plants – Science-Education-Research

Was Darwin concerned about cold radiation from above?

Can the cold be radiated, just like heat?

Keith S. Taber

Plants have a mechanism to protect themselves from cold radiation

Let me begin by acknowledging I am a great admirer of Charles Darwin who surely did more than anyone else to hasten the transition from botany and zoology just being branches of natural history to becoming part of an integrated scientific biology. I wanted to make that point, because I suspect that Darwin may have held an alternative conception which will likely seem to most readers quite bizarre. I may be wrong (and am very open to be enlightened, if so) but I suspect that Darwin thought cold could be radiated – that is, that there is cold radiation just as there is, say infra-red radiation or beta radiation or cosmic radiation.

Do, as Darwin suggested, some plants have a mechanism to protect their leaves from nightime radiation?

My evidence for this is modest, but it is really the only sense I can make of something Darwin wrote. Of itself, this limited textual evidence could easily be dismissed, except I have also read things written by other historical scientists that seem to treat 'cold' as an entity in its own right alongside 'heat'. So, James Hutton (sometimes called the 'father of 'geology') referred to cold as if it was something active in itself ('we are but limited in the art of increasing the cold of bodies') and Johannes Kepler also wrote as if cold was a distinct agent in its own right ('cold will force its way through gaps') and indeed one early supporter of the chemical atom went as far as to suggest that the atoms of cold are tetrahedral.

Read about some historical scientific ideas we would now consider misconceptions

Darwin – adventurer and recluse; and conservative revolutionary

Darwin is most famous, perhaps, for three things:

spending almost five years on a natural history collecting expedition aboard HMS Beagle after accepting the position of the Captain's companion ¹ during a voyage to better survey coasts around South America;
from his observations of geological and biological phenomena during the voyage (including a lot of time he was ashore while the surveying was being carried out) coming to a new perspective on the origin of species, based on a process which facilitated evolution – natural selection;
many years later publishing his ideas in a book known as the Origin of Species.

It is often suggested that the long delay (as Darwin turned from an adventurous young man climbing volcanoes and exploring jungles, to a reclusive middle-aged family man who seldom left his home town) was due to Darwin's awareness that his theory contradicted literal aspects of Biblical faith, and would likely lead to him being labelled an 'atheist' (something largely undesirable at the time when adherence, at least apparently, to the Anglican Church's articles of faith was often considered a prerequisite for being included in polite society) and cause tensions in his otherwise loving marriage to the devout Emma (who by the time of their wedding was already worried that Charles's scientific scepticism might put his immortal soul in danger).

There is likely something in that, but the reality is not that Darwin put off sharing his work deliberately, but rather that after the Beagle returned, Darwin effectively spent the rest of his life testing and developing his ideas. He wanted to develop a water-tight and well supported argument. (Indeed, he would not have published Origins when he did, as he felt he was only part way in drafting a much more detailed account, had he not learnt from Alfred Russel Wallace that he had hit upon much the same principle as Darwin's 'natural selection'.)

His vast collections from the Beagle Voyage (that needed to be described and catalogued for publication) kept him busy enough for some considerable time after his return. He then followed up testing out his ideas against as much evidence as he could access. Darwin famously corresponded with naturalists (and gardeners and farmers and anyone who he thought could provide relevant data) all around the world, and got them to send him observations and specimens. He consulted with various scientific experts in areas where he knew his own knowledge was not cutting edge. And he carried out his own experiments at home (for example, on whether plant seeds could survive extended periods in salt water). And he wrote to a range of periodicals about his findings, as well as passing on interesting information from his overseas correspondents.

Plants move their leaves to avoid radiation

And it was in a couple of his published letters that I read his description of how some plants change the positions of their leaves (a common enough phenomenon) along with Darwin's suggestion that in certain cases plants repositioned their leaves at night to protect them from radiation. He refers to work he had undertaken with support from one of his sons, Francis Darwin. In the first letter to Nature in March 1181, Darwin Sⁿ writes, how a correspondent of his from Brazil had written to tell him of

"…striking instances of … plants, which place their leaves vertically at night, by widely different movements; and this is of interest as supporting the conclusion at which my son Francis and I arrived, namely, that leaves go to sleep in order to escape the full effect of radiation. In the great family of the Graminere the species in one genus alone, namely Strephium, are known to sleep, and this they do by the leaves moving vertically upwards; but Fritz Müller finds in a species of Olyra…that the leaves bend vertically down at night.

Two species of Phyllanthus (Euphorbiacere) grow as weeds near Fritz Müller's house; in one of them with erect branches the leaves bend so as to stand vertically up at night. In the other species with horizontal branches, the leaves move vertically down at night, rotating on their axes, in the same manner as do those of the Leguminous genus Cassia. Owing to this rotation, combined with the sinking movement, the upper surfaces of the opposite leaflets are brought into contact in a dependent position beneath the main petiole; and they are thus excellently protected from radiation, in the manner described by us. On the following morning the leaflets rotate in an opposite direction, whilst rising so as to resume the diurnal horizontal position with their upper surface exposed to the light."

The 'us' who had previously described this kind of movement being Charles and Francis Darwin. Their theory was then that at least some plants 'sleep' at night (an interesting notion in itself), and protect their leaves from radiation by changing their position.

When I first read this I was a little confused. Certainly 'sunlight' contains high energy frequencies which can potentially damage tissues (but, of course, is also essential for photosynthesis, so avoiding the sun's radiation during the day would be counter-productive). Darwin also refers to some leaves taking positions to protect them from the direct effect of strong sunlight which makes sense if we assume that there is sometimes more than sufficient light to support photosynthesis, given strong sunlight may both cause radiation damage and encourage faster transpiration. But that was not going to be an issue at night.

Perhaps Darwin meant cosmic rays? But no, as his letter preceded their discovery by several decades. The same was true for the radioactivity found naturally in soils and the atmosphere – but even if Darwin had known about that, it is not clear how the position of leaves would make much difference. So, what kind of radiation could damage the leaves at night?

Darwin goes on to report that

"Fritz Müller adds that the tips of the horizontal branches of this Phyllanthus curl downwards at night, and thus the youngest leaves are still better protected from radiation."

This seems to suggest that whatever radiation Darwin was concerned about originated above, in the sky. A few weeks later, Darwin wrote to Nature again reporting that "FRITZ MUELLER [sic] has sent me some additional observations on the movements of leaves, when exposed to a bright light". There follow more observations on the various positions that leaves take up in some specified plants when they 'sleep' – but no more explanation of what Darwin thinks the leaves are being protected from.

So, I took a look at the book Darwin (1880) had written with assistance from Francis, about movement in plants, to see if there were any references there to 'radiation'. There it is suggested

"The leaves of various plants are said to sleep at night, and it will be seen that their blades then assume a vertical position through modified circumnutation, in order to protect their upper surfaces from being chilled through radiation."

Now, of course, leaves will radiate heat away from the plant on a cold night. Any body that is above absolute zero will radiate according to its temperature, and will consequently cool by this process if it is radiating faster than absorbing radiation (that is, in effect if it is in an environment colder than itself). Reducing exposed surface area (curling up, to reduce radiation away) or moving to be surrounded by other leaves at the same temperature (to increase absorption of incident radiation) would reduce cooling in this way: so, was this what Darwin was suggesting?

Perhaps – but this is not clear from Darwin'saccount. He is certainly concerned about damage done by frost when plants are exposed to low temperatures on cold nights. However, to my reading his phrasing in places seems to point less at reducing the heat emitted by the leaves, and more about avoiding or limiting exposure to radiation (of cold?) from the sky. Here are some pertinent extracts so readers can make up their own minds:

"The fact that the leaves of many plants place themselves at night in widely different positions from what they hold during the day, but with the one point in common, that their upper surfaces avoid facing the zenith [i.e., directly above], often with the additional fact that they come into close contact with opposite leaves or leaflets, clearly indicates, as it seems to us, that the object gained is the protection of the upper surfaces from being chilled at night by radiation. There is nothing improbable in the upper surface needing protection more than the lower, as the two differ in function and structure. All gardeners know that plants suffer from radiation. It is this and not cold winds which the peasants of Southern Europe fear for their olives. Seedlings are often protected from radiation by a very thin covering of straw; and fruit-trees on walls by a few fir-branches, or even by a fishing-net, suspended over them. There is a variety of the gooseberry, the flowers of which from being produced before the leaves, are not protected by them from radiation, and consequently often fail to yield fruit. … This view that the sleep of leaves saves them from being chilled at night by radiation, would no doubt have occurred to Linnaeus, had the principle of radiation been then discovered…

We doubted at first whether radiation would affect in any important manner objects so thin as are many cotyledons and leaves, and more especially affect differently their upper and lower surfaces; for although the temperature of their upper surfaces would undoubtedly fall when freely exposed to a clear sky, yet we thought that they would so quickly acquire by conduction the temperature of the surrounding air, that it could hardly make any sensible difference to them, whether they stood horizontally and radiated into the open sky, or vertically and radiated chiefly in a lateral direction towards neighbouring plants and other objects. …

But in every country, and at all seasons, leaves must be exposed to nocturnal chills through radiation, which might be in some degree injurious to them, and which they would escape by assuming a vertical position. …

…there can be no doubt that the position of the leaves at night affects their temperature through radiation to such a degree, that when exposed to a clear sky during a frost, it is a question of life and death. We may therefore admit as highly probable, seeing that their nocturnal position is so well adapted to lessen radiation, that the object gained by their often complicated sleep movements, is to lessen the degree to which they are chilled at night. It should be kept in mind that it is especially the upper surface which is thus protected, as it is never directed towards the zenith, and is often brought into close contact with the upper surface of an opposite leaf or leaflet. …

If a cotyledon or leaf is inclined at 60° above or beneath the horizon, it exposes to the zenith about one-half of its area; consequently the intensity of its radiation will be lessened by about half, compared with what it would have been if the cotyledon or leaf had remained horizontal [see my figure below]. This degree of diminution certainly would make a great difference to a plant having a tender constitution. … when the angular rise of cotyledons or of leaves is small, such as less than 30°, the diminution of radiation is so slight that it probably is of no significance to the plant in relation to radiation. For instance, the cotyledons of Geranium Ibericum rose at night to 27° above the horizon, and this would lessen radiation by only 11 per cent.: those of Linum Berendieri rose to 33°, and this would lessen radiation by 16 per cent."

I am not sure what to make of this. In places it seems clear that Darwin knows it is the leaves that are radiating away heat. Yet he makes much of the angle to the open sky, as if the leaves need protecting from something originating there. Changing the angle of a leaf from the horizontal would certainly reduce the surface area exposed to any radiation from above, but in itself makes no difference to the intensity of radiation emitted by the leaf. So, in places, the treatment seems based on the leaf's assumed exposure to incoming radiation rather than on any factors that might reduce the heat emitted.

Darwin thought that a plant could reduce potential damage by radiation on a cold night by re-orientating its leaves to reduce the surface area exposed to the sky above.

Of course, Darwin was not a physicist, but he was widely read and a deep thinker. He seems to be reporting a mechanism by which plants might be protected from the effects of low temperatures by repositioning their leaves – but his explanation in terms of radiation does not seem to work. If he is referring to the leaves radiating (and in some places, that certainly seems to be the case), then repositioning of the leaves does not of itself directly change that (though it might, for example, move them nearer the ground where the air may be not so cold); and if the critical factor is the apparent area of exposed leaf from directly above the plant, then this suggests a concern with something (cold?) radiated from the sky above – as the leaf will continue to emit the same level of radiation regardless of its relative angle to the sky.

Perhaps my difficulty in making sense of Darwin's explanation here is because his thought was in a kind of transitional or hybrid state? We see this in historical accounts of the development of science, and also in the classroom as learners undergo conceptual change (as, for example, when having learned that ionic bonding is the effect of lattice forces between oppositely charged ions, but still thinking that an ionic bond was a transfer of an electron from one atom to another).² The French philosopher (and former school science teacher) Gaston Bachelard argued that scientists inevitably retain in their thinking vestiges of historical scientific notions that have nominally been refuted and discarded.

The mechanisms that Darwin describes might indeed reduce the NET thermal radiation from leaves, despite the radiation emitted being unchanged, if repositioning leaves increased the amount of radiation absorbed. (Positioning leaves in warmer air, or in positions better protected from cold breezes, will have reduced losses – but not by reducing the amount of radiation emitted.³)

Darwin seems aware that the (relatively warmer) leaves radiate away heat in the cold night, but at some level he seems to hold a vestige of an earlier historical notion (from a time before temperature was understood in molecular terms), and when it was common to understand phenomena in terms of contrasting qualities and properties (hot-cold and wet-dry being critical opposites in archaic ideas about the elements, the heavenly bodies, and medicine). So, at one time, levity was seen as property in its own right, acting in an opposite way to gravity; and rarity considered as a property in its own right having an opposite sense to density. So, thinking of cold as an entity (not just a lack of heat, or a low temperature) which had active effects fitted in a long-standing tradition of thought.

Even if Darwin did not actually, explicitly, think cold existed as something that could be radiated in its own right, his account of the importance of leaves changing their angle to sky above them on a cold night does certainly seems to have vestiges of a notion of cold as an active agent radiating down from above.

Work cited:

Darwin, C. (1881). Movements of plants. Nature, 23 409.
Darwin, C. (1881). The movement of leaves. Nature, 23, 603-604.
Darwin, C. with Darwin, F. (1880) The Power of Movement in Plants. London: John Murray.

Note

¹ Although Darwin acted as a ship's naturalist, this role would normally have fallen to the ship's surgeon. Captain Fitzroy wanted someone who he could dine with, and engage in intelligent conversation, and by social convention at the time this should be someone of the right status – a gentleman. This was likely a sensible precaution on such a long voyage (even without knowing with hindsight that much later – after Governing New Zealand and establishing weather forecasts – Fitzroy would commit suicide). One might wonder whether none of the other officers on the ship came from a 'suitable' background; but a good Captain was probably also aware of the risks for maintaining ship's discipline of fraternizing with members of his crew.

² See for example: Taber, K. S. (2000) Multiple frameworks?: Evidence of manifold conceptions in individual cognitive structure, International Journal of Science Education, 22 (4), pp.399-417. https://doi.org/10.1080/095006900289813 [Download this paper]

³ For example, if two (relatively warm) leaves move to have their surfaces adjacent, then each will absorb some of the radiation emitted by the other, reducing each leaf's net heat loss. If a leaf is in a breeze then the air around it is constantly being renewed, whereas in still air the warmer leaf will raise the temperature of the surrounding air, and although diffusion will still slowly occur, this warmer air will offer some level of insulation.

A discriminatory scientific analogy

Animals and plants as different kinds of engines

Keith S. Taber

Specimens of two different types of natural 'engines'.
Portrait of Sir Kenelm Digby, 1603-65 (Anthony van Dyck: From Wikimedia Commons, the free media repository)

In this post I discuss a historical scientific analogy used to discuss the distinction between animals and plants. The analogy was used in a book which is said to be the first major work of philosophy published in the English language, written by one of the founders of The Royal Society of London for Improving Natural Knowledge ('The Royal Society'), Sir Kenelm Digby.

Why take interest in an out-of-date analogy?

It is quite easy to criticise some of the ideas of early modern scientists in the light of current scientific knowledge. Digby had some ideas which seem quite bizarre to today's reader, but perhaps some of today's canonical scientific ideas, and especially more speculative theories being actively proposed, may seem equally ill-informed in a few centuries time!

There is a value in considering historical scientific ideas, in part because they help us understand a little about the path that scientists took towards current scientific thinking. This might be valuable in avoiding the 'rhetoric of conclusions', where well-accepted ideas become so familiar that we come to take them for granted, and fail to appreciate the ways in which such ideas often came to be accepted in the face of competing notions and mixed experimental evidence.

For the science educator there are added benefits. It reminds us that highly intelligent and well motivated scholars, without the value of the body of scientific discourse and evidence available today, might sensibly come up with ideas that seem today ill-conceived, sometimes convoluted, and perhaps even foolish. That is useful to bear in mind when our students fail to immediately understand the science they are taught and present with alternative conceptions that may seem illogical or fantastic to the teacher. Insight into the thought of others can help us consider how to shift their thinking and so can make us better teachers.

Read about historical scientific conceptions

Analogies as tools for communicating science

Analogies are used in teaching and in science communication to help 'make the unfamiliar familiar', to show someone that something they do not (yet) know about is actually, in some sense at least, a bit like something they are already familiar with. In an analogy, there is a mapping between some aspect(s) of the structure of the target ideas and the structure of the familiar phenomenon or idea being offered as an analogue. Such teaching analogies can be useful to the extent that someone is indeed highly familiar with the 'analogue' (and more so than with the target knowledge being communicated); that there is a helpful mapping across between the analogue and the target; and that comparison is clearly explained (making clear which features of the analogue are relevant, and how).

Read about scientific analogies

Nature made engines

Digby presents his analogy for considering the difference between plants and animals in his 'Discourse on Bodies', the first part of his comprehensive text known as his 'Two Discourses' completed in 1644, and in which he sets out something of a system of the world.¹ Although, to a modern scientific mind, many of Digby's ideas seem odd, and his complex schemes sometimes feel rather forced, he shared the modern scientific commitment that natural phenomena should be explained in terms of natural causes and mechanisms. (That is certainly not to suggest he was an atheist, as he was a committed Roman Catholic, but he assumed that nature had been set up to work without 'occult' influences.)

Before introducing an analogy between types of living things and types of engines, Digby had already prepared his readers by using the term 'engine' metaphorically to refer to living things. He did this after making a distinction between matter dug out of the ground as a single material, and other specimens which although highly compacted into single bodies of material clearly comprised of "differing parts" that did not work together to carry out any function, and seemed to have come together by "chance and by accident"; and where, unlike in living things (where removed parts tended to stop functioning), the separate parts could be "severed from [one] another" without destroying any underlying harmonic whole. He contrasted these accidental complexes with,

"other bodies in which this manifest and notable difference of parts, carries with it such subordination of one of them unto another, as we cannot doubt but that nature made such engines (if so I may call them) by design; and intended that this variety should be in one thing; whole unity and being what it is, should depend of the harmony of the several differing parts, and should be destroyed by their separation".
Digby emphasising the non-accidental structure of living things (language slightly tidied for a modern reader).

Digby was writing long before Charles Darwin's work, and accepted the then widely shared idea that there was design in nature. Today this would be seen as teleological, and not appropriate in a scientific account.A teleological account can be circular (tautological) if the end result of some process is explained as due to that process having a purpose. [Consider the usefulness as an 'explanation' that 'oganisms tend to become more complex over time as nature strives for complexity'. ²]

Read about teleology

Scientists today are expected to offer accounts which do not presuppose endpoints. That does not mean that a scientists cannot believe there is purpose in the world, or even that the universe was created by a purposeful God – simply that scientific accounts cannot 'cheat' by using arguments that something happens because God wished it, or nature was working towards it. That is, it should not make any difference whether a scientist believes God is the ultimate cause of some phenomena (through creating the world, and setting up the laws of nature) as science is concerned with the natural 'mechanisms' and causes of events.

Read about science and religion

Two types of engines

In the part of his treatise on bodies that concerns living things, Digby gives an account of two 'engines' he had seen many years before when he was travelling in Spain. This was prior to the invention of the modern steam engine, and these engines were driven by water (as in water mills). ³

Digby introduces two machines which he considers illustrate "the natures of these two kinds of bodies [i.e., plants and animals]"

He gives a detailed account of one of the engines, explaining that the mechanism has one basic function – to supply water to an elevated place above a river.

His other engine example (apparently recalled in less detail – he acknowledges having a "confused and cloudy remembrance" ) was installed in a mint in a mine where it had a number of different functions, including:

producing metal of the correct thickness for coinage
stamping the metal with the coinage markings
cutting the coins from the metal
transferring the completed coins into the supply room.

These days we might see it as a kind of conveyor belt moving materials through several specialist processes.

Different classes of engine

Digby seems to think this is a superior sort of engine to the single function example.

For Digby, the first type of engine is like a plant,

"Thus then; all sorts of plants, both great and small, may be compared to our first engine of the waterwork at Toledo, for in them all the motion we can discern, is of one part transmitting unto the next to it, the juice which it received from that immediately before it…"
Digby comparing a plant to a single function machine

The comments here about juice may seem a bit obscure, as Digby has an extended explanation (over several pages) of how the growth and structure of a plant are based on a single kind of vascular tissue and a one-way transport of liquid. ⁴Liquid rises up through the plant just as it was raised up by the mechanism at Toldeo

The multi-function 'engine' (perhaps ironically better considered in today's terms as an industrial plant!) is however more like an animal,

"But sensible living creatures, we may fitly compare to the second machine of the mint at Segovia. For in them, though every part and member be as it were a complete thing of itself, yet every one requires to be directed and put on in its motion by another; and they must all of them (though of very different natures and kinds of motion) conspire together to effect any thing that may be for the use and service of the whole. And thus we find in them perfectly the nature of a mover and a moveable; each of them moving differently from one another, and framing to themselves their own motions, in such sort as is more agreeable to their nature, when that part which sets them on work hath stirred them up.
And now because these parts (the movers and the moved) are parts of one whole; we call the entire thing automaton or…a living creature".
Digby comparing animals to more complex machines (language slightly tidied for a modern reader)

So plants were to animals as a single purpose mechanism was to a complex production line.

Animals as super-plants

Digby thought animals and plants shared in key characteristics of generation (we would say reproduction), nutrition, and augmentation (i.e., growth), as well as suffering sickness, decay and death. But Digby did not just think animals were different to plants, but a superior kind.

He explains this both in terms of the animal having functions that be did not beleive applied to plants,

And thus you see this plant [sic] has the virtue both of sense or feeling; that is, of being moved and affected by external objects lightly striking upon it; as also of moving itself, to or from such an object; according as nature shall have ordained.

but he also related to this as animals being more complex. Whereas the plant was based on a vascular system involving only one fluid, this super-plant-like-entity, had three. In summary,

this plant [sic, the animal] is a sensitive creature, composed of three sources, the heart, the brain, and the liver: whose are the arteries, the nerves, and the veins; which are filled with vital spirits, with animal spirits, and with blood: and by these the animal is heated, nourished, and made partaker of sense and motion.

A historical analogy to explain the superiority of animals to plants

[The account here does not seem entirely consistent with other parts of the book, especially if the reader is supposed to associate a different fluid with each of the three systems. Later in the treatise, Digby refers to Harvey's work about circulation of the blood (including to the liver), leaving the heart through arteries, and to veins returning blood to the heart. His discussion of sensory nerves suggest they contain 'vital spirits'.]

Some comments on Digby's analogy

Although some of this detail seems bizarre by today's standards, Digby was discussing ideas about the body that were fairly widely accepted. As suggested above, we should not criticise those living in previous times for not sharing current understandings (just as we have to hope that future generations are kind to our reasonable mistakes). There are, however, two features of this use of analogy I thought worth commenting on from a modern point of view.

The logic of making the unfamiliar familiar

If such analogies are to be used in teaching and science communication, then they are a tactic we can use to 'make the unfamiliar familiar', that is to help others understand what are sometimes difficult (e.g., abstract, counter-intuitive) ideas by pointing out they are somewhat like something the person is already familiar with and feels comfortable that they understand.

Read about teaching as 'making the unfamiliar familiar'

In a teaching context, or when a scientist is being interviewed by a journalist, it is usually important that the analogue is chosen so it is already familiar to the audience. Otherwise either the analogy does not help explain anything, or time has to be spent first explaining the analogy, before it can be employed.

In that sense, then, we might question Digby's example as not being ideal. He has to exemplify the two types of machines he is setting up as the analogue before he can make an analogy with it. Yet this is not a major problem here for two reasons.

Firstly, a book affords a generosity to an author that may not be available to a teacher or a scientist talking to a journalist or public audience. Reading a book (unlike a magazine, say) is a commitment to engagement in depth and over time, and a reader who is still with Digby by his Chapter 23 has probably decided that continued engagement is worth the effort.

Secondly, although most of his readers will not be familiar with the specific 'engines' he discusses from his Spanish travels, they will likely be familiar enough with water mills and other machines and devices to readily appreciate the distinction he makes through those examples. The abstract distinction between two classes of 'engine' is therefore clear enough, and can then be used as an analogy for the difference between plants and animals.

A biased account

However, today we would not consider this analogy to be applicable, even in general terms, leaving aside the now discredited details of plant and animal anatomy and physiology. An assumption behind the comparison is that animals are superior to plants.

In part, this is explained in terms of the plants apparent lack of sensitivity (later 'irritability' would be added as a characteristic of living things, shared by plants) and their their lack of ability in getting around, and so not being able to cross the room to pick up some object. In part, this may be seen as an anthropocentric notion: as humans who move around and can handle objects, it clearly seems to us with our embodied experience of being in the world that a form of life that does not do this (n.b., does not NEED to do this) is inferior. This is a bit like the argument that bacteria are primitive forms of life as they have evolved so little (a simplification, of course) over billions of years: which can alternatively be understood as showing how remarkably adapted they already were, to be able to successfully occupy so many niches on earth without changing their basic form.

There is also a level of ignorance about plants. Digby saw the plant as having a mechanism that moved moisture from the soil through the plant, but had no awareness of the phloem (only named in the nineteenth century) that means that transport in a plant is not all in one direction. He also did not seem to appreciate the complexity of seasonal changes in plants which are much more complex than a mechanism carrying out a linear function (like lifting water to a privileged person who lives above a river). He saw much of the variation in plant structures as passive responses to external agents. His idea of human physiology are also flawed by today's standards, of course.

Moreover, in Digby's scheme (from simple minerals dug from the ground, to accidentally compacted complex materials, to plants and then animals) there is a clear sense of that long-standing notion of hierarchy within nature.

The great chain of being

That is, the great chain of being, which is a system for setting out the world as a kind of ladder of superior and inferior forms. Ontology is sometimes described as the study of being , and typologies of different classes of entities are sometimes referred to as ontologies. The great chain of being can be understood as a kind of ontology distinguishing the different types of things that exist – and ranking them.

Read about ontology

In this scheme (or rather schemes, as various versions with different levels of detail and specificity had been produced – for example discriminating the different classes of angels) minerals come below plants, which come below animals. To some extent Digby's analogy may reflect his own observations of animals and plants leading him to think animals were collectively and necessarily more complex than plants. However, ideas about the great chain of being were part of common metaphysical assumptions about the world. That is, most people took it for granted that there was such hierarchy in nature, and therefore they were likely to interpret what they observed in those terms.

Digby made the comparison between increasing complexity in moving from plant to animal as being a similar kind of step-up as when moving from inorganic material to plants,

But a sensitive creature, being compared to a plant, [is] as a plant is to a mixed [inorganic] body; you cannot but conceive that he must be compounded as it were of many plants, in like sort as a plant is of many mixed bodies.

Digby, then, was surely building his scheme upon his prior metaphysical commitments. Or, as we might say these days, his observations of the world were 'theory-laden'. So, Digby was not only offering an analogy to help discriminate between animals and plants, but was discriminating against plants in assuming they were inherently inferior to animals. I think that is a bias that is still common today.

Work cited:

Digby, K. (1644/1665). Two Treatises: In the one of which, the nature of bodies; In the other, the nature of mans soule, is looked into: in ways of the discovery of the immortality of reasonable soules. (P. S. MacDonald Ed.). London: John Williams.
Digby, K. (1644/2013). Two Treatises: Of Bodies and of Man's Soul (P. S. MacDonald Ed.): The Gresham Press.
Taber, K. S. & Watts, M. (2000) Learners' explanations for chemical phenomena, Chemistry Education: Research and Practice in Europe, 1 (3), pp.329-353. [Free access]

Notes:

¹ This is a fascinating book with many interesting examples of analogies, similes, metaphor, personification and the like, and an interesting early attempt to unify forces (here, gravity and magnetism). (I expect to write more about this over time.) The version I am reading is a 2013 edition (Digby, 1644/2013) which has been edited to offer consistent spellings (as that was not something many authors or publishers concerned themselves with at the time). The illustrations, however, are from a facsimile of an original publication (Digby, 1644/1645: which is now out of copyright so can be freely reproduced).

² Such explanations may be considered as a class of 'pseudo-explanations': that give the semblance of explanation without actually explaining very much (Taber & Watts, 2000).

³ The aeolipile (e.g., Hero's engine) was a kind of steam engine – but was little more than a novelty where water boiled in a vessel with suitably directed outlets and free to rotate, causing it to spin. However, the only 'useful' work done was in turning the engine itself.

⁴ This relates to his broader theory of matter which still invokes the medieval notion of the four elements, but is also an atomic theory involving tiny particles that can pass into apparently solid materials due to pores and channels much too small to be visible.

Some particles are softer than others

Keith S. Taber

Bill was a participant in the Understanding Science Project. Bill was a Year 7 student when he told me that previously, when he had been in primary school, "we did a lot about plants, and – inside them, how they produce their own food". As he had been talking to me about learning about particles (e.g. Gas particles try to spread out and move apart), I asked if there was any link between these two topics.

Okay. What about particles, we were just talking about particles, do you think that's got anything to do with particles?
Well in the plant, there is particles.
Are there?
'cause it's a solid.
Ah. So there'll be particles in that then?
Yeah.
Is it all solid, do you think?
Inside the stem is, 'cause going up the stem there would be water, so that's a liquid. And, it also uses oxygen, which is a gas, to make its food, so. I think so.
So it would be solids, liquids and gases?
Mm, I think some.
But they've all got some particle in them, they are all made up of particles.
Yeah.
Okay.

As Bill had talked to me earlier about there being particles in a gas when ice was melted, and then boiled, I wanted to see if he though the particles in different substances were the same:

Erm. Do you think that the particles in the – oxygen's a gas isn't it?
Yeah.
Do you think the particles in the oxygen gas, are the same as the particles in the steam that you said was a gas, in your experiment you did earlier?
Erm, I don't think so, no.
You think they'd be different sort of particles?
Yeah, they're different gases.
Okay. And in the solid part of the plant, do you think the particles that make up the solid part of the plant, are the same as the particles that make up this table, that's a solid?
Well, the particles, plants are soft, some plants are soft, and you, when you squeeze them they're, they feel soft and erm, but the table is hard so I think that the particles would be slightly different, but they would have, because they hold this different shape, and they would, they would be {pause} erm {pause} then they would, ob¬, then they would be softer as well.
So the softer, the plant which is softer, > > would have softer particles?
< Yeah. < I think so yeah
And the harder wood, made of harder particles?
I think so.

Here Bill offered evidence of a very common alternative conception about the particle theory. A key feature of particle theory is that chemists use particle models to explain the properties of substances macroscopically (what can be observed directly) in terms of the very different nature and properties of conjectured 'particles' (quanticles) at a submicroscopic level.

Yet after learning about these 'particles', students commonly 'explain' macroscopic properties of substances and materials by suggesting that the particles of which they are made up themselves have the property to be explained – being hard, sharp, colourless, conducting, etc.

Are plants solid?

Keith S. Taber

Bill was a participant in the Understanding Science Project. Bill (a Year 7 pupil) told me Bill talked about how in his primary school he had studied "a lot about plants, and – inside them, how they produce their own food", and how "inside, it has leaves, inside it, there is chlorophyll, which stores [sic] sunlight, and then it uses that sunlight to produce its food."

Bill had been talking to me about particles, and I asked if plants had anything to do with particles:

Well in the plant, there is particles….'cause it's a solid…. inside the stem is, 'cause going up the stem there would be water, so that's a liquid. And, it also uses oxygen, which is a gas, to make its food, so. I think so.

I suspect that Bill's reference to the plant being "a solid" would seem unproblematic to many people, especially as Bill recognised the presence of water (a liquid) and oxygen (a gas) as well.

There is however a potential issue here. The model of states of matter and changes of state taught in school strictly refers to reasonably pure samples of particular substances (so water is a liquid at normal temperatures, and oxygen is a gas – although strictly speaking the air in which it is found is a mixture which is not best considered 'a gas'). A plant (like an animal) is a complex structure which cannot be considered as a solid (and indeed living things were separated out in distinct substances, water would make up much of the content).

If the scientific model of solids, liquids and gases is applied beyond the range of individual substances, this is sometimes unproblematic. To consider the air as a gas, or the sea as a liquid, is not usually a problem as it is clear what this means in everyday discourse. But of course it is not possible to find 'the' boiling point of complex mixtures such as these.

However a wooden stool is only a solid in the everyday sense, certainly not in a scientific sense, and to refer to animals or plants as solids does considerable violence to the concept. (BBC Bitesize – please note!*)

(* Read 'Thank you, BBC: I'll give you 4/5')

There are particles in everything – but maybe not chlorophyll

Keith S. Taber

Bill was a participant in the Understanding Science Project. Bill (a Year 7 pupil) told me that "solids they stay same shape and their particles only move a tiny bit". He explained that the 'particles' were "the bits that make it what it is", although "you can't see them" as "they're very, very tiny". Later he commented that "they are microscopic".

Although it is very common for such particles to be said to be 'microscopic', a better term would be 'nanoscopic'. Microscopic suggests visible under a microscope, and the particles referred to here ('quanticles') are actually submicroscopic." The term microscopic could therefore be misleading, and it is known that often when students first learn about particles in science they often have in mind small grains of powder or dust.

Bill explained that "there is particles in everything". Bill was able to talk a lot about particles in solids, liquid and gases and explain what happened during melting.

Later in the same interview Bill talked about how in his primary school he had studied "a lot about plants, and – inside them, how they produce their own food", and how "inside, it has leaves, inside it, there is chlorophyll, which stores [sic] sunlight, and then it uses that sunlight to produce its food."

I asked Bill if plants had anything to do with particles:

Well in the plant, there is particles….'cause it's a solid…. inside the stem is, 'cause going up the stem there would be water, so that's a liquid. And, it also uses oxygen, which is a gas, to make its food, so. I think so.

Bill explained that "…in the leaves it is chlorophyll which is a green substance, so that would make, give it its colour".

Do you think chlorophyll is made of particles?
Hm, don't know.

So it seemed that although 'there is particles in everything', Bill did not seem to feel this meant that he could apply the particle idea to all substances. This could be an example of a fragmentation learning impediment: that is, where learning in one area is not recognised as relevant in studying other subjects or topics.

Plants store sunlight

Keith S. Taber

Bill was a Y7 student participating in the Understanding Science project. He used the idea of energy in talking about some aspects of his science. So when considering melting "the particles in (a solid), would have the energy, to move about more, and then it would melt down, because of its melting point, and go into a liquid". Although he could not explain what energy was, he knew "it gives something – the energy to move, it will make something else move or something". He remembered having done some work "where we had to make elastic band powered, 'cause the elastic band stored the energy to make it move", so energy could be stored.

Bill also told me about how in his previous school "we did a lot about plants, and – inside them, how they produce their own food". He explained that "inside, it has leaves, inside it, there is chlorophyll, which stores sunlight, and then it goes, then it uses that sunlight to produce its food. It also uses water from the roots, and the soil, and oxygen in the air. So it needs sunlight, oxygen and water to make its food and live."

However, Bill did not relate this process to the notion of energy, and see that the 'storing sunlight' might have been like the energy stored in an elastic band:

Interviewer: We were talking about energy just now.
Bill: Yeah
I: Do you think that's got anything to do with energy? That process you just talked about?
B: Hm, erm, (pause, c.3 seconds) I'm not sure

So Bill did not make the connection between storing energy, and what he interpreted from his science lesson as 'storing sunlight'. This appears to be an example of a fragmentation learning impediment.