Hales, Stephen 1671-1761

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Reverend Stephen Hales was an English physiologist, chemist, inventor and country vicar. In 1709 he resigned a fellowship at Corpus Christi College, Cambridge University, to become a perpetual curate at Teddington, now part of London. He studied physiology using as the foundation of his work the anatomical presentation of Grew.

For forty years while still a curate he devoted his leisure time to research in botany and zoology. His memoirs were published in a collected form as the Statical Essays, the first dealing with problems in plant physiology and the second with corresponding problems in animal physiology. Vegetable Staticks (1727) is the classic work in plant physiology. He is considered the founder of experimental plant physiology.

Hales did little vague theorizing and the greater part of his work is a record of successive experiments. An attempt to stop bleeding in a badly pruned grape vine by means of a piece of bladder tied over the wound gave him the idea of manometer.

He found that root pressure showed a daily periodicity and was affected by changes in temperature. He noticed that leaves "perspired," gave off water and proceeded to measure the amount of transpiration and to compare it with the amount absorbed by the root. He studied variations in the quantity of water transpired over a 24-hour period and demonstrated reduction in transpiration at night.

Hales also had definite notions of the part that the leaves played in plant nutrition, and studied leaf structure. He contended that "plants very probably draw through their leaves some part of their nourishment from the air" and that leaves also absorbed light.

He had a scientific mind of the highest order and is ranked along with Grew and Malpighi as outstanding leaders in botany and physiology up to the end of the 18th Century. Sachs, the great German physiologist in his History of Botany stated, "Hales made his plants themselves speak."