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Transpiration is usually taught as a definition: water lost as vapour from the aerial parts of a plant. Stephen Hales turned it into a measurement. In the 1720s this English clergyman weighed plants, measured leaves with a thread grid and worked out how fast water moves through a stem. This post goes through his sunflower experiment with his own numbers and shows how to check them.
Who was Stephen Hales?
Hales (1677 to 1761, per the Internet Archive catalogue record of his book) signed himself on the title page of *Vegetable Staticks* as a Fellow of the Royal Society, Rector of Farringdon in Hampshire and Minister of Teddington. The book, published in 1727, describes experiments "read at several meetings before the Royal Society". A contemporary summary by J. T. Desaguliers appeared in the Philosophical Transactions in 1727.
"Staticks" here means the science of weighing. Hales's method was to weigh everything.
How did Hales measure water loss from a sunflower?
His Experiment I began on 3 July 1724. He grew a large sunflower, over 3 feet high, in a garden pot and covered the pot with a sheet of thin lead, cemented so that no vapour could escape from the soil. A small glass tube let air in, and a second tube with a cork let him water the plant.
He weighed the pot and plant morning and evening on fifteen days between 3 July and 8 August. At the end he cut off the plant, sealed the stump and found that the porous pot itself lost 2 ounces every 12 hours, which he subtracted. His results:
Greatest loss in 12 hours on a very warm dry day: 1 pound 14 ounces (30 ounces).
Middle rate for 12 hours of day: 1 pound 4 ounces (20 ounces).
A dry warm night: about 3 ounces. With dew: nothing. With heavy dew or light rain, the pot gained 2 to 3 ounces.
He then cut off every leaf, sorted them into five size groups, and measured one leaf from each group under a lattice of threads with quarter-inch squares. Total leaf surface: 5616 square inches, or 39 square feet.
Worked example: how thick a layer of water did the leaves lose?
Hales converted 20 ounces of water to 34 cubic inches, using his figure of 254 grains for a cubic inch of water. Then he spread that volume over the plant.
Over the leaves:
Depth = volume ÷ area = 34 ÷ 5616 ≈ 0.0061 inch, which is about 1/165 of an inch.
In metric (our conversion):
34 cubic inches × 16.39 cm³ per cubic inch ≈ 557 cm³, a little over half a litre.
5616 square inches × 6.452 cm² per square inch ≈ 36,230 cm², about 3.6 m².
Depth = 557 ÷ 36,230 ≈ 0.0154 cm ≈ 0.15 mm in 12 hours.
Over the roots: Hales estimated the root surface at 2286 square inches. 34 ÷ 2286 ≈ 1/67 inch. So water entered the roots faster than it left the leaves, in the ratio 165 : 67, which he rounded to nearly 5 : 2. The ratio is just the leaf area divided by the root area, because the same volume passes through both.
Common mistakes:
Mixing up the 12-hour day figure (20 ounces) with the 24-hour figure (Hales used 22 ounces for a full day).
Forgetting to square the conversion factor: 1 inch = 2.54 cm, so 1 square inch = 6.45 cm², not 2.54.
How strong is the push from the roots?
In a later set of experiments, Hales fixed mercury gauges to cut vine stems in the spring "bleeding" season. In one vine, the sap raised mercury 38 inches, which he equated to about 43 feet of water. He noted this was nearly five times the force of blood in the great crural (thigh) artery of a horse, which he had also measured.
Check it: 38 inches = 96.5 cm of mercury. Mercury is about 13.6 times denser than water, so 96.5 × 13.6 ≈ 1312 cm, about 13 m or 43 feet of water. As pressure, p = ρgh = 13,600 × 9.8 × 0.965 ≈ 1.29 × 10⁵ Pa above atmospheric.
Where is transpiration in the syllabus now?
NCERT Class 10 Science, Chapter 5 (Life Processes) defines transpiration, explains that it pulls water up through the xylem, and notes that root pressure matters more at night while transpiration pull dominates in the day. That is Hales's sunflower and his vine in one paragraph. The current rationalised NCERT Class 11 Biology contents have no separate chapter on transport in plants, so check your board and the current NEET-UG syllabus to see how much is examined. Our guide to Life processes for Class 10 covers the chapter, and Jan Ingenhousz continues the story of what leaves do with light.
What can a student take from Hales's way of working?
Measure, do not just describe. A balance and a thread grid turned a vague idea into numbers.
Subtract the background. The pot's own water loss was his control.
Compare across systems. He set plant against man, and sap pressure against blood pressure, to make the numbers mean something.
If unit conversions like these trip you up, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Next step: redo the leaf-depth calculation for Hales's hottest day, 30 ounces, and check you get about 1/110 of an inch.
