Hales, Stephen

English Physiologist 1677-1761

Stephen Hales was a preeminent scientist of the late eighteenth century and the founder of plant physiology. Born in Kent, England, in 1677, Hales grew up in an upper-class Kent family and was educated at Cambridge University. Though he received no formal training in botany during college, Hales obtained a solid background in science, including physics and mechanics. Upon graduation from Cambridge, Hales moved to Teddington, a town on the Thames River in England, where he lived the rest of his life.

Hales has been called the first fully deductive and quantitative plant scientist. He made many significant discoveries concerning both animal and plant circulation. Crucially, Hales measured plant growth and devised innovative methods for the analysis and interpretation of these measurements.

Hales's most original contribution was his transfer of application of the so-called statical method he and others had used on animals to plant specimens. The basis behind the statical method was the belief that the comprehension of living organisms was possible only through the precise measurements of their inputs and outputs. Thus the way to understand a human being would be to measure the fluids and other materials that had entered and left it. In the case of a tree, a statistician would measure changes in the amount and quality of the water it consumed and the sap it contained.

In 1706, under the influence of Isaac Newton's new mechanics, Hales tried to figure out the mechanism that controlled animal blood pressure by experimenting on dog specimens. At the same time, Hales had the idea that the circulation of sap in plants might well be similar to the circulation of the blood in humans and other animals. As he was exploring animal circulation, Hales grew increasingly interested in plant circulation. He wrote later in his book Vegetable Staticks of his first circulation experiments: "I wished I could have made the like experiments to discover the force of the sap in vegetables."

After a decade of quiet research and study, Hales did indeed devise such experiments on plants. He attached glass tubes to the cut ends of vine plants. He then watched sap rise through these tubes, and he monitored how the sap flow varied with changing climate and light conditions. In 1724, Hales completed Vegetable Staticks (quoted above), wherein he distinguished three different aspects of water movement in plants. These he called imbibition, root pressure, and leaf suction.

The prevalent notion among Hales's contemporaries was that the movement of plant sap was similar to the circulation of human blood, which was discovered by William Harvey in 1628. Crucially, Hales demonstrated that this theory was false. Instead, he demonstrated the constant uptake (absorption) of water by plants and water's constant loss through transpiration (evaporation into the air). Drawing on this principle, Hales made many exact and careful experiments using weights and measures. All of these he repeated using different types of plants (willows and creepers, for example) in order to verify his conclusions. Thus, from his beginnings as a physiologist, Hales went on to create a mechanics of water movement.