Hales initiated a new stage in physiological experimentation with his "statical" methods, which were characterized by precise quantitative measurements, repetition and the used of controls, and were based on the assumption that that the known laws of matter operated in the bodies of plants and animals as well as in non-living materials. In his investigations of plant physiology, described in Vegetable Staticks, Hales studied the movement of water in plants, determining that leaf suction is the main force by which water is raised through a plant, and showing that plants lose water constantly via transpiration through their leaves. He also demonstrated that plants do not have a true circulation, and developed techniques to measure the varying rates of growth in different plant structures.
Vegetable Staticks is the first volume of Hales's Statical Essays, the second volume of which (Haemastaticks) appeared in 1733. Haemastaticks ecords Hales' invention of the manometer, with which he was the first to measure blood-pressure. His work is the greatest single contribution to our knowledge of the vascular system after Harvey, and led to the development of the blood-pressure measuring instruments now in universal use.
In the course of his work Hales indirectly discovered vasodilatation and vasoconstriction. Concluding that the force of the arterial blood in the capillaries could not be sufficient to produce muscular motion, he suggested a force regulated by the nerves, and perhaps electrical. "Hales was therefore the first physiologist to suggest, with some evidence to support it, the role of electricity in neuromuscular phenomena" (Dictionary of Scientific Biography). Digital facsimile from the Biodiversity Heritage Library at this link.