By studying the relationship between green plants, oxygen, carbon dioxide, and light, Ingenhousz discovered the major, externally observable structures that contribute to the process of photosynthesis. It would remain for later scientists to understand the internal chemical reactions at the heart of the process.

The understanding that green plants Plant physiology
Physiology;plants synthesize their own food is relatively recent, as is an appreciation of the importance of this process. In photosynthesis, a plant uses two simple, inorganic raw materials, water Water;in photosynthesis[photosynthesis] and carbon dioxide, Carbon dioxide;in photosynthesis[photosynthesis] and, in the presence of light, produces carbohydrate (which constitutes plant food), releasing oxygen Oxygen;in photosynthesis[photosynthesis] gas as a waste product. Light Light;and photosynthesis[photosynthesis] provides the energy for this process. At the time of Jan Ingenhousz’s discoveries, photosynthesis was only beginning to be understood. [kw]Ingenhousz Discovers Photosynthesis (1779)
[kw]Photosynthesis, Ingenhousz Discovers (1779)
[kw]Discovers Photosynthesis, Ingenhousz (1779)
Photosynthesis
Botany
[g]England;1779: Ingenhousz Discovers Photosynthesis[2370]
[c]Biology;1779: Ingenhousz Discovers Photosynthesis[2370]
[c]Science and technology;1779: Ingenhousz Discovers Photosynthesis[2370]
[c]Chemistry;1779: Ingenhousz Discovers Photosynthesis[2370]
Ingenhousz, Jan
Priestley, Joseph
Senebier, Jean
Lavoisier, Antoine-Laurent

Aristotle and other ancient Greeks had believed that plants obtain all of their nutrition from the soil, analogously to the way animals ingest their food. This belief persisted until the Enlightenment, in the seventeenth and eighteenth centuries, when intensive experimentation and discoveries led to a series of insights into photosynthesis. In the early seventeenth century, Jan van Helmont Helmont, Jan van concluded from an experiment that water rather than soil was the source of the gain in dry weight by growing plants. He was correct that water played a role but incorrect in concluding that water was the sole factor. In the early eighteenth century, Stephen Hales Hales, Stephen correctly surmised that some of a plant’s nutrition was derived from “air.”

Ingenhousz’s discoveries came later in the eighteenth century, as scientists were making great advances in the understanding of chemistry, especially the composition of air. Chemists of the time, such as Joseph Priestley and Antoine-Laurent Lavoisier, were replacing old ideas with new concepts and terminology. Their chemical dissection of the air revealed that it was composed of various gases, including carbon dioxide, oxygen, hydrogen, and nitrogen. These gases were as yet imprecisely understood, however, and they were given names such as “pure air,” “dephlogisticated air,” or “vital air” (oxygen) and “impure air,” “vitiated air,” or “fixed air” Fixed air (carbon dioxide) (carbon dioxide). Advances in the understanding of photosynthesis both benefited from and contributed to the growing knowledge of gases and their roles in chemical reactions.

Ingenhousz’s research on plants was inspired by experiments conducted by Priestley. In 1771, Priestley had discovered that air that had been made “impure” (oxygen-poor, in modern terms) by the burning of a candle or the respiration of a mouse could be “restored” by a sprig of mint so that it was again capable of supporting combustion and respiration. By showing that animals inhale “pure air” and plants release it, Priestley had discovered the interdependence of plants and animals, mediated by gases. Priestley was troubled by inconsistency in his results, however.

During the summer of 1779, Ingenhousz conducted more than five hundred experiments on plants. He repeated and extended the work of Priestley, performing many trials on detached leaves immersed in water. Substitution of leaves for the whole plants used by Priestley allowed Ingenhousz to draw conclusions that would have been elusive using whole plants, which are composed of both green and nongreen parts. Ingenhousz analyzed the gas composition of the bubbles that collected on the surfaces of the submerged leaves to determine whether they were “pure air” (oxygen) or “impure air” (carbon dioxide).

The title page of Jan Ingenhousz’s Experiments upon Vegetables, in which he published his findings relating to photosynthesis.

(Library of Congress)

Ingenhousz confirmed Priestley’s observations and demonstrated, in addition, that light is required for plants to produce oxygen. Ingenhousz showed that, under brilliant illumination, plants could restore “impure air” within several hours, rather than the several days that Priestley had often found. Ingenhousz attributed Priestley’s inconsistent results to variation in the degree of illumination of Priestley’s plants from experiment to experiment. Ingenhousz also identified leaves as the portion of plants affected by light and showed that the part of the Sun’s radiation that affects them is visible light, not heat.

In addition, Ingenhousz discovered that, although the green parts of plants give off oxygen in sunlight, they emit carbon dioxide in shade and at night and that the nongreen parts of plants emit carbon dioxide in both dark and light conditions. Thus, he provided evidence that plants, like animals, perform respiration. Respiration In a modern, cellular sense, Cellular biology respiration is the process whereby plants, animals, and some other organisms use oxygen to break down organic compounds in order to obtain energy and molecular building blocks. In the process, they release carbon dioxide, the raw material for photosynthesis, and thus they complete what is now known as the “oxygen cycle.” Ingenhousz showed that, overall, the amount of oxygen taken up by green plants in respiration is far smaller than the amount released through photosynthesis.

Ingenhousz immediately published the results of his summer’s work in Experiments upon Vegetables: Discovering Their Great Power of Purifying the Common Air in the Sunshine and of Injuring It in the Shade and at Night
Experiments upon Vegetables (Ingenhousz) (1779). Subsequently, Priestley claimed that he had discovered the light requirement before Ingenhousz had. Thus began a long-running quarrel between the two men over the priority of their claims. Most scholars, however, credit Ingenhousz with the breakthrough.

Working at about the same time as Ingenhousz, Swiss naturalist Jean Senebier repeated and extended Ingenhousz’s experiments. Senebier showed that plants must have access to carbon dioxide in order to liberate oxygen and that the amount of oxygen liberated is related to the amount of carbon dioxide available to the plant. Using Senebier’s findings, Ingenhousz subsequently established that plants retain weight from the carbon in the carbon dioxide they absorb. Ingenhousz thereby disproved the idea that the carbon in plants is absorbed through the roots, from humus in the soil. His finding explained the disappearance of carbon dioxide during photosynthesis. He published these research results in the second of his two works on photosynthesis, An Essay on the Food of Plants and the Renovation of Soils
Essay on the Food of Plants and the Renovation of Soils, An (Senebier) (1796).

Ingenhousz’s work laid the groundwork for further research on photosynthesis. In 1804, Nicholas de Saussure Saussure, Nicholas de discovered that a growing plant gains more in dry weight than just the weight of the carbon dioxide it absorbs. He correctly reasoned—in a throwback to van Helmont—that water also contributes to the increase in dry matter of the plant during photosynthesis.

Although Ingenhousz discovered the requirement for light, he did not determine the function of light in photosynthesis. Robert Mayer, Mayer, Robert a physicist, demonstrated in the mid-nineteenth century that the amount of energy that is liberated by the combustion of the organic matter produced in photosynthesis is equivalent to the amount of light energy that the plant has absorbed. This finding showed that photosynthesis is a mechanism for converting the radiant energy of the Sun into a stored, chemical form of energy. The organic molecules produced in photosynthesis are used for energy and as building blocks for other organic molecules, both plants and the animals that eat them. The oxygen released by photosynthesis is essential to plant and animal respiration, making photosynthesis critical to life on Earth.

Late in the nineteenth century, the overall chemical equation for photosynthesis was formulated, stating that carbon dioxide and water, in the presence of light, yield glucose and oxygen. The early twentieth century brought the insight that the oxygen released in photosynthesis is derived from the splitting of water, not from carbon dioxide as Ingenhousz had thought. As of the early twenty-first century, at least fifty intermediate steps in photosynthesis had been identified, and the discovery of many more was fully anticipated.

• Galston, Arthur W. Life Processes of Plants. New York: Scientific American Library, 1994. Chapter 1 of this easy-to-read volume includes historical material. Illustrations, index, bibliography.
• Gest, Howard. “A ’Misplaced Chapter’ in the History of Photosynthesis Research: The Second Publication, 1796, on Plant Processes by Dr. Jan Ingen-Housz, M.D., Discoverer of Photosynthesis.” Photosynthesis Research 53 (1997): 65-72. Describes Ingenhousz’s final publication on photosynthesis, which shows how his original concepts were modified by intervening breakthroughs in chemistry.
• Govindjee, J. T. Beatty, H. Gest, and J. F. Allen, eds. Discoveries in Photosynthesis. Berlin, Germany: Springer, 2005. Details the entire history of photosynthesis research.
• Ingenhousz, Jan. An Essay on the Food of Plants and the Renovation of Soils. 1796. Reprint. Oquawka, Ill: J. Christian Bay, 1933. Ingenhousz’s final publication on plant physiology.
• _______. Experiments upon Vegetables: Discovering Their Great Power of Purifying the Common Air in the Sunshine and of Injuring It in the Shade and at Night. London: P. Elmsly and H. Payne, 1779. Ingenhousz’s first publication on plant physiology.
• Nash, Leonard K. Plants and the Atmosphere. Cambridge, Mass.: Harvard University Press, 1952. A detailed analysis of the complex history of the discovery of basic photosynthetic processes, from circa 1650 to 1804.
• Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York: W. H. Freeman, 1999. Comprehensive botany textbook; chapter 7, on photosynthesis, includes historical material.

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Botany