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Botany in the 17th & 18th Centuries

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll trace Western thought about plant physiology and discuss how plants gain weight, and how we know. We'll talk about how scientists figured out the mystery of photosynthesis and who they were, and then you can test your knowledge with a quiz.

How Do Plants Grow?

The history of science can be fun. Just like today's scientists, many historical scientists lived lives full of intrigue and even some brushes with fame. But perhaps it's even more exciting to take a second look at all the ideas we take for granted, and see how they arose.

It's been observable since ancient times that plants grow, but where do they get their increased mass from? Before the 1600s, it seemed logical that a plant gained weight by 'eating' soil. In the early 1600s, though, Flemish alchemist J.B. van Helmont performed a famous experiment showing that this was not the case. He planted a five-pound willow tree in 300 pounds of dry soil. Five years later, the willow weighed over 160 pounds, but the soil only lost two ounces of weight.

J.B. van Helmont
J.B. van Helmont

At the time, scientists believed that it was water that increased the plant's weight. We now know that it was carbon fixation - the process by which plants turn the carbon from carbon dioxide into sugar. Van Helmont had plenty of time to work on his theories about botany, because he was under house arrest by the Spanish Inquisition at the time. However, it would be a while before chemistry caught up to van Helmont's ideas. The first person to suggest that plants might extract some part of their mass from the air was Stephen Hales, a friend of Alexander Pope.

In the late 1700s, English clergyman Joseph Priestley would use plants in his experiments on chemistry. Priestley was interested in learning what substances air was made of. He found that when he burned a candle in a glass jar, the air would be depleted so that a mouse could no longer breathe. But he then found that if he contained plants in glass jars, the plants would release oxygen (or, as he called it, dephlogisticated air). This would enable the mouse to breathe.

It should be noted that Priestley had many unorthodox beliefs (for the time) about religion and politics. At one point, his home and his church were ransacked by an angry mob! He eventually immigrated to the United States for his own safety.

An unflattering political cartoon of Priestley.
An unflattering political cartoon of Priestley.

Later, a Swiss pastor named Jean Senebier furthered van Helmont's ideas by showing that plants needed carbon dioxide (fixed air, in the language of the time) to produce oxygen. He did this by submerging leaves in water, and comparing them to leaves submerged in water that had been saturated by carbon dioxide.

Jan Ingenhousz, a vaccine enthusiast and friend of Benjamin Franklin, was disturbed by the fact that Priestley couldn't reliably replicate his experiment and get the same results. Ingenhousz determined what was missing from Priestley's account: sunlight! Ingenzhousz determined that plants only produced oxygen when sunlight was present. He further learned that plants produce carbon dioxide at night. Jan Ingenhousz is considered to have discovered photosynthesis, one of the most important chemical processes on Earth. He may be the most important scientist that no one has heard about!

A portrait of Jan Ingenhousz, the most important scientist no one knows about.
Jan Ingenhousz

There were other scientists involved in the study of botany during this time. For example, Swiss scientist Nicolas de Saussure was the scientist who determined that water was necessary for photosynthesis as well.

Practical Plant Uses

Only in the last few hundred years have people been interested in learning about plants for their own sake. However, people have studied plants since prehistory for their importance as food, medicine, and dyes.

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