Why do plant roots grow down and shoots grow up?
― They get it right even if you plant the seed upside down
Have you ever buried a morning glory seed in the soil without worrying which way up it went? Even so, the root always grows down and the shoot always grows up. Plants have no eyes and no brain. And yet they know which way is down. The clue turns out to be "heavy grains" inside their cells.
Think back to planting morning glory or kidney bean seeds at school. Didn't your teacher say "it doesn't matter which way up you plant it"? And sure enough, no matter which way you bury it, a shoot comes up just fine.
You may have seen rice plants flattened after a typhoon. A few days later, even though the stem is still lying down, just the tip has lifted up, as if the plant had raised its head.
Plants can't move from place to place, yet they can still correct their orientation. Where does this "power to right itself" come from?
There are two main reasons
Cells at the root tip and in part of the stem contain small grains heavier than their surroundings. Gravity pulls these grains down, and they sink to the bottom of the cell. Wherever they settle is "down." This is how the plant knows which way is which.
The side where the grains have sunk accumulates more of a growth-regulating substance. In the stem, this substance boosts growth; in the root, it does the opposite and suppresses it. Because the same lopsided distribution produces opposite responses, the direction of bending is also reversed.
In other words, plants have two separate systems: one for sensing up and down, and another for turning that sense into movement. Let's look at each in turn.
How does a plant know which way is "down"?
At the very tip of a root sits a cap-like structure called the root cap. Cells on its inside are packed with heavy grains loaded with starch. Being heavier than the surrounding fluid, they slowly sink to the bottom of the cell.
It takes time for the grains to sink. Roughly speaking, once a root is tipped sideways, the grains finish settling within a few minutes, bending begins after ten-odd minutes, and a bend visible to the eye takes a few hours. Plants seem to react slowly not because sensing is slow, but because it takes time for the cells to actually elongate.
If you bury a seed upside down, the cells end up upside down too. Even so, the grains simply settle to the bottom all over again. Wherever on Earth you are, wherever the grains settle is "down." The plant relies on nothing but this "settling point" to know its own orientation.
Take a look at Figure 1. The left panel shows a seedling right after being tipped on its side. The right panel shows it some time later, once it has corrected its orientation.
Why do roots and shoots bend in opposite directions?
Once the grains have settled, more of a growth-regulating plant hormone is carried to that same side. So far this holds true for both roots and stems. What differs is what happens next.
Stem cells grow more the more of this substance they have. In a stem lying on its side, the lower side grows faster, so the stem arches upward.
Root cells, however, stop growing when there's too much of the same substance. In a root lying on its side, growth on the lower side is suppressed, and only the upper side keeps growing. As a result, the root bends downward. The same signal gets exactly opposite replies.
One more important point is where the bending happens. Gravity is sensed by cells at the tip, but the actual stretching and bending happens a little further back, in a section that is still soft. Because the sensing spot and the moving spot are separate, the signal has to travel a few millimetres as a flow of the substance.
This is also why a flattened rice plant rights itself by bending partway up the stem. The hardened base doesn't move at all. Only the soft section still left near a node stretches again to straighten the plant's posture.
People often assume shoots grow upward because they're heading toward the light. But it's pitch dark underground. Until a seedling breaks the surface, it decides its direction without using light at all. The tendency to bend toward light only kicks in once it's above ground.
When plants are grown somewhere with almost no effective weight, such as the International Space Station, reports show that the direction roots grow in becomes inconsistent. For a plant, gravity is a "baseline" it takes utterly for granted.
Summary
Plants sense up and down through grains sinking inside their cells, and change how they grow based on how much of a substance collects on the sunken side. Roots and shoots bend in opposite directions because their response to that same substance is reversed. And you don't need to worry which way up you plant a seed, because the plant can correct its own orientation.
Plants aren't "looking" downward.
Inside their cells, grains are simply, quietly sinking.
You can read about how trees carry water all the way to their treetops in "Why can trees pull water up to 100 metres high?", about how trees change the way they grow season by season in "Why does a tree gain exactly one ring a year?", and about how the same growth substance arranges seeds into a pattern in "Why are sunflower seeds arranged in a spiral?". We also cover a case where a plant moves quickly not through growth but by shifting water in and out in "Why does a touch-me-not close its leaves the instant you touch it?".
You can find more on how plants change their insides to suit the season in "Why is timber felled from autumn to winter?" and "Are in-season vegetables really more nutritious?".
- Place a few kidney bean or soybean seeds on a damp sheet of kitchen paper, put it in a clear container, and cover it. Keep it indoors out of direct sun, and add water so it doesn't dry out.
- Once the roots have grown 2–3 cm, turn the container on its side (rotate it 90 degrees). Press the seeds gently in place with the paper so they don't shift.
- Every half day, look from the side and photograph how just the tip of the root bends downward. If a shoot has emerged, it should bend upward instead.
Only the "last few millimetres" of the root tip bends. Any part that has already hardened won't move. Mark where the bend starts, and you'll see clearly where the growing is actually happening.
Want to go deeper? ― Terms, formulas, and links to the curriculumLabels show whether a section is roughly middle-school level or university level
- JHScovered in middle-school science
- HScovered in high-school biology
- HS+advanced high-school content, or textbook sidebar material
- Univ.not covered in high school — university-level plant physiology
- Researchnot yet settled even at university level — an active research question
JHSTerminology: this phenomenon has a name
- Gravitropism: the tendency of a plant organ to bend in a fixed direction using gravity as its cue. Roots bend toward gravity; stems bend away from it.
- Auxin: a growth-regulating plant hormone. It boosts cell elongation in stems, but suppresses it in roots once it becomes too concentrated.
- Amyloplast: a small starch-filled grain inside a cell. Heavier than its surroundings, it sinks and acts as a weight signalling which way is down. Also called a statolith.
- Root cap: the cap-like structure covering a root's tip. It protects the tip as it pushes through soil, while also housing the cells that sense gravity.
JHSHSCheck with a formula: how fast does a root actually grow?
Only a very short section at the root tip bends. So how fast is that tip actually advancing? Let's take a seedling root's growth rate as 1 cm per day and convert it into other time units. Pay attention to how the units line up.
| As a symbol equation | L ＝ v × t |
| In words | length grown ＝ growth rate × time elapsed |
| Where it comes from | This is just the definition of speed — "speed = distance travelled ÷ time taken" — rewritten for length. We treat the root tip as advancing at roughly a constant rate for this estimate. |
| Symbol and meaning | Growth rate of a seedling root, in centimetres per day |
| Growth rate (varies by species and temperature) | about 1 cm per day |
| Length of one day, in hours | 24 hours |
| Days in a month | 30 days |
| Growth per hour, in cm | 1 ÷ 24 ≒ 0.042 |
| Converting cm to mm | 0.042 × 10 ＝ 0.42 |
| Growth over a month, in cm | 1 × 30 ＝ 30 |
That works out to about 0.42 mm an hour, and about 30 cm over a month. Human hair is often said to grow about 1 cm a month, so the root is advancing through the soil roughly 30 times faster than that. Plants may look motionless, but underground they're moving at quite a clip.
HSHS+How does the lopsided distribution actually arise?
HSHigh-school biology teaches tropisms as "a response that bends in a fixed direction relative to a stimulus." A response to light is phototropism; a response to gravity is gravitropism. Both are explained by an uneven distribution of auxin.
HS+Auxin is carried from cell to cell in one fixed direction only. Which face of the cell the carrier proteins line up on determines which way it flows. In a cell that has sensed gravity, these carriers are thought to regroup onto the lower face, tilting the flow of auxin toward that side.
Univ.From a weight signal to a chemical cue
University-level plant physiology treats this phenomenon as three stages: "sensing gravity," "converting the signal," and "responding." In the sensing stage, the sunken grains are thought to trigger the response by pressing against membranes or the cell's internal scaffolding. That pressure signal is then thought to be converted into shifts in calcium ion movement and internal acidity, which finally rearrange the carrier proteins. It's a process that translates a physical quantity — force — into a chemical cue. The reason roots and stems respond oppositely to the same auxin concentration is explained by each organ having a different growth-response curve to that concentration. The idea that sinking grains sense gravity is known as the starch-statolith hypothesis, and the idea that bending results from uneven auxin distribution is known as the Cholodny–Went hypothesis; together, these two form the foundation of today's explanation.
📖 Go further: Gravitropism (Japanese Wikipedia)
ResearchWhat's still not fully understood
- What exactly do cells use to "sense" the sunken grains? We can observe the grains settling, but the component that actually detects their weight or contact has not yet been fully identified.
- Responses in parts that lack the grains. Even plants with almost no starch grains show a weak gravity response. Researchers are debating whether some other mechanism is layered on top.
- Growing plants in space. How plants grow where there's almost no effective weight is being studied partly to make growing food possible on long space missions, but not every difference from growth on Earth has been explained yet.
In other words, even this article only describes "what's understood so far." That grains sink has been confirmed, but the path from there onward is still being rewritten.
Links to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science ・ plant structure and function / force and weight | Which way roots and shoots grow, what "gravity" means |
| HS | Biology ・ plant responses to environment (tropisms and auxin) | Why roots and stems bend in opposite directions |
| HS+ | Biology, advanced / textbook sidebar | Auxin carrier proteins and the direction of flow |
| Univ. | Plant physiology ・ cell biology | The three stages from sensing to response |
| Research | Plant gravity responses, growing plants in space | What's still not fully understood |
| ― | Everyday connections | Which way up to plant a seed, flattened crops righting themselves |
- The Botanical Society of Japan (日本植物生理学会), "Everyone's Plaza: Plant Q&A" (explanatory material on gravitropism and auxin)
- Charles Darwin and Francis Darwin, The Power of Movement in Plants (1880, a classic record of observations on tropisms)
- High-school biology textbook (unit on plant responses to environment, section on tropisms and auxin)
- Publicly available materials from the Japan Aerospace Exploration Agency (JAXA) and NASA on plant-growth experiments in space
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. Growth rates and the strength of responses vary greatly by plant species, temperature, and moisture. When making your own observations, please do not remove seeds or seedlings from the wild without permission.