from Nature Education, with apologies

This post could fit well in the ‘How Stuff Works’ series, always a useful resource, but I doubt if they’ve done a piece on today’s subject. Maybe I’ll check later.

I’ve been reading a book called The hidden life of trees, by Peter Wohlleben, a Chrissy present from a good friend. One of its shortest chapters is titled ‘The mysteries of moving water’. The reason for its brevity is essentially that there’s as yet no solution to the mystery of how water gets from the soil to the leaves of a tree, or any plant for that matter. At least, according to Wohlleben.

This strikes me as amazing, if true. After all, it’s a simple, everyday scenario for any home gardener. You notice on a hot summer day that the leaves of your capsicum plant are wilting. You apply a two-litre dose of H2O to the base, et voilà, within an hour or two (I don’t know, I’ve never timed it), those leaves have become as turgid as much of my writing. And it just may cross your mind that it’s pretty miraculous how plants can do that. But if it’s true that we don’t know how plants manage such an everyday miracle, surely working it out is Nobel Prizeworthy for any ambitious team of botanico-chemists out there, or whatever.

Of course it’s much more likely that botanists have been trying to solve this mystery for decades – isn’t it? But before I look into it, here’s what Wohlleben says in his book:

…water transport is a relatively simple phenomenon to research – simpler at any rate than investigating whether trees feel pain or how they communicate with one another – and because it appears so uninteresting and obvious, university professors have been offering simplistic explanations for decades… Here are the accepted answers: capillary action and transpiration.

Upon reading this I tried to recall what I knew of these terms. With capillary action I drew a blank, though I feel sure I knew about it once. Transpiration, though, was clear enough: it was like perspiration, the evaporation of water from the leaves, rather than the skin (or is perspiration the secretion of water through the pores rather than the evaporation? Later). So transpiration is only about the movement of water from the surface of a leaf to the atmosphere by means of solar energy; it surely has nothing to do with movement through the stem or trunk, though the loss of water from the leaves is presumably a signal to the plant to draw up more water from the earth, but how can we talk of signals when a plant has no brain or command centre to receive them? And how can water be ‘drawn up’ when it has no muscle power or other obvious energy source?

As to capillary action, Wohlleben explains:

Capillary action is what makes the surface of your coffee stand a few fractions of an inch higher than the edge of your cup. Without this force, the surface of the liquid would be completely flat. The narrower the vessel, the higher the liquid can rise against gravity. And the vessels that transport water in deciduous trees are very narrow indeed: they measure barely 0.02 inches across. Conifers restrict the diameter of their vessels even more, to 0.0008 inches. Narrow vessels, however, are not enough to explain how water reaches the crown of trees that are more than 300 feet tall. In even the narrowest of vessels, there is only enough force to account for a rise of 3 feet at most.

Needless to say, plenty of research has been done on the subject of water transport in plants, but I have to agree with Wohlleben that there’s a lot that’s missing. The key to the process is a material called xylem, a structure made from hollow, dead, reinforced cells. Here’s how a BBC science site tries to explain it:

Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what’s lost.

As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Water doesn’t flow upwards, however. It has to be pumped up, or sucked, as we do when we apply our lips and energy to a straw. The BBC also describes the whole process as transpiration, which just seems wrong to me. Obviously much transpires here, but it isn’t just transpiration. What?

What obviously needs explaining is where the energy comes from to draw the water up against gravity, and how the plant ‘knows’ that water needs replenishing.

A more comprehensive, and richly referenced, attempt at an explanation is provided by Nature, the well-known science magazine, on one of its educational websites. There we’re told that ‘plants retain less than 5% of the water absorbed by roots for cell expansion and plant growth’. This is fascinating, as is the reason for the lack of retention – photosynthesis. Water is lost to the atmosphere from the leaves’ stomata, which are like our pores. These stomata are used to absorb CO2 for the photosynthesis of sugars, but their openness to CO2 increases the transpiration rate, so there’s a tricky balance between the two – water loss versus CO2 and sugar gain.

The xylem mentioned above doesn’t reach down all the way to the base of the root system. First the water must pass through several cell layers that act as a filtration system. But how does it do this? What is the force being applied and where does it come from? The Nature article gives this complex explanation:

The relative ease with which water moves through a part of the plant is expressed quantitatively using the following equation:

Flow = Δψ / R,

which is analogous to electron flow in an electrical circuit described by Ohm’s law equation:

i = V / R,

where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, V is equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm’s law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).

Got that? I may be wrong, but isn’t this just an analogy? Don’t analogies tend to break down with a little bit of analytic pressure? The idea of hydraulic conductance is clearly drawn from electrical conductance, but electrical conductance relies on a power source, doesn’t it? What is the plant’s power source? Yes, I can see that certain parts of the plant have a greater resistance to the water’s mostly upward movement than others, and that this resistance is measurable by examining the time it takes for water to pass through the different parts with their particular structure and chemistry, but it says nothing about the energy source. In Ohm’s law, V, voltage is the amount of power, which comes from a source of that power, such as a battery. In the above analogy, Δψ is described as the water potential difference that drives flow. I’m possibly being dumb, but how does that happen? What’s meant by ‘water potential difference’?

The Nature article, I must say, is very good at telling us about the materials and obstacles negotiated by water molecules on their journey. First they pass through the root’s epidermis, then the cortex and the endodermis and then on to the xylem. They travel by an apoplastic pathway (more of that next time), or else a cell-to-cell pathway (C-C), and the role of ‘water-specific protein channels embedded in cell membranes (i.e., aquaporins)’ is mentioned, but this role is apparently still much of a mystery. Anyway, the xylem continues into the petiole, to which the leaves are attached, and then into the mid-rib, the main central vein of the leaf. From there the water passes into the smaller branching veins of a dicot leaf, which also contain tracheids – elongated xylem cells for the transport of water and mineral salts. It’s from this network of veins that transpiration takes place.

So I’m learning a lot, but the ‘water potential gradient’ and how it pulls or pushes water upwards, that’s still very much a mystery to me. But there’s more to come.


Peter Wohlleben, The hidden life of trees, Collins 2017


Ok, the usual update on Trump’s downfall. Some are saying that the Mueller enquiry is winding up (and I’m not talking about GOP hardheads), but I’m hoping not, because I reckon the financial stuff alone will take years to wade through properly. In the meantime though, I’m hoping that more really dramatic developments occur to light a fire under Trump’s capacious backside, sooner rather than later. The latest news is that the Mueller team are looking at the cover-up re Trump Jr’s meeting with Russian agents. So maybe the cover-ups and the endless obstructing will lead to some justice action soon, while the ‘follow the money’ aspect will continue for some time, and hopefully do the really lasting and permanent damage to the Trump horrorshow.


Written by stewart henderson

February 1, 2018 at 11:13 pm