Stephen Hales (1677-1761).
Often referred to as the founder or father of plant physiology (Devlin 1975; Hopkins 1999; Egerton 2008), Hales pioneered quantitative physiology.
He was born in Kent, England, and studied theology at Cambridge, but also read mathematics, physics, and botany.
He became a clergyman but also devoted much of his time to scientific pursuits, particularly in the conduct of experiments in plant physiology.
His most important work, Vegetable Staticks (published in 1727), was in plant physiology.
He suggested that plants derive something from the atmosphere (Rook 1964), a guess that future researchers have proven correct and has become an essential component of the process of photosynthesis in plants.
He observed that the volume of air just above the water surface decreased when a plant was grown in a closed atmosphere and concluded that air was “being imbibed into the substance of the plant.”
He also noted that light enters leaf surfaces and flowers and may play a role in plant growth and development (Govindjee and Krogmann 2004).
In addition, Stephen Hales developed instruments and techniques for the quantification of plant processes.
Among his experiments that contributed significantly to the enrichment of plant physiology are
(1) started on July 3, 1724, using a sunflower 3.5 feet tall as an experimental plant, he measured the plant’s total leaf area, total root length, and total area of the root system (Egerton 2008);
(2) he measured the amounts of water absorbed by the roots and that given off by leaves in the process of transpiration (he used the word ‘perspiration’),
(3) he measured the rate of water ascent in plant stems and the pressure responsible for the upward movement of water now called root pressure;
and (4) he also pioneered the method of studying stem growth by marking different parts of the plant stem and comparing their rates of growth (Rook 1964).
Charles Bonnet (1720-1793).
A Swiss naturalist, he pioneered in 1754 a method of measuring rates of photosynthesis that is now regularly used in schools worldwide (Govindjee and Krogmann 2004).
He pursued investigations to identify the functions of leaves and observed that when these were submerged in water, they produced bubbles.
These bubbles were eventually identified as oxygen (Aikman 1894; Govindjee and Krogmann 2004).