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.
