Intimations of Photosynthesis: Mint and Cucumber
IN 1772 JOSEPH PRIESTLEY – preacher, dissenter and radical chemist – felt that he should inform ‘my friends, and the public, that I have for the present, suspended my design of writing the history and present state of all the branches of experimental philosophy’, because – an ageless author’s complaint – he saw ‘no prospect of being reasonably indemnified for so much labour and expence’. Such a titanic work by one of the brightest minds of the eighteenth century would have been something to behold. But what we got instead more than compensated: a study of the properties of ‘different kinds of air’ which included details of an exquisitely simple experiment that was to become one of the most important in the history of botany.
The nature of air obsessed Enlightenment scientists, including the group that Jenny Uglow called ‘The Lunar Men’, which included, in addition to Priestley, the poet Erasmus Darwin (Charles’s grandfather) and the engineer James Watt. Many questions challenged their imaginations and experimental savvy. Which part of the air enabled animals and plants to breathe and grow, and which extinguished life? Did burning a substance release ‘phlogiston’ into the atmosphere, or remove some element from it? Priestley had been experimenting with ways of ‘restoring’ air which had been ‘exhausted’ by burning or breathing, and announces early in the substitute for his unwritten grand history (Experiments and Observations on Different Kinds of Air) that ‘I have been so happy, as by accident to have hit upon a method of restoring air, which has been injured by the burning of candles, and to have discovered at least one of the restoratives which nature employs for this purpose. It is vegetation.’ To be precise it was a sprig of mint. It is striking how comfortable were the whole lineage of pre-professional scientists – from Newton hunkered down in his farmhouse bedroom with a homemade rack of prisms, to Charles Darwin feeding carnivorous plants with the remains of his dinner – using the humdrum materials of domestic life in their experiments. I don’t think this was entirely due to their being, willy-nilly, amateurs working at home. As the last non-specialists they must have also felt that what they were investigating was still existentially part of the warp and weft of their ordinary lives.
In the summer of 1771 Priestley had put a sprig of mint in a glass jar which he’d stood upside down in a bowl of water, presuming the plant would exhaust the air in the way that a breathing animal would. But months later it was still growing strongly, and he found that the air in the jar would neither extinguish a candle ‘nor was it at all inconvenient to a mouse, which I put in it’. Over the weeks the root began to decay, and the mint put out ever smaller successions of leaves ‘all the summer season’. He took care to extract any dead leaves in case they began to putrefy and affect the air. He also found that mint leaves would restore the air in a jar where a candle had burnt out. He tried different sorts of flame (wax, spirits of wine, brimstone matches), different plants (groundsel, cabbage) and pieces of plant. Spinach, splendidly, gave the best results, restoring a jar of ‘burned air’ in two days. But essential plant oils or bunches of leaves didn’t work. ‘The restoration of air,’ he concluded, ‘depended upon the vegetating state of the plant.’ He begins to sense that plants affected the air in the opposite manner to animal respiration, and many hapless mice later, concludes that one mouse could live perfectly well in a jar of air made noxious by the breathing (and subsequent asphyxiation) of another of its kind simply by the insertion a sprig of mint for eight or nine days. After more timed and quantified trials later he was able to conclude that it
cannot but render it highly probable, that the injury which is continually done to the atmosphere by the respiration of such a number of animals, and the putrefaction of such masses of both vegetable and animal matter, is, in part at least, repaired by the vegetable creation … [and] if we consider the immense profusion of vegetables upon the face of the earth, growing in places, suited to their nature, and consequently at full liberty to exert all their powers, both inhaling and exhaling, it can hardly be thought, but that it may be a sufficient counterbalance.
The idea that sunlight, the motion of fluids through plants and their ‘respiration’ of air were in some way connected was not entirely new. Nor was the derision poured on these ideas by many outside science’s inner circle. In 1727 Stephen Hales, rector of Farringdon in Hampshire (a parish adjacent to Gilbert White’s Selborne), published the conclusions of his researches into ‘vegetable statics’. Hales argued that the sun’s heat caused water to be driven through a plant’s sap vessels, and pass into the leaves to be ‘perspired’. As so often in eighteenth-century science, thinking by analogy (in this case making a comparison with the way a warmed liquid expands and ‘rises’, as in a thermometer) grasped the overall effect, but not the exact mechanism. In fact water is transpired in trees by suction, as the change in pressure due to water loss from the leaves is transmitted down to the roots. In the 1735 edition of Jonathan Swift’s surreal satire on the pretensions of contemporary science, Gulliver’s Travels, Hale’s experiments are the target of one of the funniest lampoons in the book. In Swift’s Grand Academy of Lagado (a take-off of the Royal Society), one of the utopian scientists ‘had been Eight Years upon a Project for extracting Sun-Beams out of Cucumbers, which were to be put into Vials hermetically sealed, and let out to warm the Air in raw inclement Summers’.
The ‘sunlight distillery’ from Gulliver’s Travels. An illustration by Milo Winter from a 1912 edition.
If Priestley had read Swift he was undeterred, and his own hermetically sealed vials proved to his satisfaction that the growth of green leaves (doubtless on cucumber plants as well as mint) was enabled by their exchange of common and burnt air, not yet named as oxygen and carbon dioxide. He’d also recognised a truth about the relation between vegetable and animal life, and established a fundamental principle of ecology. Benjamin Franklin, who had seen some of Priestley’s flourishing plants during a visit from America, took Priestley’s conclusion one practical, and prophetic, step further: ‘I hope this will give some check,’ he wrote to Priestley, ‘to the rage of destroying trees that grow near houses, which has accompanied our late improvements in gardening, from an opinion of their being unwholesome.’ The President of the Royal Society sensed deeper global and egalitarian implications. ‘In this the fragrant rose and deadly nightshade cooperate,’ wrote Sir John Pringle in 1774; ‘nor is the herbage, nor the woods that flourish in the most remote and unpeopled regions, unprofitable to us, nor we to them; considering how constantly the winds convey to them our vitiated air, for our relief, and their nourishment.’
Jan Ingenhousz quoted these words when he reported his own experiments in 1779, which showed that oxygen (it had been named by then) was given off only in sunlight, followed three years later by Jean Senebier’s demonstration that the exhalation of oxygen in light depended on the intake of carbon dioxide. The basic principles of photosynthesis, the process that underpins all life on earth, had been established.
The central dynamic of photosynthesis – the conversion of sunlight into plant tissue via the fixing of atmospheric carbon dioxide – has been an inspiring motif for poets and writers ever since, the central idea being passed on from generation to generation, much as the molecules of life are. Erasmus Darwin, who kept an acute eye on developments in science, wove the discovery into his epic hymn to botanic creation, The Economy of Vegetation, in 1784. Erasmus felt obliged in his poetry (not, fortunately, in his prose) to animate botanical processes through the elaborate affairs of mythological humans. If you ignore the florid posturing, and the fact that he got the gas wrong, there is something energising in his thunderous and precocious image of a living earth:
SYLPHS! From each sun-bright leaf, that twinkling shakes
O’er Earth’s green lap, or shoots amid her lakes,
Your playful band with simpering lips invite
And wed the enamoured OXYGEN to LIGHT …
Close-up of a mint leaf taken through an electron scanning microscope. The blue spheres in the midst of the chlorophyll-rich, photosynthesising tissue are oil glands, which give mint its characteristic scent.
Whence in bright floods the VITAL AIR expands,
And with concentric spheres involves the lands;
Pervades the swarming seas, and heaving earths,
Where teeming Nature broods her myriad births;
Fills the fine lungs of all that breathe or bud,
Warms the new hearts, and yes the gushing blood;
With life’s first spark inspires the organic frame,
And, as it wastes, renews the subtle flame.
Coleridge thought Erasmus ‘a wonderfully entertaining and instructive old man’, and his own take on photosynthesis echoes the pre-Gaian fervour in this stanza, though the science is both tougher and more metaphysical. The following passage, which I’ve condensed slightly, is buried in an appendix to Coleridge’s densely theological tract The Statesman’s Manual (1816) and hints at the key Romantic belief that the processes of the imagination resonated with those of organic growth itself:
… I seem to myself to behold in the quiet objects, on which I am gazing, more than an arbitrary illustration, more than a mere simile, the work of my own Fancy! I feel an awe, as if there were before my eyes the same Power, as that of the REASON – I feel it alike, whether I contemplate a single tree or a flower, or meditate on vegetation throughout the world, as one of the great organs of the life of nature. Lo! – with the rising sun it commences its outward life and enters into open communion with all the elements, at once assimilating them to itself and to each other. At the same moment it strikes its roots and unfolds its leaves, absorbs and respires, steams forth its cooling vapour and finer fragrance, and breathes a repairing spirit, at once the food and tone of the atmosphere, into the atmosphere and feeds it. Lo! – at the touch of light how it returns an air akin to light, and yet with the same pulse effectuates its own secret growth, still contracting to fix what expanding it had refined … Thus finally, the vegetable creation, in the simplicity and uniformity of its internal structure symbolising the unity of nature, while it represents the omniformity of her delegated functions in its external variety and manifoldness, becomes the record and chronicle of her ministerial acts, and increases the vast unfolded volume of the earth with the hieroglyphics of her history.
Forty years on, the usually sentimental writer on gardens and botanical taste, Shirley Hibberd, is moved to a soaring passage of insight and rigour (but which never loses sight of Victorian values) while contemplating photosynthesis:
The atom of charcoal which floated in the corrupt atmosphere of the old volcanic ages, was absorbed into the leaf of a fern when the valleys became green and luxuriant; and there … [t]hat same atom was consigned to the tomb when the waters submerged the jungled valleys. It had lain there thousands of years, and a month since was brought into the light again, in a block of coal. It shall be consumed to warm our dwelling … ascent into a curling wreath to revel in a mazy dance high up in the blue ether; shall reach earth again, and be entrapped in the embrace of a flower: shall live in the velvet beauty on the cheek of the apricot; shall pass into the human body … circulate in the delicate tissues of the brain; and aid, by entering some new combination, in enducing the thoughts which are now being uttered by the pen.
It’s a passage which uncannily prefigures the journey of the carbon atom in Primo Levi’s majestic The Periodic Table, written 120 years later. In this the atom also experiences many photosynthetic embodiments, including a bunch of grapes, a cedar tree (from which it’s chewed out by a wood-boring insect) and the grass which eventually produces a glass of milk. The atom in the milk is drunk by the writer and – just as in Hibberd’s narrative – takes part in the neural synapse which guides his pen: ‘a double snap, up and down between two layers of energy, guides this hand of mine to impress on the paper this dot, here, this one’.
Unravelling the process of photosynthesis in plants was arguably the most important development in the history of biology. Most forms of life on earth depend on this transformation of the sun’s energy into living tissue. As the ethnobotanist Tim Plowman remarked, contemplating twenty-first-century revelations about plant communication, ‘Why should that impress us? They can eat light, isn’t that enough?’
Will we be more or less impressed when we are technologically able to replicate photosynthesis, to restore air or ‘eat light’ ourselves?