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Leaves make sugar, but roots, fruits and growing buds need it. Something must carry sugar through the plant, sometimes downwards and sometimes upwards. The explanation in your textbook, pressure flow, was proposed by a forester in 1930 and waited more than eight decades for a direct test. This post explains the idea, the man behind it, and the 2016 measurements, with arithmetic you can check.
Who was Ernst Münch?
The Neue Deutsche Biographie records that Ernst Münch was born on 26 November 1876 in Ruchheim near Ludwigshafen and died on 9 October 1946 in Lechbruck. He studied forestry in Aschaffenburg and Munich, worked from 1904 to 1910 as an assistant in the botany section of the Bavarian forestry research institute, and earned his doctorate in 1909 on the disease susceptibility of woody plants.
He then worked as a practising forester, studied resin tapping and resin flow, and kept doing research in plant physiology. In 1921 he became professor of forest botany at Tharandt. In 1930 he published "Die Stoffbewegungen in der Pflanze" (the movement of substances in the plant), which set out his pressure flow theory. In 1933 he moved to the University of Munich.
What is the pressure flow hypothesis?
The eLife paper by Knoblauch and colleagues summarises Münch's idea in plain terms:
At a source, such as a mature leaf, sugars are loaded into the phloem sieve tubes, making the sap more concentrated.
Water moves in from the neighbouring xylem by osmosis, raising the pressure inside the tubes.
At a sink, such as a root tip or fruit, sugars are removed and used, so the concentration and pressure fall.
The pressure difference pushes the whole solution along the tube from source to sink.
According to the paper, Münch proposed that this happens without extra energy input along the transport path. Energy is spent at loading, not in between.
Why was it so hard to test?
The authors note that the hypothesis gained wide acceptance mainly for its simplicity and plausibility rather than experimental evidence. Phloem flow stops the moment you cut a plant, so most lab methods destroy what they try to measure. Critics also asked whether narrow sieve tubes, crossed by sieve plates, could let sap flow fast enough over the length of a tall tree, especially as measured pressures in trees seemed lower than in small herbs.
What did the 2016 experiments measure?
Knoblauch and colleagues grew morning glory (Ipomoea nil) vines 7.5 m long and measured each term in the flow equation they used, U = kΔp/(ηL), where U is flow speed, k is tube conductivity, Δp is the pressure difference, η is viscosity and L is length.
Flow speed: 123 µm/s, measured with radioactive carbon dioxide fed to one leaf
Sap viscosity: 1.7 mPa s, about an 18 percent sucrose solution
Conductivity: about 1 µm², from detailed tube anatomy
Pressure in source sieve tubes: 1.08 MPa; in root tip sink cells: 0.59 MPa
Rearranging gives the pressure gradient needed: Δp/L = Uη/k.
Δp/L = (123 × 10⁻⁶ m/s × 1.7 × 10⁻³ Pa s) ÷ (1 × 10⁻¹² m²)
= 2.09 × 10⁵ Pa/m, or about 0.21 MPa per metre
The farthest sink was no more than 2 m from a source, so the plant needed about 0.21 × 2 = 0.42 MPa. The measured difference was 1.08 − 0.59 = 0.49 MPa. Supply exceeds demand, so pressure flow can do the job. As a sense of speed, 123 µm/s is 123 × 10⁻⁶ × 3600 ≈ 0.44 m per hour.
When the team made plants longer, sieve tube conductivity and pressure rose with distance between source and sink. They concluded their results give strong support to the Münch hypothesis.
How does this appear in your syllabus?
NCERT Class 10, Chapter 5 (Reprint 2026-27) describes it without the name: sucrose is moved into phloem using energy from ATP, which raises osmotic pressure, draws in water, and moves material to tissues with less pressure; in spring, sugar stored in roots or stems moves to growing buds. Transport in plants is not in the NEET-UG 2026 syllabus, so check your own board's current syllabus.
Common mistakes:
Saying phloem only moves sugar downwards. Direction depends on where the source and sink are.
Saying ATP pushes the sap along the whole tube. ATP is used at loading; flow follows the pressure gradient.
Mixing units. Convert µm to m and mPa s to Pa s before you multiply.
What can a student take from Münch?
Münch was a working forester who kept asking physiological questions about the trees he managed. His hypothesis was simple enough to state in four lines and specific enough to be tested, even if testing it took 86 years, from 1930 to 2016. Pressure gradients are easier to grasp when you can watch them change; the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For the other half of plant transport, read about Dixon and Joly's cohesion-tension theory.
Try this today: redo the gradient calculation with a viscosity of 3.4 mPa s and decide whether the measured 0.49 MPa would still be enough over 2 m.
