Robert Brown was a Scottish botanist whose name lives on in physics, even though he spent his career studying plants. His most famous findings came from patiently watching tiny things through a small single-lens microscope. Two of his observations, a restless jiggling and a dark spot inside cells, each became foundations of modern science decades after he made them. In his own day, though, he was best known for cataloguing the plants of Australia.
How Brown Discovered Brownian Motion
In 1827 Brown looked at pollen grains of a wildflower called Clarkia pulchella in water.1APS News. August 1827: Robert Brown and Molecular Motion in a Pollen-filled Puddle Inside the grains he saw oblong granules about 5 microns long, roughly a twentieth of a hair’s width, in constant motion.2University of Virginia. Brownian Motion Smaller round grains moved even more vigorously.3University of Virginia. Brownian Motion He checked that the movement was not caused by currents in the water or by evaporation.4University of Virginia. Brownian Motion
Under the eyepiece, each particle trembles and darts in a short, jerky zigzag, changes direction without warning, and never settles, even if you watch for hours. That differs from the slow, steady drift of everything in one direction that you get when a slide is tilted or the water is drying out. This motion has no overall direction, and neighbouring particles move independently. Smaller particles jiggle more briskly than larger ones, which matches Brown’s note about the small round grains. Today this motion is called Brownian motion, and you can see it yourself with dust or milk fat droplets under a basic microscope.
At first Brown suspected the particles moved because they were alive, like a plant version of sperm.5University of Virginia. Brownian Motion But pollen from dead plants jiggled just as much.6University of Virginia. Brownian Motion He then found the motion in fragments of fossilized wood.7University of Virginia. Brownian Motion Then he found it in things that were never alive: crushed window glass and dust from a stone of the Sphinx.8University of Virginia. Brownian Motion The jiggling was a property of tiny particles themselves, not of life.
The cause is that water molecules are in constant, random thermal motion and strike a tiny particle from every side. At any instant the hits from one side are very slightly stronger than those from the other, so the particle is nudged one way, then another. Brown could not know this. Molecules were far too small for his lens, so what he saw was the particle’s response to invisible bumps. He concluded only that the motion belonged to small particles in a fluid, and he never claimed to have explained it.
Naming the Cell Nucleus
While studying orchid tissue, Brown noticed a small dark round body inside each cell.9Microscopy Today. Living Images from the Birth of Microscopy He called it the nucleus, a name he coined around 1828 to 1831.10Microscopy and Analysis. The Clarity of Images from Early Single-Lens Microscopes Captured on Video He also showed the body was present in a variety of plant cells, not just orchids.11The Scientist. Sketching out Cell Theory, circa 1837
In orchid tissue the nucleus looks like a small, roundish, often darker spot sitting inside each cell, usually near the centre. Cells are transparent, so without good lighting and careful focus the spot can blend into its surroundings. Brown’s contribution was not only seeing it but recognising it as the same kind of structure from cell to cell rather than random debris.
Today we know the nucleus holds most of the cell’s DNA, the instructions for building and running the cell, and directs which instructions are used. Plant and animal cells both have one, though some specialised cells, such as mature human red blood cells, lose theirs. Brown had no idea of its function. To him it was a structure worth naming. Earlier microscopists may have glimpsed similar bodies, but Brown’s name and his demonstration of how common it was are why he is credited.
Seeing Cells in Motion
Brown also watched the fine hairs on the stamens of Tradescantia, the spiderwort plant.12Infocus. Charles Darwin and Robert Brown – their microscopes and the microscopic image There he saw the living contents of cells flowing around inside them.13Infocus. Charles Darwin and Robert Brown – their microscopes and the microscopic image Modern tests with his own lens show the flow with striking clarity.14Infocus. Charles Darwin and Robert Brown – their microscopes and the microscopic image
The stamen hairs are chains of large, see-through cells, so tiny granules inside can be watched moving along the cell in streams. The movement follows paths, often circulating around the cell. That is unlike the random trembling of Brownian motion, which happens in dead or nonliving material too. The flowing of a cell’s contents is an organized flow driven by the living cell, and it needs the cell to be alive and supplied with energy. It slows or stops when the cell is damaged or cooled.
Mapping the Plants of Australia
Brown studied medicine in Edinburgh but did not graduate.15Encyclopedia of Australian Science and Innovation. Brown, Robert He became botanist on HMS Investigator under Matthew Flinders, exploring the coasts of Australia, then called New Holland.16Encyclopedia of Australian Science and Innovation. Brown, Robert He sailed with collector Peter Good and artist Ferdinand Bauer.17Encyclopedia of Australian Science and Innovation. Brown, Robert Good gathered plants, and Bauer made accurate drawings that preserved features dried specimens lose. The ship left in July 1801, reached southwest Australia in December and Port Jackson, now Sydney, in May 1802.18Encyclopedia of Australian Science and Innovation. Brown, Robert
In 1810 he published the Prodromus Florae Novae Hollandiae, the first attempt to survey Australian plants.19Wikipedia. Prodromus Florae Novae Hollandiae et Insulae Van Diemen It described over 2,040 species, more than half for the first time.20Wikipedia. Prodromus Florae Novae Hollandiae et Insulae Van Diemen
Describing one new species took a lot of work. A specimen had to be collected with its flowers, and ideally fruit, because those features separate close relatives. It was then pressed and dried so it would survive the long voyage home. Back in Britain, Brown compared it with known plants to find out whether it was truly new, then wrote a detailed description. A formal name let any botanist anywhere know exactly which plant was meant, and the shared catalogue let later botanists build on his work.
Few European botanists had seen Australian plants, so many specimens fitted no known group and forced Brown to think about classification. The Prodromus gave later botanists a starting list to correct, extend and build upon.
Rafflesia and Brown’s Museum Career
A flower of extraordinary size was found in Sumatra in 1818 on a journey by Sir Stamford Raffles.21Transactions of the Linnean Society of London. An account of a new genus of plants, named Rafflesia The specimen had to reach Britain before it could be studied properly. Brown recognised that it fitted no existing genus, described it as a new one, and named it Rafflesia in honour of Raffles.22Transactions of the Linnean Society of London. An account of a new genus of plants, named Rafflesia
Rafflesia has the largest single flower of any plant, up to about a metre across. It smells like rotting meat, which attracts the flies that carry its pollen. It has no leaves, stems or true roots and lives as a parasite inside a host vine, appearing only when it flowers.
Brown became librarian to the naturalist Joseph Banks.23Wikipedia. Robert Brown (botanist, born 1773) When Banks died in 1820, Brown inherited his library and herbarium, a collection of dried pressed plants.24Wikipedia. Robert Brown (botanist, born 1773) In 1827 the collection went to the British Museum with Brown as its keeper.25Natural History Museum. Botany He became the first Keeper of the museum’s botany department and held the post until his death in 1858.26Wikipedia. Robert Brown (botanist, born 1773)
Dried specimens let scientists recheck species centuries later. Banks’s and Brown’s plants became part of today’s Natural History Museum in London.
How Brown’s Discoveries Shaped Later Science
In Berlin, Matthias Schleiden was influenced by Brown’s studies of the nucleus.27British Society for the History of Medicine. Matthias Schleiden and the Genesis of Cell Theory Schleiden saw the nucleus as found in all plants and argued that a plant is a community of cells.28encyclopedia.com. Advances in Cell Theory In 1837 he mentioned Brown’s nucleus to Theodor Schwann over dinner.29The Scientist. Sketching out Cell Theory, circa 1837 Schwann, hearing it described, was reminded of something he had seen in animal tissue. He checked, recognized the same structure in animal embryo tissue, and the two confirmed cells with nuclei in both.30The Scientist. Sketching out Cell Theory, circa 1837
That tie between plants and animals is what made the idea a general theory rather than a plant-only finding. Schwann defined true cells by having a nucleus, building cell theory on Brown’s start.31encyclopedia.com. Advances in Cell Theory Cell theory says all living things are made of one or more cells, and that the cell is the basic unit of structure and life. It underlies modern biology and medicine, since diseases and treatments are understood by what happens to cells.
Brownian motion had a longer wait. Albert Einstein later produced a theory explaining it as the result of molecular collisions. Jean Perrin’s experiments confirmed the theory using tiny particles of a yellow pigment.32Archive for History of Exact Sciences. Einstein–Perrin dilemma on the Brownian motion (Avogadro’s number) resolved? This was the proof of molecules that convinced even the last sceptics.33Archive for History of Exact Sciences. Einstein–Perrin dilemma on the Brownian motion (Avogadro’s number) resolved? Nearly 80 years passed between Brown’s observation and Einstein’s 1905 explanation.
Could a Simple Microscope Really See All This
A simple microscope uses one small, strongly curved lens held close to the eye, like a very powerful magnifying glass. Compound microscopes of the period used several lenses and tended to produce coloured fringes, which is where the idea came from that they were poor instruments. Brown’s lenses survive at the Linnean Society of London, which is why they could be tested again.
Some people later doubted that Brown’s pollen observations were truly Brownian motion, since pollen grains also contain things that might move for other reasons.34APS News. August 1827: Robert Brown and Molecular Motion in a Pollen-filled Puddle Microscopist Brian J. Ford argued that Brown’s writings show he studied particles in a closed setting and understood turbulence.35APS News. August 1827: Robert Brown and Molecular Motion in a Pollen-filled Puddle Ford recreated the demonstration with the original lens and filmed it.36APS News. August 1827: Robert Brown and Molecular Motion in a Pollen-filled Puddle The motion was clearly visible inside the pollen grains.37The Microscope. Brownian Movement in Clarkia Pollen: A Reprise of the First Observations Most scientists now accept that Brown’s observations were genuine Brownian motion.38APS News. August 1827: Robert Brown and Molecular Motion in a Pollen-filled Puddle
The nucleus has had its own test. A BBC programme tried to show Brown’s view of orchid cells with his lens and saw almost nothing.39Microscopy Today. Living Images from the Birth of Microscopy But careful adjustment of the same lens on the same tissue clearly showed nuclei and leaf pores.40Microscopy Today. Living Images from the Birth of Microscopy Good results depended on light, mounting and careful focusing, so skill mattered as much as equipment. Other recreations showed nuclei and even the smaller bodies inside them, using oil lamps and daylight.41Journal of Biological Education. The nucleus and the simple microscope The belief that rainbow-coloured blur ruined simple microscopes is a misconception.42Info Microscopes. Charles Darwin and Robert Brown – their microscopes and the microscopic image