It wasn’t until the 17th century that a better understanding of photosynthesis began to emerge but, frankly, even then progress was halting.
The first breakthrough came from a Flemish alchemist whose medical experiments, apparently inspired by the dreams of the Archangel Raphael, had to be hidden from the Spanish Inquisition. Jan Baptist van Helmont was a physician primarily focused on the study of human physiology. He introduced the mystical term “gas” into medical literature. He discovered that “fermenting must”, carbon dioxide in today’s parlance, made the air in wine cellars unbreathable and was carried by the blood flowing through the veins and expelled through the lungs.
In the 1620s, van Helmont focused on testing the dogma that soil was consumed by plants. He grew a young willow tree in a covered pot, filled it with two hundred pounds of carefully weighed, dehydrated soil, and then watched the tree grow. Five years later, he discovered that the tree had gained 169 pounds in weight, while the soil weighed only two ounces less. He concluded, partly correctly, that plants consume water instead of soil. One can quibble with the exact methods, but this was the first modern reference to a structured, oft-repeated experiment that tested what plants needed to grow.
Like so many scientific discoveries, the idea had probably been around for much longer. Leonardo da Vinci had conducted a similar experiment with pumpkin seeds which was later discovered in his unpublished notebooks. Da Vinci, in turn, may have been inspired by Nicholas of Cues, who suggested the experiment in his book De staticis experimentalis in 1450. And Nicholas of Cues himself may have been inspired by a Greek text dated between 200 and 400 what called Recognitions. Ideas can hang around for thousands of years before someone actually tests them.
Van Helmont got us halfway there by proving that plants don’t eat soil. Ironically, for someone who had studied the biological function of carbon dioxide, he completely missed the fact that his tree was consuming gases and light. Perhaps if the Spanish Inquisition had not placed him under house arrest for blasphemy, his findings could have been debated and repeated sooner. Instead, they went unnoticed until 1648, when his book Ortus Medicinae was published posthumously by his son.
In his obscure way, Erasmus Darwin stumbled upon what exactly happens with photosynthesis and plant growth.
Another advance in human knowledge nearly happened in 1679 when a French physicist and priest, Edme Mariotte, developed a theory that plants could get some of their food from the air. He was a gifted man who was present at the founding of the French Academy of Sciences, but unfortunately he did not publish his findings. In effect, Speech of the nature of air, plant vegetation. New discovery affecting sight was not printed until 250 years later, in 1923. Note to young scientists: please publish, disseminate, test, repeat and discuss your findings; don’t expect your ideas to magically emerge. And even then, ideas sometimes have to circulate for a long time before they are accepted.
Instead, we had to wait until the 18th century for the next milestone on this slow journey of scientific discovery. Stephen Hales, an English clergyman whose many accomplishments included the first measurement of blood pressure, began studying a process called transpiration – or water loss from leaves. He surmised that “plants most likely draw through their leaves some of their food from the air”.
Blowing and huffing through his homemade lung-powered bellows and bizarre ‘re-breathing’ machines, he experimented with inverted bottles and observed that the volume of air above the water’s surface decreased. when a plant was grown in a closed atmosphere. He concluded that the air “was soaked in plant substance”. In his 1727 book, vegetable static,5 he even conjectured that light could be a source of energy for plants.
These ideas began to percolate across Europe. In 1779, a Dutch doctor called Jan Ingenhousz decided to test the idea that plants exchange air by submerging the leaves under water in sealed bottles and then waiting for bubbles to form. Bottle after bottle of drowned leaves testified to the failure of these early experiments until a ray of sunlight accidentally illuminated one of his bottles. In a few minutes, he observed the formation of the long-awaited bubbles.
Ingenhousz, rooted in the scientific tradition of the time, is surprised that light has something to do with it. He repeated the experiment with many different plants in dark and light bottles. He even tested whether it was the thermal heating of his chimney or visible sunlight that caused the bubbles to form. Eventually he deduced that light was necessary for the formation of bubbles and concluded that the gas released was “fire air”, soon to be called oxygen. He then tested which gases were emitted in the dark and described them as “damaging the air” as opposed to those emitted during the day, which “purified” the air. Today we know that factories removed carbon dioxide from waste air during the day.
By the end of the Enlightenment we were really making progress and the scientific understanding of photosynthesis was about to take a giant leap forward, thanks to Erasmus Darwin. He was a polymath physician, pathologist and botanist, as well as the grandfather of the most famous Charles Darwin. He was also an abolitionist of the slave trade, a supporter of female education (especially for his illegitimate daughters), and the author of bizarre poems such as “Les amours des plantes”. It was in this poem in 1789 that he first addressed heavily hidden concepts of biological evolution.
This poem, found in Part II of Darwin’s book The Botanical Garden, is preceded by an “Apology” which describes how it presents scientific theories and information through the deployment of mythical beings – gnomes, sylphs, nymphs and salamanders, as well as deities of Egypt, Rome and Greece – in the hope of making them more accessible. With no index or discernible structure, the poem happily jumps from topic to topic.
For example, the first canto contains the following order of subjects: “Hesperian Dragon, Electric Kiss. Halo around the heads of Saints, Electric Shock, Lightning from Clouds. Cupid extracts the Lightning from Jupiter, the Phosphoric Acid and the Vital Heat produced in the Blood. The Great Egg of the Night. West wind unimpeded. The Botanical Garden consists of two books, each with four cantos, and the whole thing is a savage confusion, interspersed with additions, arguments, footnotes and lists of findings and scientific data.
However, in “Note 5” on the rays of the sun, Erasmus Darwin adds the following to Canto 1.I.136:
Some modern philosophers are of the opinion that the sun is the great fountain from which the Earth and the other planets derive all the phlogiston [burnable material] that they own; and that it is formed by the combination of the solar rays with all the opake [sic] bodies, but especially with the leaves of plants, which they supposed to be organs capable of absorbing them. And as animals get their nourishment from plants, they also get their phlogiston in a secondary way from the sun. And finally, as great masses of the mineral kingdom, which have been found in the thin crust of the earth where human labor has penetrated, were evidently formed from the recreations of animal and vegetable bodies, these are also supposed to have thus derived their phlogiston. of the sun.
The extra notes added to the poem continue with wildly improbable theories about how the sun works. Yet, in his obscure way, Darwin had stumbled upon what exactly happens with photosynthesis and plant growth.
Emerging from this mix of archangels, alchemy, sylphs and experiments with home-made devices was a new understanding that plants absorb air and water to produce usable energy. . Crucially, this early biology had also allowed people to see that sunlight was necessary for the process of turning water and gases into food and oxygen. This is the fundamental principle of photosynthesis. Although this new idea took hundreds of years to develop, it finally dispelled the misunderstanding that human beings had worked with for millennia: that plants eat soil.
There is another lesson in these stories. Even at a time when communications were frustratingly slow and books were published posthumously by eccentric investigators, ideas could still migrate from country to country, develop and spark structured experimentation. Of course, the equipment was rudimentary, the messages were hidden in obscure analogies, and progress was painfully slow. Of course, many other scientists have confused the debate with a scholarly repetition of the status quo. Yet the lessons of these experiences were repeatable and demonstrable, and slowly they began to transform our understanding of nature.
Consider for a moment the danger of the old belief that the earth was consumed by hungry green creatures called plants. This vision has made the world a finite, declining and pessimistic place. By contrast, learning that plants turn sunlight, water and air into food, we discovered that the world was not over – that there was ‘new’ production. And that this plant production complements our animal consumption: they inhale what we exhale. Instead of a world in which plants and animals ate both a limited amount of food and land on the planet, we understood that expansion and growth were possible.
Extract How Light Creates Life: The Hidden Wonders and Life-Saving Powers of Photosynthesis © Raffael Jovine, 2022. Reprinted with permission from the publisher, The Experiment. Available wherever books are sold. experimentpublishing.com