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."
