Select Page

CONTEXT

This article describes plant senses by comparing them to human senses. A related article, Plants make sense describes the influence of plants on the evolution of human senses.

Plant sense

Plants comprise around 99.7% of the terrestrial biomass of planet Earth, and that’s a whopping 450 gigatonnes.[6] Though they lack a nervous system and brains, it is evident that their (un-conscious) evolutionary survival strategies have worked well. Plants surround our cities and our dependence on them is total. Indeed, there is strength in the argument that plants domesticated humans rather than the other way around. By adopting the sedentary lifestyles necessary for agriculture, humans followed the path of civilization. Later, modern industrial societies – built using coal, oil, and gas – were founded on ancient plant energy.

Plants, then, are the paradigmatic quiet achievers – and yet we tend to ignore them. Why? We need a careful diagnosis and appropriate treatment.

Well, they are immobile, speechless, and non-threatening organisms that decrease in significance as humanity moves into its cities. Sure, plants have ornamental attributes but, all-in-all, they are a bland background to lives occupied with more important matters.

There is a simple fact here that we easily forget. Plants can exist without us, but we cannot exist without plants.

Our indifference to plants is a cognitive bias sometimes called ‘plant blindness‘,[4] although ‘plant awareness disparity’ has been suggested as a more politically correct expression (avoiding ableism and a disability metaphor) because it encourages the educational correction of our emphasis on action-packed animals (zoocentrism and zoochauvinism).[5]

One way that education[7] can increase our empathy for plants is to accentuate our close evolutionary connection. This has been described as ‘anthropomorphizing’ plant lives.[8] But ‘anthropomorphization’ is not only an extraordinarily long word, it is also used in a derogatory way.

In this article, I examine the idea of ‘anthropomorphizing’ plants as a method for bridging the gap between ourselves and our green relatives.

Evolution

We no longer believe that each species of organism was uniquely placed on Earth by God. Only in the last 150 years have we recognized that everything in the universe emerged from a point source at the Big Bang 13.7 billion years ago, and that the entire community of life arose from a common ancestor about 3.5 billion years ago. This means that all the matter in the universe is connected to a common origin, and its living component is tightly connected by a common heredity.

So how are we related to plants, in an evolutionary sense?

Multicellular animals evolved a little more than 0.5 billion years ago with the last common ancestor of plants and animals, estimated from molecular clock data, dating back about 1.6 billion years.  Though we may 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 many similar metabolic pathways.

What emerges from this characterization is a continuum of varying organic complexity, but all with a common ancestor and a history of radiation and diversification by environmental adaptation.

Anthropomorphism – human-talk

We accept that consciousness can exist in degree: that dogs, fish, and even worms have consciousness, albeit consciousness that is different from our human consciousness. We could use a specialist term for each of these kinds of consciousness but, instead, we accept the semantic breadth of the word ‘consciousness’, while using our human consciousness as a reference point. By thinking about consciousness in human terms we are being anthropomorphic. But that does not mean there is no consciousness in other organisms. All it means is that their consciousness is different from ours.

Although nature exists in graded complexity – a staged continuum – we like our ideas to be clear and distinct. It is simpler when properties are either present or absent, rather than present by degree. If we regard consciousness as a strictly human property, then it (and its associated properties – like reason, learning, memory, and purpose) cannot be present in other organisms. So, what are we to call these ‘similarities’ or gradations that occur in nature? Well, we could give them all separate names in an elaborate system of nomenclature. But that is too complicated. Instead, we are inclined to describe them, confusingly, as ‘metaphor’. It is only ‘as if’ they were human. They are the anthropomorphisms of human-talk.

We ignore the reality of ‘gradations’ in nature by saying that it is only ‘as if’ they were human. In a strange inversion of reasoning that is anthropocentrically based, we perform a mental leger-de-main. We assume that because only our human minds can understand or grasp these similarities, then these same similarities can only exist in our minds. We are conflating non-human and non-existence. Unfortunately, by describing biological ‘similarities’ or ‘gradations’ in this way, we are not just diminishing their significance, but denying their reality. 

Since these gradations are real in nature, we find ourselves constantly using ‘anthropomorphic’ language – partly because of our anthropocentric bias and the  occasional use of genuine metaphor, partly because we do not have the vocabulary to express clearly what we are trying to say, and partly because we do not have referents for these words – we do not know what it is like to have the consciousness of a fish. Without words to describe these gradations of meaning we inevitably revert to human-talk. This creates both semantic and conceptual confusion.

Human-talk will not go away because it attempts to encompass similarities that exist in nature, not just in human minds. In practice we have simply adopted anthropomorphic words that are, by convention, of greater and lesser acceptability in scientific discourse. Just as we see in evolution an increasing differentiation of structures so, in language, we have a parallel differentiation in meaning.

From now on this article will use ‘human-talk’ for the reasons outlined above, but mainly because practical alternatives are scarce and human-talk can be regarded as a signal for verbal homologies.

The following is a glossary, not of metaphor, but some of the language of graded biological reality expressed in human-talk.

Adaptation                  –    learning, intelligence
Natural selection       –   self-correction, reason
Autonomous activity –   behaviour, agency, consciousness
Function                       –   purpose
Information storage  –   memory
Recording variables   –   representing, forming a cognitive map, thinking

Sensing the world

Organisms have persisted on Earth for about 3.5 billion years (in affirmation of the biological axiom of survival and reproduction) because they have adapted to their surroundings.

We humans recognize about six major ways of sensing the world around us, but other forms of life has explored an almost infinite number of ways of doing this. How do we get our bearings on everything there is around us outside our bodies?

Living organisms use many strategies to survive, reproduce, and flourish and, since life has persisted for 3.5 billion years, these sensory adaptations have worked for those organisms that exist in the world today.

Here we want to compare the strategies adopted on two lines of evolution, one resulting in anatomically modern humans, and the other the flowering plants.

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’.

We have separated ourselves from plants in many ways, not least of which is a vocabulary that makes us ‘different’. Here is some difference-talk . . .  words that separate us from plants.

Behaviour, talking, communicating, sentience (sensation), intention, value, memory and hindsight, learning, foresight, reason, valuation, purpose, self-awareness.

We humans are sentient creatures that can think abstract thoughts and reflect logically on our own existence. This is why, over 2000 years ago, philosopher Aristotle described us as the uniquely ‘rational animal‘. It was reason, he claimed, that made humans distinct from other sentient creatures.

Aristotle endorsed a ‘ladder of life’ representation of existence that was accepted and modified slightly by Christianity. It was a powerful representation of the world that still lingers today.

God was at the top, humans just below, and the rest of the living world arranged downwards in decreasing importance and complexity leading to inert rocks and, perhaps, an underworld. This was a ladder of graded complexity and moral worth. Plants were moral inferiors. They lacked mobility and sentience and were therefore correctly subjugated, placed on Earth by God for the benefit of humankind.

Mobility

Motility exposes an organism to a variety of environments. To obtain food from diverse environments means identifying sources (sensory system) while negotiating potential threats to life. The sensory system must convey all this information about the environment around the body (nervous system), and there must be some means of coordinating that information into a beneficial response (central nervous system or brain). We understand this explanation of animal construction by adopting an ‘intentional stance’ (see below) and describe the animal’s response as its ‘behaviour’.

The presence of a nervous system with a brain is a major biological difference between plants and animals which, at its most sophisticated extreme, has resulted in consciousness (sentience) and rational thought.

Even so, the environmental challenges are broadly the same for both plants and animals, the language we use to distinguish between their different evolutionary approaches tending to obscure their similarity.

The following account compares the responses of plants and animals to the same environmental challenges.

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.

Feeling

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 ,[8]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.

The 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.

Seeing

Sight or vision is the capability of the eyes to focus and detect images of visible light and generate electrical nerve impulses for varying colors, hues, and brightness.  Visual perception is how the brain processes these impulses – recognising, differentiating and interpreting visual stimuli through comparison with experiences made earlier in life.

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[2] (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.

Smelling

Smell or olfaction is our ability to detect scent – chemical, odour molecules in the air.  Our olfactory system begins in our nose which has hundreds of olfactory receptors.  Odour molecules possess a variety of features and, thus, excite specific receptors more or less strongly.  This combination of excitement is interpreted by the brain to perceive the ‘smell’.

How olfactory information is coded in the brain to allow for proper perception is still being researched and the process is not completely understood, however, what is known is that the chemical nature of the odorant is particularly important, as there may be a chemotopic map in the brain.

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.[3] (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. [4] 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.[5] (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.

Tasting

Taste, or gustation, refers to the capability to detect the taste of substances such as food, certain minerals, and poisons, etc. The sense of taste is often confused with the “sense” of flavour, which is a combination of taste and smell perception.

Humans receive tastes through sensory organs called taste buds concentrated on the upper surface of the tongue. There are five basic tastes: sweet, bitter, sour, salty and umami.

Tasting

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 .[6] (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.[7] (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’.[8] (C. pp. 62-68)

Hearing

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 …

Drinking

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,[1]

Hearing

Hearing, or audition, is the ability to perceive sound by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear.   As with sight, auditory processing relies on how the brain interprets, recognises and differentiates  sound stimuli.

Touching

Touch, or somatosensory, is a perception resulting from activation of neural receptors, generally in the skin including hair follicles and a variety of pressure receptors respond to variations in pressure (firm, brushing, sustained, etc.).

The somatosensory system is a diverse sensory system that is spread through all major parts of our body. At its simplest, the system works when activity in a sensory receptor is triggered by a specific stimulus (such as heat); this signal eventually passes to an area in the brain uniquely attributed to that area on the body and this allows the processed stimulus to be felt at the correct location.

Gravity – vestibular system

The vestibular system explains the perception of our body in relation to gravity, movement and balance. The vestibular system measures acceleration, g-force, body movements and head position.  Examples of the vestibular system in practice include knowing that you are moving when you are in an elevator, knowing whether you are lying down or sat up, and being able to walk along a balance beam.

Proprioception

Proprioception is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement.  This sense is very important as it lets us know exactly where our body parts are, how we are positioned in space and to plan our movements.  Examples of our proprioception in practice include being able to clap our hands together with our eyes closed, write with a pencil and apply with correct pressure, and navigate through a narrow space.

intention, valuing, memory and hindsight, learning, foresight, reason, valuation, purpose, self-awareness.

The following account examines the conceptual distance that exists between humans and plants.

Positional orientation

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.

Energy

Life processes are sustained using energy. In simple terms, organisms have solved this energy requirement in two ways – by finding it (motile animals seeking food) or making it (plants capturing sunlight and converting it to chemical energy). This gives us a primary distinction between animals and plants that has generated momentous evolutionary consequences. 

Hylomorphism

Time has not eroded the admiration and respect we feel for our own human mental achievements. The human brain has the capacity for hindsight and foresight, language, and abstract reasoning. It was our mental faculties that helped us create complex social organization and the technology that has given us mastery over other organisms and planet Earth. Perhaps it was not plants, as implied in the introduction above, but our human capacity for reason.

We know that the brain is just physical material, matter. Like the rest of our body, it is made of stardust, and yet it is organized in a way that has resulted in it becoming aware of itself – and how amazing is that?

In the admiration we feel for ourselves and our self-conscious brains, we have forgotten that it was the imaginative and creative power of mindless evolution, of nature itself, that provided us with our brains. The human construction of computers pales into insignificance when compared with this brilliant feat of creation.

We did not invent the brain, mindless nature did. Nature did this from scratch. It did it by itself. It did it without reflection, and it did it without the assistance of another mind or mental tools. It did it without awareness.

This article examines the ill-defined boundary that exists between the conscious and unconscious, both in the world itself, and in the language that we use to describe it. It investigates our anthropocentric emphasis on our uniquely human subjectivity and how this has ignored our debt to nature by building linguistic and conceptual barriers between ourselves and other organisms. And it examines the way our human-centredness has coloured our scientific assumptions about the nature of reality.

In what follows, I use scare quotes to indicate words whose meaning is inextricably linked to human consciousness and whose use, when applied to non-human organisms is ambiguous but, by convention, treated as metaphor.

Reality & consciousness

Our human understanding of the world, our ‘reality‘, is what our uniquely human sensory and cognitive apparatus communicates to us about what is ‘out there’ in the external world. Being a uniquely human experience of the world does not mean that what we make of the world is mistaken or illusory, but it is, nevertheless, our human interpretation, our uniquely human experience.

In spite of all our exceptional mental capacities, we have cognitive and perceptual limitations. So, for example, we cannot experience the range of smell or sound that is experienced by dogs, see with the optical acuity of an eagle, or do complex arithmetic like a computer. Our reality is different from that of a cow or fish, and despite our exceptional mental gifts, there are other, sometimes superior, ways of sensing particular aspects of the world.

Venus Flytrap

Dionaea muscipula – Venus Flytrap – showing trigger hairs

Courtesy Wikimedia Commons
NoahElhardt – Accessed 26 April 2018

The nature of the matter

It is only in the last 150 years or so that our best science has acknowledged the evolutionary continuity and connectedness of all matter, firstly, because of its ultimate origin from a point source at the Big Bang 13.7 billion years ago (a theory that only became scientifically accepted in the early 20th century) and, secondly, because of the origin, and continuity, of the entire community of life as  derived from a single common ancestor and explained in Darwin’s On the origin . . . of 1859.

We humans have, for millennia, distinguished between four basic kinds of matter based on degree of physical complexity and their mental attributes or lack thereof: the inanimate, the un-conscious, the conscious, and the rationally self-aware.

1. INANIMATE – the world of non-living material objects as distinct from the ANIMATE or living (semi-autonomous) organisms that are the product of natural selection.

In the animate world we distinguish between three kinds of organisms based on their mental attributes.  and characterized by the emergence of :

2. UN-CONSCIOUS – most of the community of life – those organisms that we do not regard as being ‘conscious’ and certainly including all plants

3. CONSCIOUS – Sentient animals – the subset of the community of life with a central nervous system that we believe is conscious, aware, and sensitive to pleasure and pain.

4. RATIONAL CONSCIOUSHomo sapiens, human beings – those sentient animals with self-awareness, the capacity for hindsight and foresight, language, abstract reasoning, and capacity for complex social organization and sociality.

A significant characteristic of these four groups is that, though very different at their extremes, in nature they grade into one-another at their boundaries. So, for example: it is debatable whether viruses are ‘living’ or not; we are unsure whether we worms or snails are ‘conscious’; and there are aspects of the behaviour of chimps and bonobos that are ‘reason-like’, and so on.

Domain language

These four states of matter intergrade in nature, but we differentiate them by using domain-specific and technical language as science tries to improve our representation of the world by using precisely defined or new words for categories that enhance our predictions, understanding, and explanations and thus facilitating our management of the world around us.

So, what is this domain language, how does it work, and how might we express the intergrading of domains?

Origin

Physics has provided us with an account of the ordering processes (the laws and physical constants) that, following the Big Bang, determined the formation of elements, compounds, planets, stars, and galaxies.

Selection & purpose

The universe of inanimate matter is not random and chaotic, it displays order that is amenable to scientific investigation. Laws and physical constants place ‘selective’ constraints on possible outcomes as the universe undergoes change after the Big Bang. Processes have a deterministic character. Natural selection is a mindless designer. Insofar as even non-conscious selection leads to a restricted range of outcomes it is in this sense ‘purposive’. Though the words ‘selection’ and ‘purpose’ seem inappropriate in this context there is a nuanced similarity to application in the context of life.

Patterns are common in inanimate nature. The repeated sifting of wave and tide produces ripples and neatly graded pebbles on the seashore. But with natural selection something further was added to this kind of process. Before Darwin, the obvious design that we see all around us in nature was regarded as conclusive proof for the existence of God. By providing a naturalistic explanation for design in nature, Darwin removed the necessity for God. He showed how we get design from ignorance, from mindless trial and error. However, design in nature was intimately associated in peoples’ minds with God’s purpose as introduced to the world at the Creation. The new opposing certainty that emerged during the Scientific Revolution swept away purpose and design with God. The possibility that nature itself might exhibit purpose and design (attributed to Aristotle and his natural teleology) was denied. Purpose was associated with conscious intention and Aristotle accused of both inserting a supernatural entelechy into the world, and mistakenly reading human intentions into nature. The baby of real natural design, and natural purpose was thrown out with the bathwater of religion.

But Darwin had not removed purpose and design from nature, what he did was provide a naturalistic explanation of how they had arisen out of an ignorant process. Natural selection, as a selective process that promotes the flourishing of living organisms by producing functional adaptations, presented a new question … ‘What is it for: what is its purpose?‘. Here was a process in nature whose explanation demanded a move ‘from narrative to justification‘ (Dan Dennett): it is the appearance of a mechanism of ‘self-correction’. The association of this question with ends or goals, Aristotle’s telos or final cause, was still resisted by scientists after Darwin. Purpose was regarded as a product of the conscious mind and therefore the functional explanations that saturate biology are being used heuristically, they are useful metaphor. The purpose in nature is therefore apparent, not real. On the contrary, purpose in nature is real, even though it is not conscious purpose.

Principle 3 – natural selection itself is a mindless and ignorant process, but its products demostrate complex functional adaptations of great sophistication. As natural reasons of a functional kind they resemble human conscious purpose and warrant the title pre-conscious purpose since they are not unaware of those ends</span

Design

In a similar way, the ‘selective’ action of physical laws and constants results in the physical order that is investigated by science, that also has the nuanced characterisstics of ‘design’, as we see for example in the orderly pattern of the elements in the periodic table.

Value

Value has simple origins where there is ‘benefit’ … where there are ‘interests that matter’. It is difficult to detect any value-like process in the inanimate world except for a crude ‘preference’ for the formation of one kind of structure rather than another, and one kind of process or event rather than another.

The inanimate world ‘evolved’ in the sense that it became more complex. From an initially unstructured plasma at the Big Bang eventually arose elements and compounds aggregated into planets, stars, and galaxies. Even in the inanimate world the ordering activity of laws and physical constants presents us with a rudimentary or primordial semblance of ‘selective’ or ‘purposive’ process.

2. Living matter

Living matter is matter of the universe with unique properties that are the product of a special kind of process, natural selection. Superimposed on the ordering processes of laws and constants there is the ordering that results from matter that has acquired the capacity to replicate with variation that is subject to ‘selection’ based on fitness to surroundings (differential reproduction). Aristotle drew attention to the inner potential of animate as opposed to inanimate nature by remarking that ‘If the art of shipbuilding was in the wood then we would have ships by nature‘, the point being that the telos of a ship is imposed from outside, the telos of an organism derives from within ‘by nature’.

Reasons & explanations

The products of natural selection gave rise to an additional kind of why question. Not only can we ask, along with Aristotle (his four causes) ‘What was its origin?’, ‘How does it work?’, and ‘What is its structure and composition?’ and ‘What does it do?’ but, since living organisms are products of a selection process related to fitness to surroundings (adaptation) we can now also ask ‘What is it for?”. That is, organisms as products of the process of natural selection exhibit functional adaptations. Biologists investigate not only reasons relating to organic matter itself, the forms it takes, what it does, and how it does it, but also by using reverse engineering they examine the functional reasons that are fulfilled. ‘You dont need a mind to act for reasons’ and ‘a computer does not have to learn arithmetic’ (Dennett).

Principle 2 – Functions are different from natural reasons. One way of describing them is as the pre-conscious reasons specific to non-conscious living matter. Importantly, preconscious reasons do not imply foresight: they have arisen as a consequence of events that occurred in the past

Origin

We do not know precisely how life originated. We assume that somehow cyclical processes acting on inanimate matter of a particular kind produced replicating units capable of incorporating change as they replicate, in a kind of feedback loop. Over many replications there was a differential persistence of particular characteristics under the influence of surrounding conditions. The first processes of self-organization, of natural selection, had created life of the most rudimentary kind. Replicating pre-conscious matter was now on a path to symbiotic eucaryotic cells, multicellularity, a Cambrian explosion of forms, organic complexity, and consciousness.[2] Natural selection had produced a form of replicating matter which, by changing in relation to its surroundings over many generations, had developed some independence, some autonomy from its surroundings, a separate identity or ‘self’. This was, almost certainly, a new relationship in the universe – that between an organism and its environment.

Selection & purpose

Design

That there is design in nature cannot be doubted. That there is orderly symmetry, pattern and design in the structure and function of everything from a single cell to a leaf or the human brain is self-evident. The sometimes difficult point to understand is that this design arose out of an ignorant process (unconscious natural selection) so it is unintelligent design. The fact that it is unintelligent design does not mean that it is not really design at all … that it is only apparent design. The design in nature is real, even though it is not deliberate design.

Value

As living creatures we humans intuitively value existence and life over death and non-existence. We understand that functional adaptations benefit replicators by increasing their chances of survival, and insofar as functions can operate more or less efficiently in promoting flourishing, we say that they are for better or worse. Normativity (value) therefore arrived in the world with a selective process that coud be more or less efficient in promoting flourishing, with the arrival of things that matter to an organism’s well-being.. Whether a function operates more or less efficiently is not a matter of human judgement, it is a fact of the world, it is natural normativity. As humans we are aware of those factors that can challenge or improve our lives and we can also discern their pre-conscious precursors.

Principle 4 – when the special kind of natural reason called a function can differentially influence the flourishing of an organism then they are spoken of as ‘purposes’. These are not conscious purposes they are natural purposes

Principle 5 – value (normativity) arrived in the world when natural reasons produced functional reasons relating to the benefit or flourishing of organisms. This natural normativity does not entail awarenessof value

In summary, there is a special class of natural reasons in the world that we refer to as functions to indicate that they are products of a selection process (natural selection). This is pre-conscious or natural normativity and it accentuates the existence a form of matter that exists with a degree of autonomy from the matter in which it persists. Since natural selection produces functional adaptations that are more or less efficient in promoting the flourishing of organisms, there is also the introduction of pre-conscious purpose, design, and normativity.

We can now see how, with the origin of life came not only natural ‘selection’, but natural ‘purpose’, natural ‘design’, and natural ‘evaluation’. Also, just as evolution grades organisms by complexity so there is a parallel gradation in our notions of purpose, design, value and the gradual transition from natural reasons to pre-conscious functions and conscious intentions.

Simple reasons/purpose/design – those arising in inanimate matter e.g. ‘there is a simple reason why the Earth orbits the Sun’. Explanations tend to follow the non-conscious language of causation and interactions are impersonal.
Pre-conscious reasons/purpose/design – those arising in living matter from the benefit-conferring process of natural selection e.g. ‘there is a pre-conscious reason why the spider builds a web’. Explanations tend to follow the pre-conscious teleological language of function. and interaction can involve benefit and harm.
Conscious reasons/purpose/design – those arising in self-conscious living matter as human conscious reasons e.g. ‘I go walking for a reason’. Explanations tend to follow the language of conscious intention and interactions entail moral responsibility

Causation, function, and intention

The views expressed so far in his article present a particular perspective of scientific metaphysics. There are four kinds of matter recognized by humans as transitions in the physical continuum emerging from the Big Bang: from the inanimate to the living, from the living as the un-conscious to the conscious living, then from the conscious-living to the rational-conscious. These domains of existence we describe using distinctive domains of discourse: the causation of the inanimate world (matter), the function of the biological world (living matter), feelings of conscious nature (living matter that is self-aware), and the intentions of humans (living matter that is rational and self-reflective).

In the animal kingdom we find a gradation of consciousness. Trying to match our existing language to this metaphysics is a minefield. But by an inversion of reasoning we can regard teleology not so much as treating non-human organisms in a human way but as recognizing consciousness as an extension to the purposiveness of all life. On this view the philosophical error lies not so much in failing to recognize the primacy of consciousness, but in failing to recognise the primacy of nature. Philosophy has filled libraries with the books and articles attempting to expunge teleological language from biology. Perhaps it would do better to find a language that puts purpose back into nature with consciousness its adjunct.

Plants

Consciousness-talk emphasizes 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 and mindless.

There seems 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 don’t ‘think’: they also can’t ‘know’, and they don’t ‘feel’. Using this kind of consciousness-talk for plants is metaphorical fantasy at best; just a convenient shorthand way of making scientific investigation more human-like.

And yet, as we shall see, while all of this is true it presents plants to us in an extremely anthropocentric and demeaning way that overestimates the power of conscious deliberation, while at the same time under-rating un-conscious purpose. Remember . . . it was the mindless purposes of un-conscious nature that gave us our bodies and brains in the first place: we humans have not achieved anything remotely so miraculous.

Regarding the biological processes of non-human nature as ‘ignorant’ is just a preliminary error of human judgement. There are many sophisticated structures and processes found in plants that challenge those found in humans because plants, like humans, have successfully adopted their own, albeit very different, strategies (functional adaptations) to survive, reproduce, and flourish under the same broad environmental pressures.

Un-conscious purposes

While plants do not have subjective intentions, they do have un-conscious purposes: like all living beings, plants have agency.

There was a time, not too long ago, when mind was treated as something beyond body – the mind transcended the body and was even considered akin to a soul that departed the body at death.

Nowadays wonderful and mysterious though reason and consciousness are, we accept the unity of mind and body, that whatever we ‘feel’ is inextricably associated with our brains. There is no separation of one world of mental processes from another world of physical processes. Our minds are not separate from nature – we share with plants a common physicality.

From this perspective the view that subjective intentions are qualitatively different from the ‘ignorant’ mechanistic processes of the rest of nature loses its strength.   Both plants and sentient creatures have scientifically investigable reasons for their behaviour. Subjectivity (intentionality and consciousness) is as much functional adaptation as photosynthesis.

Principle 10 – The significance of subjectivity lies more in what it can do, than what it is

Plants are not conscious, but they bring with them the inherited ‘wisdom’ of evolution. To respond to their environments, they must be able to sense them – to have a sensory system. They are not ‘totally devoid of any sensory apparatus whatever‘.

Let’s look first at commonalities across the living world before giving special attention 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.

Hearing –
Sounds of music being of recent evolutionary time are only likely to be to be incidentally influential, if at all.

Commentary

This theme is then explored further by treating human and plant senses as functional analogues.

Is consciousness fundamental to existence because our minds create the world we live in or are the smallest fundamental particles the building blocks of the universe . . . or what?

The ideas presented in this article are difficult because they challenge the conventional scientific assumptions about the relationship between ourselves, other organisms, and the world – and therefore scientific reality. It is an article about metaphysics. Even if you disagree with the principles presented in this article there are plenty of tantalizing ideas to ponder. In this commentary I’ll try to re-state and summarize the main points.

Media Gallery

The development of human consciousness and intellect is usually associated with the evolution of a nervous system and central nervous system that arose in response to the environmental information processing needed when organisms were mobile.

This timelapse video of opening flowers shows that although plants do not change environments, they certainly make the most of their umweldt. We see plants in human animal time – perhaps timelapse allows us to see plants in plant time, including their longer-term purposive mobility.

Key points

short-termism compounds plant blindness by narrowing the narrative of deep time . But plants do behave, they just do it on a different time scale to humans, making it difficult for us to perceive without a great deal of patience.

Blooming Flower Timelapse

Next Observer – 2019 – 2:40

First published on the internet – 1 March 2019

Dionaea muscipula – Venus Flytrap
Showing trigger hairs
Courtesy Wikimedia Commons – NoahElhardt – Accessed 26 April 2018

Venus Flytrap
Print Friendly, PDF & Email