Darwin’s Phototropism Experiment — How Does a Plant Know Where the Light Is?
Every plant on a windowsill seems to know something that it has never been taught: where the light is.
Leave a young plant beside a window and, given enough time, its shoots begin to lean towards the glass. Turn the pot around and, remarkably, the plant gradually changes direction again.
We usually accept this without thinking very much about it.
But there is a wonderful biological question hidden inside that simple observation:
How does a plant know where the light is?
Plants have no eyes, no brain and no nervous system remotely like ours. Yet they can detect the direction of light and alter their growth accordingly.
More than 140 years ago, Charles Darwin and his son Francis investigated this problem using an experiment so simple that a version of it can still be carried out on a kitchen table, windowsill or in a small home laboratory.
And the really fascinating discovery was not merely that plants grow towards light.
It was that the part of the plant that detects the light is not necessarily the part that does the bending.
That observation eventually helped open the door to the discovery of plant hormones.
A Classic Experiment With a Very Modern Question
Charles and Francis Darwin described their investigations of plant movement in their 1880 book The Power of Movement in Plants. They studied young grass seedlings and discovered that directional light was detected principally at the tip of the emerging shoot. Covering that tip with an opaque cap could prevent the normal bending response, even though the region lower down the shoot remained illuminated.
Today we call growth towards light positive phototropism.
The experiment is beautifully suited to teaching because the apparatus is almost embarrassingly simple.
You need seedlings, some small caps and a light source.
But from those simple materials comes a surprisingly sophisticated idea about communication within a living organism.
Recreating Darwin’s Experiment at Home
For a practical version, I would use a fast-germinating grass such as oats or wheat.
You want the seedlings to be young enough that the emerging grass shoot, or coleoptile, is still clearly visible.
A coleoptile is the protective sheath that surrounds the first emerging shoot of grasses. It is particularly useful for experiments such as this because its growth response is relatively easy to observe.
What you need
You could use:
- oat, wheat or another suitable grass seed;
- several small pots, trays or sections of damp paper;
- compost, vermiculite or moist cotton wool;
- a cardboard box;
- a desk lamp or other directional light source;
- aluminium foil or black paper;
- transparent plastic film;
- small pieces of paper or drinking straw from which to make caps;
- ruler;
- protractor or photographs for measuring curvature;
- labels.
I would recommend growing several seedlings for each treatment rather than relying upon one plant.
Living organisms vary.
One seedling may grow rapidly while another hardly grows at all. Replication turns a demonstration into a much better scientific investigation.
Stage One — Grow the Seedlings in Darkness
Germinate the seeds and keep them in darkness until shoots have emerged.
Growing them initially in darkness has an advantage.
The shoots tend to become relatively long and pale as they search for light, making subsequent curvature easier to see.
Once you have reasonably similar seedlings, divide them into experimental groups.
The Four Key Treatments
1. The uncovered seedling
Leave the entire shoot exposed.
This is your basic control.
When illuminated from one side, it should bend towards the light.
2. Cover the tip with an opaque cap
Make a tiny cap from foil or another completely opaque material and place it over the very top of the shoot.
Do not crush the seedling.
The lower part of the shoot remains exposed to exactly the same directional light as before.
The intriguing result is that the seedling should now show greatly reduced phototropic bending.
And immediately we have a puzzle.
The lower part of the shoot can still see the light — so why doesn't it respond normally?
3. Cover the tip with a transparent cap
This is one of my favourite controls in the experiment because it answers an obvious criticism.
Perhaps putting a cap on the plant simply interferes mechanically with its growth.
So make another cap from transparent plastic.
The tip is enclosed just as before, but light can pass through it.
The plant should still show phototropic curvature.
That tells us that it is not simply the presence of the cap that matters.
Blocking the light reaching the tip is important.
4. Cover the lower shoot but leave the tip exposed
Now reverse the arrangement.
Wrap an opaque shield around part of the lower coleoptile while leaving the tip exposed.
We might expect this plant not to bend because the region where much of the bending actually occurs has been hidden from the directional light.
Yet it can still respond.
The tip detects the direction of illumination and somehow sends information downwards to the growing region.
This is the crucial observation.
The Darwins' work showed that the perception of light and the resulting growth response could occur in different regions of the plant.
The Results You Might Expect
| Treatment | Tip receives directional light? | Expected response |
|---|---|---|
| Uncovered shoot | Yes | Strong bending towards light |
| Opaque cap on tip | No | Little or greatly reduced bending |
| Transparent cap on tip | Yes | Bends towards light |
| Lower shoot covered, tip exposed | Yes | Bends towards light |
An excellent optional fifth treatment is simply to remove the tip.
Again, phototropic bending is greatly reduced.
But be careful about interpretation: removing the tip also removes actively growing tissue and potentially damages the seedling, so the opaque-cap experiment is in many ways the more elegant demonstration.
Why the Experiment Is So Clever
At first glance this looks like an experiment about light.
It is actually an experiment about information.
Imagine that we separate the plant into two regions.
At the top is the detector.
Further down is the machinery producing the response.
Something therefore has to connect the two.
The Darwins did not discover auxin in this experiment. Their work suggested that some influence was transmitted from the light-sensitive region towards the region where bending occurred. Later experiments by researchers including Peter Boysen-Jensen and Frits Went helped establish the idea of a mobile chemical growth signal and contributed to the development of our understanding of auxin.
That historical distinction is worth making.
Science often advances this way.
One experiment rarely produces the whole explanation.
Instead:
Darwin asks where the stimulus is detected.
Then someone else asks:
What carries the message?
Then another scientist asks:
What molecule is responsible?
And modern biology asks:
What receptors, proteins and genes control the whole system?
One simple plant bending beside a lamp becomes decades of biology.
So What Is Auxin Doing?
For an A-level student, this is where the experiment becomes especially useful.
One of the major auxins in plants is indole-3-acetic acid, usually abbreviated to IAA.
When a young shoot receives light predominantly from one direction, blue-light receptors called phototropins help detect this uneven illumination. This ultimately results in an unequal distribution of auxin across the shoot, with more growth-promoting auxin activity on the shaded side.
In a shoot, auxin promotes cell elongation.
So imagine the stem viewed from above:
Light side: cells elongate less.
Shaded side: cells elongate more.
If one side becomes longer than the other, the shoot cannot remain straight.
It curves.
And because the shaded side has grown more, the shoot curves towards the light.
That distinction is important.
Auxin does not somehow "pull" the plant towards the lamp.
The plant bends because of differential growth.
One side grows faster than the other.
The Plant Is Not Turning — It Is Growing Unequally
This is something students can easily overlook.
An animal can see food, turn its body and walk towards it.
A young plant cannot do that.
Once rooted into the soil, it has another solution.
It alters where growth occurs.
That makes tropisms fundamentally different from most animal movements.
When a sunflower appears to have "turned" or a seedling leans towards a window, at least some of what we are observing is the consequence of controlled differences in cell growth.
This is a wonderfully economical biological system.
No muscles.
No brain.
No conscious decision.
Just receptors, chemical signalling and changes in cell growth.
What Happens Inside the Cells?
For younger students, saying that auxin causes the shaded side to grow faster is sufficient.
For A-level students, we can go another step.
Auxin can stimulate proton pumps in cell membranes. These move H+ ions into the cell wall region, reducing the pH.
That acidic environment activates proteins including expansins, which loosen interactions within the cell wall.
The cell wall becomes more capable of expanding.
Water entering the cell maintains turgor pressure, and the cell can elongate.
So we can follow the mechanism from something enormous — a plant bending visibly towards a window — all the way down to processes occurring across microscopic cell membranes.
Directional light → photoreceptor signalling → altered auxin distribution → increased cell elongation on the shaded side → bending towards light.
That is a lovely chain of cause and effect for an A-level Biology student to practise explaining.
Turn the Demonstration Into a Proper Investigation
Rather than simply deciding whether each plant "bent" or "didn't bend", we can collect numerical data.
Photograph the seedlings from exactly the same position at regular intervals.
For example:
- immediately before illumination;
- after 2 hours;
- after 4 hours;
- after 8 hours;
- after 24 hours.
Measure the angle between the original vertical direction and the direction of the shoot tip.
You could then calculate a mean angle for perhaps five seedlings in each treatment.
That produces genuine quantitative data.
A student could graph:
Time on the horizontal axis
against
mean angle of curvature on the vertical axis.
Suddenly Darwin's Victorian botanical experiment has become an excellent modern exercise involving biology, experimental design, statistics, photography and data analysis.
One Seedling Is Not an Experiment
This is also an ideal opportunity to explain biological variation.
Suppose our uncovered plant bends 35 degrees towards the light while our capped plant bends 4 degrees.
That looks convincing.
But perhaps the second seedling was simply unhealthy.
Instead, try five or ten plants in each condition.
If most uncovered plants bend strongly and most opaque-capped plants do not, our evidence becomes much more convincing.
If doing this with an A-level student, I would consider calculating:
- mean curvature;
- range;
- standard deviation if appropriate;
- perhaps statistical significance in a larger investigation.
For a family experiment, however, photographs and a simple comparison are more than adequate.
Keep the Light Directional
One practical difficulty is that modern houses are full of stray light.
A seedling beside a lamp may also receive daylight from a window and artificial light from several other directions.
The easiest solution is a cardboard box.
Cut a window in one side and place the light beyond it.
Now most of the useful illumination arrives from one direction.
You have effectively constructed a miniature controlled environment chamber for almost nothing.
A Few Things That Can Go Wrong
This experiment is straightforward, but biology rarely behaves as neatly as a textbook diagram.
The seedlings don't bend very much
They may not have been exposed for long enough, or the directional difference in light intensity may not be strong enough.
Move the light closer, while avoiding excessive heating.
Everything bends before the experiment starts
The seedlings may already have received directional light.
Grow them in darkness or uniform lighting first.
The opaque caps fall off
They need to be extremely light.
A heavy cap can bend or damage the shoot and introduce an entirely different variable.
The capped seedlings stop growing completely
Check that the cap is not physically constricting the shoot.
This is exactly why the transparent-cap control matters.
Different seedlings give different results
Good!
That is biology.
Use more seedlings, calculate averages and discuss variation rather than hiding it.
An Excellent Investigation of Controls
From a teaching point of view, the real strength of this experiment may be its experimental design.
Ask a student:
"Why do we need the transparent cap?"
They should eventually realise that it separates two possible explanations.
If both opaque and transparent caps stopped bending, then perhaps the physical cap itself was responsible.
But if the transparent cap allows bending while the opaque cap prevents it, the evidence points towards light reaching the tip as the important variable.
Then ask:
"Why cover the bottom of the shoot?"
Because it distinguishes the region detecting light from the region producing the response.
This is experimental science at its best: changing one thing at a time to distinguish between competing explanations.
Darwin Didn't Have Our Modern Equipment
This is another aspect of these classic experiments that I find particularly appealing.
We sometimes give students the impression that important science requires enormously expensive equipment.
Modern science certainly can.
But some profound scientific discoveries began with extraordinarily simple apparatus.
Darwin did not have digital light sensors, data loggers, time-lapse cameras or image-analysis software.
Today, however, we can repeat the underlying investigation and add all of those things if we wish.
A smartphone placed on a tripod could produce a time-lapse sequence.
The resulting video would make the movement dramatically more obvious than watching the seedling directly.
Image-analysis software could measure the curvature.
A light sensor could measure illumination.
A temperature sensor could check whether the lamp is heating one side of the apparatus.
It becomes an excellent example of combining a classic scientific question with modern measurement technology.
A Family Version: Make It a Prediction Game
For children, I would not begin by explaining auxin.
I would show them four seedlings and ask them to predict what will happen.
Which plant will bend?
Perhaps draw the expected shape of each shoot before switching on the lamp.
Then return the following day.
The opaque-tip experiment is particularly satisfying because the answer is not necessarily obvious.
Most people initially assume that the part of the plant doing the bending must also be the part detecting the light.
When the experiment contradicts that prediction, we have one of the most important experiences in science:
"That isn't what I expected."
That sentence is often where interesting science starts.
A-Level Extension — Follow the History of the Signal
The experiment can also become the beginning of a much larger sequence.
Once we know the tip is important, the obvious next question is:
What travels from the tip to the rest of the plant?
That led to further classic experiments.
Peter Boysen-Jensen placed different materials between the tip and the lower part of the coleoptile. A permeable gelatin barrier allowed the response to continue, whereas an impermeable barrier could prevent it. This supported the idea of a mobile chemical signal rather than something requiring direct cellular continuity.
Frits Went's subsequent work with coleoptile tips and agar blocks helped establish the existence of a diffusible growth-promoting substance associated with auxin research.
So Darwin's experiment can become the first chapter in a scientific detective story:
Where is light detected?
How does the message move?
What is the message?
How does that message alter cell growth?
That progression is far more memorable than simply learning:
"Auxin causes phototropism."
Could We Test Colour as Well as Direction?
A particularly interesting extension would be to illuminate seedlings using different wavelengths of light.
Modern plant phototropism is strongly associated with blue-light photoreceptors called phototropins.
So we could investigate whether seedlings respond equally strongly to:
- blue light;
- red light;
- green light;
- white light.
Care would be needed because different lamps may have very different intensities. Merely placing coloured plastic over a lamp would not necessarily produce a fair quantitative experiment.
But it creates another excellent question:
Does a plant simply measure how much light there is, or can it distinguish different wavelengths?
Once again the apparently simple plant on the windowsill turns out to possess remarkably sophisticated sensory biology.
Plants Know Far More About Their Environment Than They Appear To
This experiment also challenges a very human way of thinking.
Because plants do not move around rapidly, we often treat them as passive.
They are anything but passive.
Plants continually detect and respond to:
- light;
- gravity;
- water;
- touch;
- temperature;
- chemicals;
- day length;
- damage;
- neighbouring organisms.
They do not experience these things as we do, but they possess molecular sensing and signalling systems capable of changing growth and development.
Phototropism is simply one particularly visible example.
The Real Lesson From Darwin's Seedlings
We could describe this as an experiment demonstrating phototropism and auxin.
But I think that undersells it.
The more interesting lesson is about how biology discovers hidden processes.
We cannot see a plant detecting blue light.
We cannot watch auxin molecules redistributing with the naked eye.
We cannot directly see individual cells on one side of the stem elongating faster than those on the other.
Yet by performing a carefully designed set of experiments, we can infer that something must be happening.
Cover the top.
The response disappears.
Use a transparent cover.
It returns.
Cover another region instead.
The response remains.
Each experiment eliminates an explanation and narrows the possibilities.
That is scientific reasoning.
And it can be demonstrated with little more than some grass seedlings, cardboard, foil and a lamp.
Conclusion — How Does a Plant Know Where the Light Is?
A plant does not "know" where the light is in the conscious sense.
Instead, cells near the growing tip contain molecular systems capable of detecting differences in illumination.
That information alters growth signalling, including the distribution and action of auxin.
Cells on the shaded side elongate more strongly than those on the illuminated side.
The shoot therefore curves towards the light.
What makes the Darwin experiment so memorable is the discovery that sensing and responding can happen in different places.
The tip detects.
A message travels.
Cells further down respond.
The plant bends.
And from that remarkably simple observation grew an entire field of plant physiology.
The next time a houseplant starts leaning towards a window, it is worth remembering that you are watching the result of an extraordinarily sophisticated biological signalling system.
The plant has no eyes — but it can certainly detect where the light is.
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