On December 8, 1730, Dutch physiologist, biologist and chemist Jan Ingenhousz was born. He is best known for showing that light is essential to photosynthesis and thus became one of the scientists who significantly contributed to the discovery of photosynthesis. He also discovered that plants, like animals, have cellular respiration.
Jan Ingenhousz was born in Breda. He started his studies of medicine at the University of Leuven when he was 16 years old and starting from 1753, he continued his studies at the University of Leiden. Ingenhousz started a general medical practice around 1755 in his hometown Breda. He increased his interest in techniques in inoculation against smallpox and traveled to England, where he made many valuable contacts. Ingenhousz became a master inoculator and it is said that he inoculated approximately 700 village people combating the epidemic. Due to his success in the field, Ingenhousz became Maria Theresia’s court physician and settled in Vienna. [1,2]
However, to Ingenhousz’ interests belonged not only medicine, but he also was quite enthusiastic about gaseous exchanges of plants. Experiments with plants and their energy sources presumably date back to the early 17th century. Jan Baptist van Helmont, a chemist, physicist, and physician performed his famous experiment by growing a willow tree in a pot for five years. At the end, the tree had increased in mass by more than 70kg even though the mass of the soil had changed little. He came to believe that water was the source of the extra mass and the plant’s life source. Then, in the late 1600s, John Woodward, the professor and physician at Cambridge University, attempted to design an experiment to test Van Helmont hypothesis. In a series of experiments over as many as 77 days, Woodward measured the water consumed by plants. He suggested that most of the water was “drawn off and conveyed through the pores of the leaves and exhaled into the atmosphere”. As a result, the hypothesis that water is the nutrient used by plants was rejected.
Further experiments by Joseph Priestley were devoted to the interaction of plants with air. The chemist proved that plants somehow change the composition of the air. In one of his more famous experiments, Priestley kept a mouse in a jar of air until it collapsed and he found out that a mouse that was kept with a plant would survive. He developed the hypothesis that plants restore to the air whatever breathing animals and burning candles remove. Jan Ingenhousz was the one who took Priestley’s work even further and demonstrated that plants needed the light to create oxygen (which was discovered just a few years earlier). Ingenhousz was the first to show that light is essential to the plant process that somehow purifies air fouled by candles or animals. During an experiment in 1779, the scientist inserted a candle and a plant into a transparent closed space and exposed it to the sunlight for several days. Next, Ingenhousz covered the space with a dark cloth and again let it sit for a few days and when trying to light the candle it would not work. His conclusion was, that the plant needed light in order to purify the air. [3,4]
It is assumed that Ingenhousz performed about 500 experiments to come to his conclusion, which he summarized in his book “Experiments upon Vegetables” and he is regarded the discoverer of photosynthesis. However, he probably was not really aware of what he had discovered. Ingenhousz correctly concluded that the “green parts of plants in the light of the sun produce oxygen … and that they produce carbon dioxide when in the dark.” However, he did not really know how all this happened, as Geerdt Magiels puts it. Further scientists continued to specify and describe photosynthesis including Jean Senebier, who demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light or Cornelis Van Niel, who made key discoveries explaining the chemistry of photosynthesis. 
At yovisto, you may be interested in a video lecture on Photosynthesis by Paul Andersen.
References and Further Reading:
-  Geerd Magiels, From Sunlight to Insight: Jan IngenHousz, the Discovery of Photosynthesis & Science in the Light of Ecology
-  Jan Ingenhousz at Britannica
-  The Discovery of Photosynthesis
-  Geerd Magiels, Dr. Jan Ingenhousz, or why don’t we know who discovered photosynthesis
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