Stephen Hales

The Man Who Weighed a Plant's Thirst · Clergyman & Plant Physiologist · 1677–1761

The country parson who first put numbers on a living plant — how much water it draws, how fast, and with what force.

Who was Hales?

Stephen Hales was born in Kent in 1677, took orders, and served as perpetual curate of Teddington in Middlesex for over half a century. He did his science in the parsonage garden. Before him, plants were described; Hales measured them. In 'Vegetable Staticks' (1727) he weighed potted sunflowers daily, sealing the soil so that only the plant could lose water, and established that a sunflower draws up and gives off many hundreds of grams of water a day — perhaps seventeen times what a man perspires, weight for weight. He cut vines and fitted glass tubes to the stumps, measuring the force with which sap is pushed upward from the roots — root pressure — and found it could raise a column of mercury. He measured the rate of sap flow, the pressure in stems and branches, and the water lost from leaf surfaces, and he showed that a plant's leaves take in something from the air that contributes to its substance, anticipating what the next century would name. He then did the same for animals: in 'Haemastaticks' (1733) he made the first direct measurement of blood pressure, inserting a tube into the artery of a horse and recording how high the blood rose. He invented a ventilator that saved lives in ships, prisons and hospitals, and campaigned against gin. He died at Teddington in 1761.

Major achievements

• Founded plant physiology by measuring rather than describing — 'Vegetable Staticks' (1727)
• Measured transpiration: how much water a plant takes up and gives off in a day
• Discovered and measured root pressure using glass tubes fitted to cut vine stumps
• Made the first direct measurement of blood pressure, in a horse (1733)
• Showed that leaves draw something from the air that becomes part of the plant

Life in brief

• 1677 — Born in Bekesbourne, Kent: Son of a landed family.
• 1709 — Perpetual curate of Teddington: Where he remained over fifty years.
• 1718 — Elected to the Royal Society: For his experiments on the movement of sap.
• 1727 — Vegetable Staticks: Transpiration, root pressure and sap flow, measured for the first time.
• 1733 — Haemastaticks: The first direct measurement of blood pressure.
• 1741 — The ventilator: Installed in ships, prisons and hospitals; deaths fell sharply.
• 1761 — Dies at Teddington: Aged 83, still curate of the same parish.

Explore the life of Hales — five chapters

1. Fifty Years in One Parish — A curate who applied Newton to a cabbage
2. Weighing a Sunflower Every Day — How much water a plant loses, and where it goes
3. The Push From Below — Glass tubes on a cut vine, and a failure worth studying
4. A Tube in a Horse's Artery — The first measurement of blood pressure, 1733
5. Ventilators, Gin and a Long Old Age — A clergyman who counted deaths and changed them

Read the five-chapter life of Stephen Hales →

Where Hales appears in your course

Stephen Hales has a genuine claim on 2 lessons of the Pearson Edexcel International GCSE science course built into Incandio:

• Why Transport Systems Exist, and the Two Plant Vessels — Biology: This lesson claims a plant has two separate transport systems. Hales is the man who first measured any transport in a plant at all — the quantity of water passing through a sunflower in a day, the rate sap moves, and the force pushing it up from a cut root — and he found ONE sap in one system of vessels, because the distinction between the two tissues could not be seen with the instruments of 1727. He is the standard of evidence this page's claims have to meet.
• Water Uptake and Transpiration — Biology: Everything on this page was first turned into a number by Hales. In 1727 he potted sunflowers, sealed the soil with lead and putty so water could escape only through the plant, and weighed them daily — the first measurement of transpiration ever made. He then fitted glass tubes to cut vines and measured the force pushing sap up from the roots. And he reported honestly that this force was far too weak to raise sap to the top of a tall tree, leaving open the exact question this lesson answers.

Begin

• Start a conversation with Hales — a dramatised, historically grounded AI portrayal
• Read the Historical Brief
• Explore the life of Hales — five chapters
• Find your exam topics · Meet all 208 figures