Posts Tagged ‘botany’
How do trees transport water such long distances? Part 2: the mechanism remains a mystery (to me)
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 strip – a 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.
how evolution was proved to be true
The origin of species is a natural phenomenon
Jean-Baptiste Lamarck
The origin of species is an object of inquiry
Charles Darwin
The origin of species is an object of experimental investigation
Hugo de Vries
(quoted in The Gene: an intimate history, by Siddhartha Mukherjee)
Gregor Mendel
I’ve recently read Siddhartha Mukherjee’s monumental book The Gene: an intimate history, a work of literature as well as science, and I don’t know quite where to start with its explorations and insights, but since, as a teacher to international students some of whom come from Arabic countries, I’m occasionally faced with disbelief regarding the Darwin-Wallace theory of natural selection from random variation (usually in some such form as ‘you don’t really believe we come from monkeys do you?’), I think it might be interesting, and useful for me, to trace the connections, in time and ideas, between that theory and the discovery of genes that the theory essentially led to.
One of the problems for Darwin’s theory, as first set down, was how variations could be fixed in subsequent generations. And of course another problem was – how could a variation occur in the first place? How were traits inherited, whether they varied from the parent or not? As Mukherjee points out, heredity needed to be both regular and irregular for the theory to work.
There were few clues in Darwin’s day about inheritance and mutation. Apart from realising that it must have something to do with reproduction, Darwin himself could only half-heartedly suggest an unoriginal notion of blending inheritance, while also leaning at times towards Lamarckian inheritance of acquired characteristics – which he at other times scoffed at.
Mukherjee argues here that Darwin’s weakness was impracticality: he was no experimenter, though a keen observer. The trouble was that no amount of observation, in Darwin’s day, would uncover genes. Even Mendel was unable to do that, at least not in the modern DNA sense. But in any case Darwin lacked Mendel’s experimental genius. Still, he did his best to develop a hypothesis of inheritance, knowing it was crucial to his overall theory. He called it pangenesis. It involved the idea of ‘gemmules’ inhabiting every cell of an organism’s body and somehow shaping the varieties of organs, tissues, bones and the like, and then specimens of these varied gemmules were collected into the germ cells to produce ‘mixed’ offspring, with gemmules from each partner. Darwin describes it rather vaguely in his book The Variation of Animals and Plants under Domestication, published in 1868:
They [the gemmules] are collected from all parts of the system to constitute the sexual elements, and their development in the next generation forms the new being; but they are likewise capable of transmission in a dormant state to future generations and may then be developed.
Darwin himself admitted his hypothesis to be ‘rash and crude’, and it was effectively demolished by a very smart Scotsman, Fleeming Jenkin, who pointed out that a trait would be diluted away by successive unions with those who didn’t have it (Jenkin gave as an example the trait of whiteness, i.e. having ‘white gemmules’, but a better example would be that of blue eyes). With an intermingling of sexual unions, specific traits would be blended over time into a kind of uniform grey, like paint pigments (think of Blue Mink’s hit song ‘Melting Pot’).
Darwin was aware of and much troubled by Jenkin’s critique, but he (and the scientific world) wasn’t aware that a paper published in 1866 had provided the solution – though he came tantalisingly close to that awareness. The paper, ‘Experiments in Plant Hybridisation’, by Gregor Mendel, reported carefully controlled experiments in the breeding of pea plants. First Mendel isolated ‘true-bred’ plants, noting seven true-bred traits, each of which had two variants (smooth or wrinkled seeds; yellow or green seeds; white or violet coloured flowers; flowers at the tip or at the branches; green or yellow pods; smooth or crumpled pods; tall or short plants). These variants of a particular trait are now known as alleles.
Next, he began a whole series of painstaking experiments in cross-breeding. He wanted to know what would happen if, say, a green-podded plant was crossed with a yellow-podded one, or if a short plant was crossed with a tall one. Would they blend into an intermediate colour or height, or would one dominate? He was well aware that this was a key question for ‘the history of the evolution of organic forms’, as he put it.
He experimented in this way for some eight years, with thousands of crosses and crosses of crosses, and the more the crosses multiplied, the more clearly he found patterns emerging. The first pattern was clear – there was no blending. With each crossing of true-bred variants, only one variant appeared in the offspring – only tall plants, only round peas and so on. Mendel named them as dominant traits, and the non-appearing ones as recessive. This was already a monumental result, blowing away the blending hypothesis, but as always, the discovery raised as many questions as answers. What had happened to the recessive traits, and why were some traits recessive and others dominant?
Further experimentation revealed that disappeared traits could reappear in toto in further cross-breedings. Mendel had to carefully analyse the relations between different recessive and dominant traits as they were cross-bred in order to construct a mathematical model of the different ‘indivisible, independent particles of information’ and their interactions.
Although Mendel was alert to the importance of his work, he was spectacularly unsuccessful in alerting the biological community to this fact, due partly to his obscurity as a researcher, and partly to the underwhelming style of his landmark paper. Meanwhile others were aware of the centrality of inheritance to Darwin’s evolutionary theory. The German embryologist August Weismann added another nail to the coffin of the ‘gemmule’ hypothesis in 1883, a year after Darwin’s death, by showing that mice with surgically removed tails – thus having their ‘tail gemmules’ removed – never produced tail-less offspring. Weismann presented his own hypothesis, that hereditary information was always and only passed down vertically through the germ-line, that’s to say, through sperm and egg cells. But how could this be so? What was the nature of the information passed down, information that could contain stability and change at the same time?
The Dutch botanist Hugo de Vries, inspired by a meeting with Darwin himself not long before the latter’s death, was possessed by these questions and, though Mendel was completely unknown to him, he too looked for the answer through plant hybridisation, though less systematically and without the good fortune of hitting on true-breeding pea plants as his subjects. However, he gradually became aware of the particulate nature of hereditary information, with these particles (he called them ‘pangenes’, in deference to Darwin’s ‘pangenesis’), passing down information intact through the germ-line. Sperm and egg contributed equally, with no blending. He reported his findings in a paper entitled Hereditary monstrosities in 1897, and continued his work, hoping to develop a more detailed picture of the hereditary process. So imagine his surprise when in 1900 a colleague sent de Vries a paper he’d unearthed, written by ‘a certain Mendel’ from the 1860s, which displayed a clearer understanding of the hereditary process than anyone had so far managed. His response was to rush his own most recent work into press without mentioning Mendel. However, two other botanists, both as it happened working with pea hybrids, also stumbled on Mendel’s work at the same time. Thus, in a three-month period in 1900, three leading botanists wrote papers highly indebted to Mendel after more than three decades of profound silence.
Hugo de Vries
The next step of course, was to move beyond Mendel. De Vries, who soon corrected his unfair treatment of his predecessor, sought to answer the question ‘How do variants arise in the first place?’ He soon found the answer, and another solid proof of Darwin’s natural selection. The ‘random variation’ from which nature selected, according to the theory, could be replaced by a term of de Vries’ coinage, ‘mutation’. The Dutchman had collected many thousands of seeds from a wild primrose patch during his country rambles, which he planted in his garden. He identified some some 800 new variants, many of them strikingly original. These random ‘spontaneous mutants’, he realised, could be combined with natural selection to create the engine of evolution, the variety of all living things. And key to this variety wasn’t the living organisms themselves but their units of inheritance, units which either benefitted or handicapped their offspring under particular conditions of nature.
The era of genetics had begun. The tough-minded English biologist William Bateson became transfixed on reading a later paper of de Vries, citing Mendel, and henceforth became ‘Mendel’s bulldog’. In 1905 he coined the word ‘genetics’ for the study of heredity and variation, and successfully promoted that study at his home base, Cambridge. And just as Darwin’s idea of random variation sparked a search for the source of that variation, the idea of genetics and those particles of information known as ‘genes’ led to a worldwide explosion of research and inquiry into the nature of genes and how they worked – chromosomes, haploid and diploid cells, DNA, RNA, gene expression, genomics, the whole damn thing. We now see natural selection operating everywhere we’re prepared to look, as well as the principles of ‘artificial’ or human selection, in almost all the food we eat, the pets we fondle, and the superbugs we try so desperately to contain or eradicate. But of course there’s so much more to learn….
William Bateson