Consciousness-talk emphasises the fact that a plant does not possess a brain or nervous system. Plants are brainless, thoughtless, eyeless, tongueless, noseless, earless, and therefore non-conscious. Expressed this way there is a vast chasm separating sentient and non-sentient organisms. It does not ‘make sense’ to ask ‘What is a plant’s experience of the world like?’ because without consciousness there can be no ‘experience’. Plants dont ‘think’: they also can’t ‘know’ or ‘feel’. Using this kind of consciousness-talk for plants is metaphorical fantasy at best; just a convenient shorthand way of making scientific investigation more palatable. We use metaphor all the time in science. If it helps and, preferably, if we are aware it and its limitations, then it can be useful.
Obviously plants do not see and hear in the subjective way that we do. However, there are structures and processes in plants that are comparable or analagous to the human senses because plants, like humans, must adopt ‘strategies for survival’ (functional adaptations) in response to similar environmental pressures.
It has been argued that while plants do not have subjective intentions they do have pre-conscious purposes. But this statement can also be regarded as placing undue emphasis on difference based on subjectivity. Nowadays we can accept that consciousness is intimately associated with causal physico-chemical processes: though much remains to be explained, the mind is no longer regarded as something apart from nature. From this perspective the view that subjective intentions are qualitatively different from the ‘ignorant’ mechanistic processes of the rest of nature loses its strength. Perhaps it harks back to old assumptions about the separation of mind (soul) and body, to one world consisting of mental processes and another world of physical processes. Both plants and sentient creatures have scientifically investigable reasons for their behaviour. Subjectivity (intentionality and consciousness) falls under the general heading of functional adaptation. Biologically it lies alongside other functional adaptations, it does not rise above them (see articles on reductionism).
Principle 10 – The significance of subjectivity lies more in what it can do, not what it is
Plants may not be conscious but they have the inherited ‘wisdom’ of evolution and they are not ‘totally devoid of any sensory apparatus whatever‘. Let’s look at some similarities across the living world before moving on to plant ‘senses’.
The community of life
The inanimate and animate worlds, the living and the dead, are all made out of the substance of the universe. Living organisms Like us, both plants and animals, are, literally, made out of stardust. But so much depends on the way that this stardust is organized. Organisms are matter that can metabolize, absorbing energy and maintaining a temporary individuality against the forces of inexorable entropy. When continuous replication accompanied by variation occurs in demanding surroundings then those variations that tend to harmonize or ‘fit’ with the environment (functional adaptations) tend to persist. This creates a form of selection but it is not conscious selection, it is natural selection. So far as we can tell this process only happened once so the entire community of life has diverged and radiated from the same biological stock. All organisms are related. Darwin showed that we are not uniquely different and unchanging living beings created separately and placed on earth by God. Instead we are organisms that have arisen out of universal stuff that has acquired the properties of life. And all life, all plants and animals, have evolved by descent from a common ancestor. This means that we humans are not just close relatives of the apes and chimps, we also in a broad biological way, have much in common with plants, as we shall see.
So how are we related to plants in an evolutionary sense?
The last common ancestor of plants and animals, estimated from molecular clock data, probably lived about 1.6 billion years ago. Multicellular animals evolved a bit more than 0.5 billion years ago so, although we will probably never know for sure, it is likely that our common ancestor was a single-celled organism, and modern data suggests something akin to a protozoan. We humans share many genes with plants because, as living beings with common ancestry, we share similar metabolic processes. Though evolution has followed many paths, these paths represent different solutions to the common problem of organisms adapting to environment. There is continuity through the living world.
What emerges from this characterisation is a continuum of varying organic complexity, some of which is conscious and some of which is not, but all with a common ancestor and a shared organic history of environmental adaptation. One way of adapting to surrounding conditions was to increase the environmental options for obtaining food energy by developing motility. It seems likely that the complexity of the variety of environments available to motile organisms facilitated the development of the nervous system. Motile organisms with nervous systems access the environments through their unique and limited sensory systems. But plants too must respond to their environments if they are to survive, so it does not seem too far-fetched to also describe the way they access their surroundings as a ‘sensory system’.
Plants as reasoners competing intellectually with humans
Plants have been around on Earth much longer than human beings so clearly their (pre-conscious) survival ‘strategies’ have worked, since they make up 99.7% of terrestrial planetary biomass. We generally ignore them, even though we are totally dependent on them. In spite of their pervasive presence around us and the fact that they are a major component of our food they exist as a bland background to our lives. This is a cognitive bias called ‘plant blindness‘ probably generally due to the fact that they are immobile and physically innocuous (they won’t pounce on us from behind a car) but possibly also because more of us thn ever before in history are becoming city-dwellers. But pre-conscious evolution is ‘smart’. Plants are quiet achievers that have been grossly underestimated.
There are two major ways in which plants have pre-consciously ‘outwitted’ humans.
Firstly, and most dramatically, plants produce their own life-sustaining energy. That is, they manufacture food by using water from the earth, carbon dioxide from the atmosphere, and light from the Sun while they are simply ‘standing still’ … they are photosynthetic. This is an astounding achievement comparable to the evolution of the brain. Photosynthesis produces the energy currency on which the entire natural economy runs: it is the fuel that powers the global cycle of life. Motile organisms must move around to find their food: they must hunt, because they cannot make it themselves. Their energy can only come, ultimately, from plants which are the world’s primary producers. In other words, much of the historical evolutionary effort expended by animals in developing locomotory and nervous systems was only possible because plants had previously ‘solved’ the energy problem needed for life-support. We only have brains because we did not have the pre-conscious ‘intelligence’ to photosynthesize.
Secondly it is plants that were at the core of the Neolithic Agricultural Revolution. To all intents and purposes we have cereal plants to thank or blame for our domestication, our move from wild nature to cities and civilization – probably the most significant human social transition that was a by-product of our attention to vast areas of land where we carefully tended our food plants.
But for our plant-blindness and anthropocentrism it would be blatantly obvious to engineers, architects, artists, and indeed all of us, that plant structures demonstrates a preconscious ‘intelligence’ and ‘beauty’ and functional adaptation of extreme ‘wisdom’ that far exceeds anything our conscious brains can achieve.
Perhaps by using our reasoning consciousness to study and compare the reality of plants with our own human reality we can learn a few adaptive tricks. After all, our consciousness does allow us to make reality comparisons – which is something that a plant’s ‘awareness’ of the world cannot do … at least, not in the same vivid way.
With all this in mind lets now examine the plant functional analogues of the human senses.
Our anthropocentrism has meant that we view senses from a human perspective. This may not matter provided we are aware of what we are doing. In what follows each sense will be defined in both specific human and more general biological terms. This is because our human senses must discriminate and respond to those fundamental physical components of the environment that can also impact on the lives of all living organisms: storing and passing on information from the past into the future (knowing, learning, remembering), temperature and contact (feeling), water (drinking), diurnal and seasonal changes in light (sight), chemicals in the air (smell), chemicals in solution (taste), orientation to gravity (balance), orientation to other organisms (ecology and socialisation).
What the examples that follow demonstrate is the way that nature’s grounding physiological processes are present across the range of organisms. Like physical structures they also exhibit highly complex effective and efficient functional adaptation – pre-conscious reason and purpose to the highest degree.
Ultimately sense information is translated into physiological instructions that regulate growth patterns and behaviour that can influence the capacity of the organism to survive and reproduce.
Plant blindness has resulted in plants senses being a greatly under-researched field of study. While the brain attracts many millions of dollars in research funding the objects that sustain those brains remain neglected. But this means that young botanists still have the potential to make exciting novel discoveries with many practical applications.
Knowing, learning, remembering
Without a brain, the idea of plants ‘knowing’ anything at all seems absurd. But let’s unpack what we mean by ‘knowing’ and ‘knowledge’ and ‘remembering’.
Human memory is a subject of active research. Cognitive scientists distinguish between three kinds:
procedural memory – non-verbal or instinctive adjustment to external factors (riding a bicycle)
semantic memory – the acquisition of concepts (learning maths) )
episodic memory – the recollection of special events (21st birthday party or breaking a leg). )
Only procedural memory does not require a brain.
Another arrangement into different kinds includes:
sensory memory – which filters input from the senses)
short-term memory – which can briefly hold up to seven or so objects, like numbers, in consciousness)
long-term memory – which retains memories, sometimes for life)
muscle-memory – the unconscious memorization of movements like playing a scale on the piano or tying a shoelace)
immune memory – when our immune system remembers past infections. )
Only immune memory does not require a brain.
It also helps to think of memory as involving three key processes:
encoding – memory formation)
storage – retention of information)
retrieval – the recollection of information)
This is the way we understand memory in computers. Of course, memory is largely treated as a human phenomenon this category can be widened to include plants and other non-human organisms when ‘knowing, learning, and remembering’ are defined as ‘manipulating information’. We can then usefully apply the last three categories (encoding, storing, and retrieval) to organisms in general.
Before we can have conscious memories, we need organisms and one feature of organisms that we tend to take for granted, and Aristotle did not, was the simple fact that ‘like begets like’. Humans do not give birth to fish.
We now know that the community of life almost certainly had a single beginning – that the entire complexity of life in all its complexity and diversity arose by descent with modification from a common ancestor. Only in the mid- 19th century, with Darwin’s theory of natural selection, did the mechanism for encoding this information become a central problem for biology when it was proposed that species changed, or evolved, over time.
Darwin outlined a mechanism whereby small variations in nature would be ‘selected’ (encoded into the characteristics of a particular organism) as a result of differential reproduction. That is, those variations that benefitted the organism would tend to persist under multiple replication. Beneficial variation was essentially a better ‘fit’ with the environment, improved functioning. Over thousands of generations information about the environment was being encoded in the genetic make of different organisms that gradually changed over time. The precise way thi sgenetic information remained unknownfor another century.
It was only in the 1950s that the astounding foundational structural simplicity of inheritance came to light when Watson and Crick identified the replicating double helix of DNA (present in the chromosomes found in the nucleus of every living cell) consisting of nucleotides in a particular sequence that pass from generation to generation, in egg and sperm, the coded information, as genes, that trigger the formation of those proteins that uniquely determine the physical structure and developmental processes that we associate with a particular species.
Information encoded in DNA is the plant’s blueprint for the future: here are the instructions that detail the way a new organism will unfold in a developmental process that proceeds from juvenile to mature adult as genes switch protein metabolism on and off to produce the structures by which we recognise one species rather than another.
A good example of plant memory occurs in the Venus’s Flytrap. When a fly first touches a hair an electrical potential passes from cell to cell which concentrates the ions in the cell, taking about 20 seconds to return to normal. If the hair is touched again the action potential is sufficient to trigger the trap to shut. This stops the trap shutting unnecessarily in a process that is vary similar to that which occurs in neurons.
Memory seems crucial to our human exisatence, total absence of memory is the subject of thriller movies and is hard to imagine. Pre-conscious plants have no memory at all and yet they have persisted, so how have they compensated? Like all pre-conscious organisms plants possess a rich arsenal of pre-conscious memory analogues. The important capacity from a biological standpoint is not whether we are ‘aware’ of the past but whether the organism has the capacity to incorporate past events into strategies relevant to the present. We call this process ‘learning’ but from a biological perspective From this perspective, whether we refer to these connections with the past as ‘recollection’, or ‘memory’ becomes incidental.
So how can organisms possibly possess these referencing to past events that we call ‘memory’?
Memory is one of the wonders of the biological world. Our genetic makeup, our genotype, carries information about our entire evolutionary history. Most obviously it passes this information from generation to generation as a blueprint that is unique to each kind of organism. A single generation requires a finely-tuned developmental sequence for structures and processes.
Just as humans have (conscious) reasons for their behaviour so a plant has (unconscious) reasons for developing structures that benefit its short and long-term existence. There are beneficial reasons (purposes) for functional design in nature even though these are not conscious reasons. We should not be ashamed of natural teleology even though consciousness-talk may not be appropriate to describe it.
The single key difference between plants and animals is that animals are motile: they can towards benefial influences in the environment and away from disadvantageous ones. Plants have had to confront weather, predators, and disease directly with no means of escape. We might assume that the wide diversity of sensory input needed to cope with mobility has ultimately given rise to nerve tissue. But plants, too, have evolved complex sensory and regulatory systems.
We believe we behave in certain ways due, on the one hand, to our unconscious impulses and intuitions and, on the other, our conscious deliberations. Consciousness, if we believe it has a physical basis that can be explained scientifically, is in this sense at one with all physical phenomena. Consciousness is thus an epiphenomenon of matter.
Plants, like all living organisms today, are a product of evolution extending back 3.5 billion years. Contained in their genes are the ‘memories’ of past environments since it is the historical response of every organism to its ancient environments that has produced the physical bodies they possess today. This we can speak of as genotypic ‘memory’, the inherited memory of the past. But there is also phenotypic memory, the ‘remembered’ response to the environment in the present. This will be discussed under subsequent headings.
It seems that although we have a virtually infinite store of memories the proteins involved in memory maintenance are few.
In humans the transfer of electrical signals between sensory neurons (like mechanoreceptors, nociceptors, pain receptors) and the brain are converted into mental sensations with emotional associations. Without a nervous system plants cannot experience pain or any other subjective mental states. But this does not mean that plants are indifferent to temperature or contact. The functional analogy between the ‘feeling’ plants and animals is the biological capacity to respond to temperature and contact.
Plants respond to hot and cold: they also respond to touch (especially climbing plants with tendrils, like Star Cucumber, Sicyos angulatus. Some plants are ten times more sensitive to touch than ourselves. The Venus’s Flytrap ,C p. 70, Dionaea muscipula, has trigger hairs that initiate the closing of the ‘trap’ that takes less than a tenth of a second before enzymes are secreted that dissolve the unfortunate prey. But this reaction only occurs when two hairs are touched in succession. This initiates an action potential like that which occurs during muscle contraction in a sequence of events that precludes shutting due to rain or wind and the capture of objects with no nutritional value. After closing, five or more stimulations of the trigger hairs starts the production of digesting enzymes.
Further insights were gained from the Sensitive Plant, Mimosa pudica, whose leaves droop and leaflets fold when they are touched. This movement was also generated by an action potential that radiated down the length of the leaf (affecting the concentrations of sodium, potassium and calcium ions). At the base of leaves and leaflets are cells called pulvinus cells which under usual conditions are held rigid by water pressure. The electric signal causes water to flow out of the cell which then becomes flaccid so the leaflets fold, gradually returning to normal.
There is, it is now thought, a general plant response to touch called (unfortunately) thigmomorphogenesis. The plant Arabidopsis thaliana touched several times a day in the laboratory becomes squatter and flowers later than one untouched. It has been found that the spraying of leaves alone, the physical sensation, turns on certain genes. These genes were those involved in calcium signalling and the important calcium-binding protein calmodulin , calcium being an important regulator of electrical discharge, notably cell turgor. This is currently an active field of research.
Plants do not feel pain but damage one leaf on a tomato plant and this produces a response in the unwounded leaves – the transcription of genes called proteinases and that the signal initiating this response is electrical. For example damage from insects generates an electrical signal that promotes the formation of the defence hormone jasmonic acid.
In these and many other ways plants respond physiologically to changes in temperature, and respond to contact with changes in growth and sometimes the mechanical cellular conditions like turgor.
All living organisms need water. Plants use it for photosynthesis, as a medium of chemical transfer, and for maintaining turgour or rigidity (they wilt when needing water). Sometimes it facilitates movement. More water is needed on a hot day when transpiration and evaporation have a cooling effect. The volumes of water involved can be surprisingly large. A mature oak tree can transpire more than 400 litres of water on a summer day.
Anyone who has cleared the drains around their house and garden will have noticed how roots somehow sense and grow towards water. They will also increase root growth and penetrate deeper in droughts. Much remains to be learned about the physiology of this response. Carefully designed experiments have demonstrated that drought-stressed plants can chemically communicate water stress between roots through the soil. (see publications of the Novoplansky Research team at Ben Gurion University Israel) Further, chemicals released into the soil may also be influencing flowering times and the behaviour of one plant in the presence of another, although the exact agent of communication has not been isolated,
Light is a basic physical property of the universe and one to which every organism on planet earth must adjust.
We associate the word ‘sight’ with eyes and our general optical system but from the perspective of functional analogy between plants and animals ‘seeing’ is simply the biological capacity to respond to light.
Our optical system uses light from a narrow range of the electromagnetic spectrum to create finely discriminated pictures on the retina. However, our bodies respond to light that lies beyond the visible spectrum – in the ultraviolet and infrared regions – so, in the biological sense of seeing, our bodies see invisible light!
Our eyes shave chemical protein photoreceptors in structures called rods and cones that give us a visual resolution to about 130 megapixels (p. 11) much greater than today’s digital cameras (8-12 megapixels in 2018). The rods contain rhodopsin which allows us to see at night under low light conditions (to discriminate light and shade) but not colour. Cones containing photopsins (one for each of red, blue, and green) discriminate different colours in bright light. A fifth light receptor called cryptochrome regulates our internal clock.
Plants can discriminate light intensity and colour composition, direction, and duration of its presence and absence (photoperiod).
Darwin’s second-last book was The Power of Movement in Plants (1880) which in which he worked with his son Francis. Here Darwin describes his investigation into the way plants bend towards light (phototropism). The great German plant physiologist Julius von Sachs had noted in 1864 that this response was initiated by blue light (it was later found that plants have at least one blue photoreceptor, phototropin). The Darwins demonstrated that phototropism results from the effect of light on the tip of a plant shoot, this information being transmitted to the mid-section of the shoot which then bends.
Several decades later it was discovered that plants respond to the period of exposure to light and that, depending on the length of exposure to light certain life processes, like flowering, would be initiated. Following this research plants have been divided into ‘long day’ and ‘short day’ plants. The onset of shorter days after summer would stop growth and bring the onset of flowering and later fruiting. Short day plants include chrysanthemums and soybeans while irises and barley are long day plants. It was later found that it was not the light that plants were responding to but the length of the continuous period of darkness. Even a brief period of light within the darkness could interrupt the cycle, but only if it was red light. This knowledge has enabled flower production to be timed precisely, a boon for floristry. Detailed research has shown that it is blue light that responds to light direction and red light that responds to photoperiod. Further, far-red light (that appears naturally at dusk) could cancel the effect of red light.
In the early 1960s it was found that a single chemical photoreceptor was responsible for the red and far-red effects and it was named phytochrome. Red light primes phytochrome to receive far-red which then reverses this effect. This response corresponds in nature to the last light at dusk being far-red while the first light in the morning is red. The site of the photoreceptor is not the shoot tip but the leaves and illuminating a single leaf will produce the response in the entire plant.
Later research has extended our knowledge of plant photoreceptors beyond phototropin and phytochrome. We now know that, for example, in the plant Arabidopsis there are at least 11 photoreceptors that trigger: germination, bending, flowering, photoperiod, shade responses and more.
Plant and human photoreceptors are similar in that they all consist of protein connected to a chemical dye that absorbs light. Both animals and plants contain blue-light receptors called cryptochromes and these control circadian rhythms. We think it likely that circadian clocks arose in evolution at the stage of single-celled organisms (cryptochrome is found in both bacteria and fungi today), protecting them from the potential damage when exposed to UV radiation.
Undoubtedly the greatest evolutionary adaptive achievement of plants is the way they have harnessed light to provide the energy needed to maintain the biological structure that resists the forces of entropy. Light energy is used to turn water and carbon dioxide into sugars (photosynthesis) and this food energy not only fuels the plant but provides the food energy for all animals. We wonder at the marvel of the human brain and consciousness, but our brains would not have been existed without the prior wonder of photosynthesis.
For humans ‘smelling’ is the detection of odour or scent through chemicals that influence the olfactory nerves connected to smell receptors in the nose and brain. We have hundreds of different receptors each adapted to particular volatile chemicals (e.g. menthol, putrescine) although the aromas we are aware of generally consist of a mix of several chemicals. The smell we call ‘peppermint’ includes menthol and about 30 other chemicals. (C. pp. 28-29) Olfaction is linked into our limbic system, the emotional control centre and an ancient module of our brains. Olfaction may be conscious as when we recoil from unpleasant smells but we also communicate subconsciously with chemicals called pheromones which relate to anger, fear, and mating. One well-known subconscious example is the synchronization of menstrual cycles to that of the dominant female in a group.
Since plants do not have a nervous system then we say that they cannot ‘smell’. But plants do send, receive, and respond to chemical signals. So, from the perspective of a functional analogy ‘smelling’ is the biological capacity to respond to chemicals in the air. Plant ‘smelling’ then becomes the capacity to respond to chemicals of a special kind, those which to humans have odour or scent.
The most obvious way, to us humans, that plants use aerial chemicals is the way that flowers give off perfumes and smells, pleasant or unpleasant to us humans – like the sweet smell of jasmine and the putrescent smell of Titan Lily. We notice unique smells at a market in the fruits and vegetable sections. This is all part of the complex chemical interaction that occurs with animals and insects acting as pollinators and seed-spreaders.
Plant responses to chemicals in the air were noticed by the Ancient Egyptians. A few crushed figs in a batch noticeably accelerated general ripening. Ancient Chinese found that burning incense would also encourage immature pears to ripen.  In 1924 American Frank Denny found that incense smoke contains ethylene and that ethylene gas promotes ripening, in the case of lemons at the minute concentration of 1 part per 100 million. (p. 30) Later, in 1930, Richard Gane in Cambridge found that the air around ripening apples contains ethylene and by 1931 ethylene was widely regarded as a plant hormone universally responsible for ripening. Ecologically this collective ripening would attract animals to the fruit and thus to disperse the seed. Only one receptor for volatile chemicals has been found in plants – the ethylene receptor – but there coulday be many more that convert volatile chemicals into physiological responses.
It has also been found that parasitic plants (that tap the sap flowing through phloem tissue) as young seedlings detect the presence of other plants, homing in on the stem, even if they first touch other parts of the host plant. So, for example, the chemical beta-myrcene in tomato plants attracts the parasitic plant dodder, while (Z)-3-Hexenyl acetate, found in wheat, acts as a repellent.
Leaves may become unpalatable to pests if they contain phenolic and tannic chemicals and researchers have found that damaged trees communicate to trees nearby using airborne chemical signals – effectively ‘warning’ their neighbours. The proximity of attacked leaves to undamaged leaves acted as a stimulant for the undamaged leaves to manufacture chemical protection. Aerially transmitted chemicals have been isolated and found to be methyl salicylate and the similar methyl jasmonate, leading to the conclusion that salicylic acid is a defence hormone that triggers the plant’s immune system and that soluble salicylic acid can be converted into volatile methyl salicylate and vice-versa. Plants ’taste’ salicylic acid and ’smell’ methyl salicylate since we taste soluble molecules on the tongue and smell volatile molecules on the nose.
The human sense of taste is very similar to our sense of smell, the distinction being that we smell volatile chemicals and we taste soluble ones. This difference is greatly accentuated by our smelling noses and tasting tongues. From the perspective of a functional analogy ‘tasting’ is the biological capacity to respond to chemicals in solution.
The smell of lemon sensed by olfactory receptors in our noses comes from limonene while the sour taste, its flavour, comes from citric acid acting on taste buds in our mouth and throat, these being of five main kinds: sweet, sour, salt, bitter, and umami passing to a gustatory nerve connected to the brain. Taste operates like smell with chemicals locking onto particular proteins – like sodium binding to the salt receptor and initiating a salty taste signal from the taste centers of the brain . (C. p. 50).
Plants too distinguish between different soluble chemicals especially as they are taken up through the roots which absorb water, minerals, and chemical ‘messages’ from other roots and micro-organisms.
Our intuition is that plants draw their sustenance from the soil through their roots. And indeed it was some time before it was realized that they produce both food and structural materials using photosynthesis (light fusion) which combines carbon dioxide and water to form sugars, followed by proteins and complex carbohydrates. What does come from the soil are the macronutrients nitrogen, phosphorus, potassium, calcium, magnesium and micronutrients iron, zinc, boron, copper, nickel, molybdenum, and manganese. Magnesium is found at the centre of each chlorophyll molecule like iron is found at the centre of haemoglobin in the blood. Plants don’t ‘taste’ these chemicals with ‘taste receptors’ but each cell has mineral receptors as, for example, proteins on the outside of the cell that bind and transport each of the macro- and micronutrients into the roots. In humans taste and nutrition are physically separated, while in plants nutrient-binding enables transport throughout the plant thus combining sensing, signalling, and nutrition.
The quantities of minerals absorbed is biologically regulated with sugars passing down from the leaves through the central core phloem tissue and water up to the leaves along the surrounding xylem tissue. What passes into this vascular cylinder is regulated by the surrounding endodermis. So mineral ‘tasting’ occurs first at the root surface and then at the endodermis not unlike the way a human cell maintains its mineral homeostasis. (C. p.54)
Plant ‘taste’ became of special significance to humans during the 20th century industrialized agricultural revolution with its high-yielding cultivars, high-tech irrigation, and clever use of fertilizers which depend on plant ‘taste’ for water and chemicals. In the early 20th century came synthetic fertilizers following the awarding of the Nobel Prize to Bosch and Haber for a method of converting atmospheric nitrogen to ammonia and nitric acid. The human ‘purposive’ development of dwarf cultivars with thick stems (shorter stems and fewer leaves) holding heavier grains simply enhanced the natural purposive structure of the plants to human, not plant, ends. Between 1960 and 1980 the use of synthetic KPN fertilizer increased dramatically as increased agricultural yields in the West were followed rapidly by those in Mexico, India, China, Vietnam and elsewhere. Norman Borlag was awarded the Nobel Prize in 1970 for developing high-yield cultivars that would lift much of the developing world out of poverty, notably in India and Pakistan. Some caution is warranted as P and K are non-renewable and monocultures compromise genetic diversity. Genetic engineering will further refine both cultivars and the calculation of quantities of water and nutrients needed to achieve yields. All this requires a better understanding of plant ‘taste’. (C. pp. 62-68)
Sound is a series of pressure waves propagated through air, water, or even solid materials.
Humans hear by detecting vibrations in the air received by the auditory apparatus in the ear. This converts to the movement of the ear drum and hairs in the ear connected to special auditory nerves that record volume (loudness or amplitude) and pitch (wavelength or frequency) generating electrical action potentials that pass to the brain.
If plants can ‘see’ without eyes, can they ‘hear’ without ears?
In spite of many claims that plants respond to music, most notably Mozart, although Led Zeppelin, Jimi Hendrix and other old time favourites have been given an outing – but the hard evidence is slim. Plant preferences seem to often coincide with the musical preferences of the experimenters.
In 2000 the full genome of the research plant Arabidopsis thaliana was sequenced. It took 300 researchers about four years at a cost of c. 70 million dollars to find 25000 genes consisting of c. 120 million nucleotides (wheat has the same number of genes but 16 billion nucleotides. Humans have about 22,000 genes in 2.9 billion nucleotides) . Today entire plants genomes can be sequenced in less than a week. Arabidopsis shares many genes with commercial crops, making it extremely valuable for genetic engineering.
Arabidopsis genome contains several genes involved in human diseases and disabilities while humans have several genes associated with plant development – remembering that the genes have nothing to do with biological functions, only their clinical outcome.
However it may be no coincidence that plants, being anchored in the ground, lack aural apparatus because historically they did not need hearing to the extent that motile animals did. Can you think of sounds that would confer some evolutionary advantage?
A 1973 a book by author-journalists caused a small sensation by explaining plant behaviour using emotional terms and dubious experimental evidence bordering on the occult. Rightly condemned, it was nevertheless a book that drew on our intuitive awareness of the purposive character of nature so roundly condemned at that time.
We are now more receptive now to teleological language provided the experimental evidence is sound since we recognise that much of biology is indeed reverse-engineering, the elucidation of what the structures and processes that make up an organism are ‘for’. Two examples …
Sounds of music being of recent evolutionary time are only likely to be to be incidentally influential, if at all.
Apart from movements of plants, like the Venus Fly Trap and Sensitive Plant, movement is relatively slow although time-lapse photography can reveal remarkably animal-like intentional behaviour, seasonal, circadian, and other cycles of activity. Research is suggesting that sound of running water may have a role to play in roots seeking water and sewer pipes, not always the damp itself. Sound, being vibration, may trigger pollen release for visiting bees. There may well be much left to learn about acoustic signals and their influence on plant physiology.
We humans orientate ourselves and our various limbs using proprioceptors that are found throughout the body in muscles, ligaments, and tendons – so that we can walk, catch balls, scratch itches, touch our nose with eyes closed, and maintain equilibrium while playing sports. We have noted how plant movement occurs as a response to light, gravity, and odour. In the 1930s the growth-promoting chemical auxin was isolated as the movement hormone.
The nearest we have to a specialist organ are the semicircular canals of our ears set at right angles, full of fluid, these act like a gyroscope. Another region called the vestibule has hairs and small unattached bony bodies called otoliths that sink sink under the force of gravity. Together, as a system, these provide the precise information needed to maintain balance.
Darwin was not only a great theoretician, he spent many hours carefully observing skillfully devised experiments, so he was a great experimental biologist too. One of his major works in this vein was The Power of Movement in Plants (1880). In 1758 Frenchman Duhamel noted the way plants orient themselves with root down and shoot up, no matter how they are moved (geotropism, gravitropism). This theory was strengthened by Royal Society aristocrat Thomas Knight who showed the same orientation when subjecting seedlings to centrifugal forces on a rotating wheel. Darwin demonstrated experimentally how it was the tip of the root that responded to gravity then sending a signal upwards to produce the bending response much higher. We now know that it is two kinds of tissues that respond to gravity, in the root it is the cells of the root cap and in the stem it is the cells of the endodermis. Both root and endodermis cells contain spherical statoliths that function much like the otoliths of our ears. Plants taken into space where, in the absence of gravity, they showed no gravity response.
We are most aware of movement that occurs within a particular time scale. This is presumably and adaptation to our needs as motile organisms. But we know that plants move, albeit more slowly. Their leaves unfoold and orientate themselves, flowers open and close, and stems circle and bend. We become more attuned to plant movement through time-lapse photography. No doubt we have all wondered about plant movement but it took someone like Darwin to subject it to experimental observation. He discovered that all plants move in a spiral oscillation he called circumnutation, the time taken and reach of the spiral is consistent within a species but variable between species and dependent on both genetic an environmental factors. Tulips take four hours, wheat two hours. Bean shoots have a radus of 10 cm, strawberries a few millimetres. Experiments on the space shuttle Columbia in 1983 showed that in Sunflower seedlings these gyrations continue in the absence of gravit, but gravity is needed to reach the full expression which is more complex than a simple response to statoliths.
Tropisms can counter one-anothers’ effects leading to optimal positioning. Humans and plants both use sensors to adjust balance and position. We remember our movements but can plants do the same?
Life preserves diversity in a world that favours sameness.
Causes, reasons, purposes, information – abstract objects
Plants do not ‘like’ or ‘prefer’.
Though there is the hard problem of consciousness (the difficulty in accounting for the sensation of self-awareness of ‘I’) science nevertheless proceeds on the assumption that it is possible to investigate all the activities of the brain.