Why doesn't a cactus die when you never water it?
― It only "breathes" at night
A houseplant in the same room wilts after just a few days without water. A cactus can be ignored for a month and look perfectly fine. The difference isn't how much water it stores. It's when it opens its water "exit." Ordinary plants open their pores by day — a cactus keeps them shut tight the whole time. Instead, it takes in air only at night.
You come home from a trip and check on your plants. The houseplant by the window has drooping leaves and bone-dry soil.
The small cactus next to it looks exactly the same as before you left. Its soil is even drier, yet it hasn't shrivelled at all.
You could just say "it stores water" and leave it there. But the real story is why it doesn't have to spend that stored water in the first place.
There are two main reasons
A leaf has tiny pores that let air in and out. Open them, and water escapes along with it. A cactus keeps these pores sealed during the driest hours of the day.
Keeping pores shut all day means no raw material gets in. So at night, when it's cool, the cactus opens its pores, takes in carbon dioxide, and turns it into an acid to store inside itself.
These two tricks work as a pair. Let's look at each in turn.
A plant's "pores" can't help but lose water
The surface of a leaf is covered in countless tiny openings too small to see. These are called stomata (pores). Through them, a plant draws in carbon dioxide and uses light to build sugar.
But these openings have an awkward property. The same path that lets carbon dioxide in also lets water vapour out. The inside of a leaf is damp, so the moment a pore opens, water starts escaping.
Dry daytime air makes this escape even faster. The hotter the air, the more water vapour it can hold. So a plant ends up losing the most water exactly when the light is strongest. Ordinary plants make up for this by pulling more water up through their roots. In the desert, there's no such supply to draw on.
So the cactus made a bold choice: keep its pores shut by day. Figure 1 shows night on the left and day on the right.
Storing air overnight, in the form of an acid
Keeping pores shut all day means no carbon dioxide — photosynthesis's raw material — can get in. Light alone can't make sugar without it.
So the cactus shifted its timing. It opens its pores after sunset, once the air has cooled and turned damp. Taking in the same amount of carbon dioxide costs far less water at night.
But carbon dioxide gas won't just stay put inside the plant — as a gas, it would simply drift back out. So the cactus attaches it to another substance inside itself, converting it into an acid called malic acid. Dissolved in liquid, that acid can be stored in sacs inside its cells.
Once dawn breaks, the cactus seals its pores shut. It then slowly releases carbon dioxide from its stored malic acid and runs photosynthesis entirely within its own sealed body — getting a full day's work done without exchanging any air with the outside. This day-night reversal is called Crassulacean acid metabolism.
Plants that use this trick build up acid overnight. So they're most sour right at dawn, growing milder through the day as the acid gets used up. Pineapples are known to use the same trick.
A cactus's spines are thought to be modified leaves. Wide, flat leaves catch more light, but they also lose more water. Shrink them down to spines, and there's almost no exit left for water. The job of catching light is instead taken over by the thickened stem.
In short
A cactus survives not simply because it stores lots of water. The real answer is the shift in timing itself: pores shut in the dry day, open in the damp night. You could say it's a plant that gave up having light and raw material available at the same time, choosing instead to protect its water.
Shut by day, open by night.
A cactus doesn't cut down on water — it shifts when it loses it.
For another angle on moving water, see How can trees pull water up 100 metres? If you're curious about the animal-world answer to the same problem, try How can a camel go days without drinking water?, and for the harshness of the desert itself, see Why is the desert scorching by day but freezing at night? The role of water escaping from leaves also comes up in Why are forests cooler than cities?
- Get a small cactus (or succulent) in a pot, and a houseplant of similar size in another pot. Water both the same amount, then discard any water left in the saucers.
- Weigh each pot on a kitchen scale and write down the numbers. Then place both pots side by side on the same windowsill.
- Weigh them again at the same time every three days, and record how much weight each has lost. After about two weeks, the difference should be clear.
Most of the lost weight is water leaving through the soil surface and the leaves. The cactus's line on your chart should stay nearly flat. Keep an eye on the plants and stop if either one looks unwell.
Want to go deeper? — Terms, formulas, and how this connects to the curriculumClearly labelled by level, from middle-school science to university-level courses
- MSCovered in middle-school science
- HSCovered in high-school biology basics/biology
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university-level plant physiology
- ResearchNot yet settled "textbook fact" even at university — an active research question
MSTerms: this phenomenon has a name
- Stoma (plural: stomata): A tiny opening on the surface of a leaf or stem. It's both the entry point for carbon dioxide and the exit point for water vapour. A pair of cells on either side controls how wide it opens.
- Transpiration: The process by which a plant loses water as vapour. It also helps power the pull that draws water up from the roots.
- Crassulacean acid metabolism (CAM): A method of storing carbon dioxide as an acid at night, then releasing it for photosynthesis by day. Seen in cacti and stonecrop-family plants.
- Malic acid: An acid also found in fruit. In this system, it serves as a temporary storage form for carbon dioxide.
MSHSWorking the numbers: how much more water does it take to build the same body?
How much water a plant uses to build its body varies hugely by species. Let's compare an ordinary plant with a cactus-family plant, imagining we're growing each to the same body weight.
| Water an ordinary plant uses to build 1 gram of dry mass | roughly 500 grams |
| Water a cactus-family plant uses to build the same 1 gram | roughly 50 grams |
| Target body weight for this comparison | 1000 grams |
| One household bathtub of water | roughly 200 litres |
| How many times more water | 500 ÷ 50 = 10 |
| Water an ordinary plant uses (grams) | 500 × 1000 = 500000 |
| Converted to litres | 500000 ÷ 1000 = 500 |
| Water a cactus-family plant uses (grams) | 50 × 1000 = 50000 |
| Converted to litres | 50000 ÷ 1000 = 50 |
| Bathtubs' worth for an ordinary plant | 500 ÷ 200 = 2.5 |
To build the same 1 kilogram of body, an ordinary plant needs about two and a half bathtubs of water, while a cactus-family plant needs about a quarter of one. In places where rain rarely falls, that gap becomes the difference between living and dying. These figures are rough guides that vary greatly by species and environment.
HSHS+Why is water loss smaller at night?
HSHow fast water escapes through a pore depends on the difference in water vapour content between the inside of the leaf and the outside air. The bigger the gap, the faster water leaves.
HS+The amount of water vapour air can hold rises sharply as temperature increases. Dry daytime air has a lot of "room" left to absorb more. At night, temperatures drop, that room shrinks, and humidity rises. Opening the pores by the same amount then costs far less water.
UnivWhere the acid is stored, and the trade-off it comes with
The malic acid made at night is stored in a large sac inside the cell called the vacuole. By dawn, the vacuole is known to be fairly acidic. This system comes with a cost, too: the amount of acid it can store is capped by the vacuole's size, which limits how much carbon dioxide can be taken in per day. This is thought to be why cactus-family plants grow more slowly than other plants even with plenty of water available. Surviving in dry land and growing fast are hard to have both at once.
ResearchWhat's still not fully understood
- How many times it evolved. This system shows up independently in plant groups that are only distantly related. Exactly how many times it arose over evolutionary history is still being recounted.
- The switching signal. Some plants normally run ordinary photosynthesis and switch to this system only when it gets dry. Exactly what they're sensing to trigger that switch is still being worked out in detail.
- Applying it to crops. Research continues into whether this trait could be given to crops grown in dry regions. Balancing it against growth speed is seen as the key challenge.
In other words, even this article reflects only "what's understood so far." How plants strike a balance with water is still a topic open to revision.
Curriculum connections (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science — plant structure and function | Stomata, transpiration, and photosynthesis's raw materials |
| HS | Biology basics/biology — metabolism and photosynthesis | The order of carbon dioxide uptake and fixation |
| HS+ | Biology, advanced — plant responses to environment | Why water loss is smaller at night |
| Univ | Plant physiology, ecology | The trade-off of storing acid in the vacuole, and slow growth |
| Research | Plant molecular physiology, crop science | Switching signals, applications to crops in dry regions |
| ― | Everyday relevance | Watering intervals and where to place potted plants |
- Taiz & Zeiger (eds.), Plant Physiology (植物生理学, Baifukan/培風館) — chapters on stomatal opening/closing, transpiration, and carbon fixation in photosynthesis
- Osmond, C. B., "Crassulacean acid metabolism: a curiosity in context", Annual Review of Plant Physiology, 1978
- Nobel, P. S., "Remarkable Agaves and Cacti", Oxford University Press, 1994
- Borland, A. M. et al., "Exploiting the potential of plants with crassulacean acid metabolism for bioenergy production on marginal lands", Journal of Experimental Botany, 2009
※This article is a general-audience science explainer. The figures given are approximate, meant to illustrate the underlying mechanism. Growing conditions vary greatly by species and environment. Base your actual watering on the needs of the specific plant you're growing.