Archive for the ‘plants’ Category
Animals r us, but also…
kinds of trunks
The video world that we now seem suddenly steeped in – I write from the perspective of someone who was almost 40 when the internet suddenly swept over us – has its pluses and minuses of course, and one definite plus for me is cute and often fascinating vids of non-human creatures doing smart stuff. I try not to get addicted, but it’s tough. I’m especially interested in ‘exotic’ creatures, which for me is anything I can’t encounter in my local park-lands or zoo. These videos bring such exotica into the living rooms of people like me, who worry about the time, effort and money involved in getting into the next suburb never mind darkest America or the hinterlands of Tunbridge Wells.
So I’m reading – slowly and savouringly, if that’s a word – Carl Safina’s Beyond Words, the first few pages of which – about elephants – literally brought tears to my eyes. Such exquisitely elegant and sensitive things – and that’s just the trunk.
Getting further into this inspiring book, it’s not just about elephants and other creatures whose superlative sensitivities, when brought more forcibly to our attention, are both exciting and mildly intimidating, it’s about the endless ways that complex lives can be lived. It even makes me think of the sudden surge in ‘AI’ over just the past few years, and where it will be in a few generations, as it adapts to ever more complex and challenging conditions. It ain’t going away.
But to return to conscious beings – assuming that AI hasn’t achieved consciousness quite yet – it wasn’t long ago that we preferred to believe we humans were the only ones. Safina looks at the issue early on in his book. It’s taken a long time for us to accept that we’re animals (millions, if not billions, refuse this label), and even those who have mostly believe that we’re massively exceptional, and consciousness, inter alia, is a thing that sets us apart. But what, exactly is consciousness? The neurologist Christof Koch calls it ‘the thing that feels like something’ [Safina, p21]. This subtle definition is certainly worth pondering. It doesn’t require language, it seems to me – language being another item we use to claim exceptionalism – even though we humans can name that thing – fear, anger, boredom, love, fatigue, hunger, pain etc – of which we are conscious. A hungry dog feels hungry. She’s conscious of her hunger, and that consciousness will make her start pestering her owner who’s late in feeding her.
These feelings are all in the mind. It follows that anything with a mind has consciousness. And yet we lose consciousness, when deeply asleep or under anaesthetic. Do we then lose our minds? A strange kind of thought experiment came to me recently when I noticed, without much originality, that upon waking up, it was somewhere between 6 and 7 in the morning, whereas the last time I was aware of myself it was around midnight. During that time I was ‘dead to the world’ as they say. My mind had ceased to exist. If someone had crept into my room during those hours, and gently slit my throat, would there be any real difference to my state?
Of course we know that deep sleep, or unconsciousness, isn’t the same as death, and some of us know a thing or two about REM and the three phases of non-REM sleep, but we don’t know it from experience. What we experience is the same – mostly nothingness. Non-experience. We know, because we have learned, that in those death-like states we still breathe, and blood is still pumped through our bodies. Our mind, though, has gone. We’ve ‘lost’ consciousness. It’s really quite bizarre.
Even so, it’s as common as life. Every animal sleeps, after all. Every animal loses consciousness, so every animal has a consciousness to lose. Fish sleep too, without closing their eyes, as they don’t have eye-shutters like we do. So do birds, insects, crustaceans, worms. They all lose, or greatly reduce, consciousness, so they all have consciousness to lose and recover. They all have experiences to be conscious of and learn from.
So what about plants? I mean, how low can you go? Most scientists today would agree that plant consciousness isn’t a thing, but the boundaries are certainly shifting. Early on in Daniel Dennett’s Kinds of minds he considers the development of consciousness in humans. A human’s life starts when she (or it?) becomes a fertilised egg – that’s to say, one single cell, known as a zygote. Just to confuse us, it’s also known as a diploid cell, as it contains the genetic material from two parents. Let’s accept that zygotes don’t have minds. What about blastulae? They’s mostly undifferentiated globs of cells which – well without getting into details, there’s no mind at this stage, so let’s go on to the foetal stage which starts at around eight weeks, and lasts until birth. That’s a lot of development, and presumably consciousness is one of the things that starts developing at this time, if not before…
Of course it all has to do with neural or brain development in animals. Yet many mysteries remain. Safina tells this story:
A thirty-year-old man named Roger lost about 95% of his cortex due to a brain infection. Roger can’t remember the decade before the infection, can’t taste or smell, and has great difficulty forming new memories. Yet he knows who he is, recognises himself in a mirror and in photographs, and generally acts normal around people. He can use humour and can feel embarrassed. All with a brain that does not resemble a human brain.
So it seems it doesn’t take as much as we think to make us conscious. And of course other consciousnesses, or minds, or even ways of living, can be just as impressively adaptive. Charles Darwin, in the feverish years after his Beagle adventure, was proudly intrigued by the subject, as his notebooks show:
It is absurd to talk of one animal being higher than another… People often talk of the wonderful event of intellectual Man appearing – the appearance of insects with other senses is more wonderful… Who with the face of the earth covered with the most beautiful savannahs & forests dare to say that intellectuality is only aim in this world…
Needless to say, Darwin was far more circumspect on these matters in his published work. Yet on the subject of plants he surely would’ve been chuffed to learn that, though they have no nervous system, they produce the same chemicals – including serotonin, dopamine and glutamate – essential for neurotransmission in animals like us (Safina, p23).
None of this should surprise us, I suppose, as we’ve come to learn that all life is connected. We have a relationship with every other living being on this planet, which we could trace, if we had all the time in the world…
References
Carl Safina, Beyond words: what animals think and feel, 2016
Daniel Dennett, Kinds of minds: towards an understanding of consciousness, 1996
Janet Browne, Charles Darwin, voyaging, 1995
How do plants transport water? Part 1: xylem, transpiration and a mysterious water potential difference
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.
References
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.