Most plants open their stomata during the day to take in CO2 for photosynthesis. Cacti (and other succulents) do the opposite — and it's one of the coolest adaptations in the plant kingdom.

The three pathways:

• C3 photosynthesis — the "default" pathway used by most plants (wheat, rice, most trees). Stomata open during the day, CO2 is fixed directly into a 3-carbon compound. Simple, but water-hungry — lots of moisture is lost through open stomata in daytime heat.

• C4 photosynthesis — evolved in plants like corn and sugarcane. They pre-concentrate CO2 in specialized cells before fixing it, which makes photosynthesis more efficient in hot, sunny environments and reduces water loss compared to C3.

• CAM photosynthesis (Crassulacean Acid Metabolism) — the strategy cacti, agaves, and pineapples use. These plants flip the schedule entirely: stomata open at night, when it's cooler and more humid, to take in CO2. That CO2 is stored as an acid inside the cell. Then, during the day, stomata slam shut to prevent water loss, and the plant uses stored sunlight energy to convert that stored acid back into CO2 for the actual photosynthesis reactions — all while sealed up tight.

Why it matters evolutionarily:

CAM isn't just a neat trick — it's a textbook example of convergent evolution. It's evolved independently in over 30 plant families that had zero recent common ancestry, all converging on the same solution because they were facing the same problem: extreme water scarcity. Whenever you see the same complex trait pop up repeatedly across unrelated lineages, it's a strong signal that the trait is an unusually good solution to a very common environmental pressure.

There's also a flexibility angle — some plants (like certain Clusia species) can actually switch between C3 and CAM depending on water availability, which shows these aren't rigid categories but a spectrum of strategies plants can deploy as conditions change.

Next time you see a cactus sitting in the sun looking like it's "doing nothing," remember — it did all its CO2 shopping the night before, and it's just running the process in complete isolation from the outside air.

If you want to go deeper on plant physiology like this — hormonal signaling, stress responses, and more — we cover it in depth in the courses here at Root & Leaf.