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Plants bend towards light, but they have no muscles and no nerves. Something must carry a message from the part that senses light to the part that bends. Frits Went was the first to collect that message in a block of jelly. This post traces the experiments that led to him, explains his method, works through how the Avena test measures auxin, and lists the mistakes students make.
Who was Frits Went?
Frits Warmolt Went was born in Utrecht in the Netherlands in 1903 and died in 1990, according to the Deutsche Biographie and the Dutch Biografisch Portaal, which lists a memorial biography by the Royal Netherlands Academy of Arts and Sciences. With Kenneth Thimann he wrote the book Phytohormones (1937), which is now freely readable online.
What came before Went?
The NCERT Class 11 chapter Plant Growth and Development (2026-27 reprint) starts with Charles Darwin and his son Francis, who saw that coleoptiles of canary grass bend towards one-sided light. After a series of experiments they concluded that the tip was the site of a transmitted influence that made the whole coleoptile bend.
In their 1880 book The Power of Movement in Plants, the Darwins describe covering the upper parts of canary grass (Phalaris) and oat (Avena) coleoptiles with little caps of tin foil or tubes of blackened glass. The lower, uncovered parts then did not bend towards a side light. They also showed this was not just because the covered tip could not bend first.
Phytohormones describes the next steps. Peter Boysen Jensen showed the transmission was one "of substance or of ions". Árpád Paal then showed that the stimulus could cross a layer of gelatin but not mica or platinum foil, and that a tip put back on one side of a cut stump made the stump bend even in darkness. Others, from 1921 onwards, put agar blocks mixed with tissue extracts on decapitated coleoptiles, but they saw only inhibition, never the growth-promoting substance.
What did Went do?
According to Phytohormones, Went succeeded in work published in 1926 and 1928:
He cut the tips off oat coleoptiles and stood them on small blocks of agar for a while, so that any substance could diffuse down into the agar.
He removed the tips and placed each agar block on one side of the cut stump of a decapitated coleoptile.
The stump curved away from the side carrying the block.
The side under the block grew faster, so the stump bent away from it. The substance had passed from tip to agar to stump. NCERT puts it in one line: "Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings."
How does the Avena test measure auxin?
Went's key improvement, Phytohormones explains, was to measure the angle of curvature, which proved proportional, within limits, to the concentration of the active substance. That made a quantitative bioassay, the Avena curvature test, which he and others used to find that the substance was stable to heat and light and diffused readily. Cholodny and Went then explained tropisms as an uneven distribution of this growth substance between the two sides of an organ.
Work through a sample calculation. The numbers are made up for practice and assume you are inside the proportional range.
A reference block gives a curvature of 12 degrees.
A test block gives 9 degrees.
Ratio: 9 ÷ 12 = 0.75, so the test block holds about three-quarters of the reference concentration.
A block made from half as many tips as the reference would be expected to give about 12 × 0.5 = 6 degrees.
Outside that range the rule breaks, so real experiments use standard conditions and a calibration curve.
Common mistakes in exam answers
Saying Went discovered phototropism. The Darwins described the bending and the role of the tip; Went collected and measured the substance.
Saying the stump bends towards the agar block. It bends away, because the side under the block grows more.
Confusing coleoptile and radicle. The coleoptile is the sheath around the young shoot of grasses such as oat.
Calling every growth regulator auxin. Auxin is one of five major groups in NCERT, with gibberellins, cytokinins, abscisic acid and ethylene.
For the full chapter, see our guide to plant growth regulators, and for another plant response to light, our post on Wightman Garner and photoperiodism.
What can a student take from Went?
Others had tried to extract the substance and failed. Went's advance was partly a cleaner method, letting the tip deliver the substance itself, and partly the decision to measure the bending rather than just note it. Turning a yes-or-no observation into a number is often what makes an idea testable. If you want to see a growth response change as conditions change, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Try it: draw the three steps of Went's experiment, with arrows showing which way the stump bends.
