Jan Ingenhousz
1730-1799
Dutch Plant Physiologist and Physician
Jan Ingenhousz is best known for his discovery of photosynthesis, the process by which green plants absorb carbon dioxide in the presence of sunlight and release oxygen. Through an ingenious series of experiments, Ingenhousz proved that plant leaves need sunlight rather than heat in order to produce oxygen. He also discovered that plant leaves reverse this process in the dark and release carbon dioxide. Ingenhousz's remarkable observations on plant physiology and photosynthesis were published as Experiments Upon Vegetables, Discovering Their Great Power of Purifying the Common Air in Sunshine, and of Injuring It in the Shade and at Night (1779).
Ingenhousz was born in the Netherlands in 1730. He studied medicine, chemistry, and physics at the universities of Louvain and Leiden. In 1765 he visited London and established a successful medical practice there. He became well known as an early practitioner of inoculation against smallpox. In 1768 he was called to Vienna to inoculate the family of the Austrian empress Maria Theresa. His services were in great demand and he remained in Vienna as court physician and surgeon until 1779, when he returned to London. He was elected a Fellow of the Royal Society in 1769. Although best known for his experiments on plants, Ingenhousz was interested in many other areas of science. He invented a device that could be used to generate large amounts of static electricity and made the first quantitative measurements of heat conduction in metal rods. Ingenhousz carried out many experiments on electricity, magnetism, and the relationship between plants and animals. He died in Wiltshire, England, while visiting the Marquis of Lansdowne's estate.
The experiments on photosynthesis carried out by Ingenhousz were inspired by the work of Stephen Hales (1677-1761) and Joseph Priestly (1733-1804). Hales, the founder of experimental plant physiology, published his classic treatise Vegetable Staticks in 1727. While conducting quantitative experiments on the movement of water in plants, Hales discovered that some component in the air was essential to plantgrowth. Hales, however, did not fully understand that ordinary air is made up of a mixture of distinct gases. Having isolated and characterized several gases, including "fixed air" (carbon dioxide), Priestley demonstrated that plants have the ability to restore or "revivify" air that has been "damaged" by animal life or by combustion. Green plants, in other words, produce a substance that supports animal life and combustion. Priestley, however, did not always clarify the importance of sunlight in these experiments. Instead, it was Ingenhousz who proved that green plants must be exposed to light rather than heat in order to restore oxygen to the air. He also demonstrated that only the green parts of a plant are capable of performing photosynthesis. While green leaves exposed to sunlight produce large amounts of oxygen, all parts of a plant produce small amounts of carbon dioxide in the dark; plants, like animals, perform respiration and release carbon dioxide into the air. Ingenhousz thus helped to clarify the fundamental similarities and differences between plant and animal life.
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