Cacti utilize CAM photosynthesis, a process unique to succulents.

So why would cacti evolve these two unusual features of green stems and non-photosynthetic stabby leaves? Well, these combination of traits are adaptations to dry and resource-poor environments. Big and flat leaves have lots of surface area which is typically a good thing from a plant’s perspective as it captures lots of sunlight for photosynthesis. But many plants have more sunlight than they can handle and more photosynthetic area means more water loss. All photosynthesis requires gas exchange, carbon dioxide in and oxygen and water vapor out. So for plants in water-limited and very sunny environments, like deserts, adaptations that limit photosynthetic area (and thus limit water-losing gas exchange) are likely to be advantageous and selected for.

How do cacti photosynthesize? - Quora

Does a cactus perform photosynthesis? How? | Yahoo …

What part of the cactus photosynthesizes? - Stack …

So, cacti don’t have leaves, right? Well, kind of, but those spiky bits are technically modified leaves. That is why they are, in botanical lingo, spines. . At some point early in the evolution of cacti their leaves evolved to be smaller and smaller and eventually into hardened points that did not photosynthesize. Cacti also evolved green stems which took over the photosynthetic tasks.

photosynthesis for cactus? | Yahoo Answers

Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" (CAM) as part of photosynthesis. Transpiration, during which carbon dioxide enters the plant and water escapes, does not take place during the day at the same time as photosynthesis, but instead occurs at night. The plant stores the carbon dioxide it takes in as malic acid, retaining it until daylight returns, and only then using it in photosynthesis. Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced.

It is interesting to note that the only cacti to use C3 photosynthesis is the primitive pereskia
where does photosynthesis occur in cacti - where does photosynthesis occur in cactus -

they do the photosynthesis for the cactus.

To distinguish between the Thanksgiving and Christmas cacti, look at the shape of the flattened stem segments, which are botanically called phylloclades. On the Thanksgiving cactus, these stem segments each have 2 to 4 saw-toothed serrations or projections along the margins. The stem margins on the Christmas cactus are more rounded. Note that there are no true leaves on either of these holiday cacti, so photosynthesis occurs within the green phylloclades.

CAM (Crassulacean Acid Metabolism) photosynthesis is found in cacti and succulents, ..

Spines, Photosynthetic Tricks, and Other Marvels ..

So why the spines? Why not just lose the leaves all together? The short answer is that spines are a defense against herbivores. Herbivory, or the consumption of plant matter, can be really bad news for a plant: get a bunch of your photosynthetic tissue eaten, have to grow it back, not enough resources left to produce seeds and reproduce, less offspring, lower fitness. Plants that get less damage from herbivores might have higher fitness so evolution should favor plants with traits that reduce herbivory. This is particularly true in environments where resources are scarce, like deserts, where regrowing tissue lost to herbivory is very difficult (this is called the ). Thus, plants in low resource environments, like cacti in deserts, invest very heavily in defenses, like big gnarly spines.

Stomata are microscopic pores on the plant through which carbon dioxide enters for photosynthesis. Cacti vary in size based on their species.

Henry Shaw Cactus Society - From the Digest

So as adaptation to very dry desert environments cacti evolved photosynthetic stems which presumably lessen water loss during photosynthesis. Great. But there are more photosynthetic tricks in cacti that you can’t see. Early cacti evolved an additional solution to the photosynthesis-water-loss problem, a whole new physiological mechanisms for photosynthesis called. There are a lot of differences between CAM photosynthesis and what most other plants use, but the most important part is that cacti, and other CAM plants, can leave their stomata (the little pores that allow gas exchange) closed during the heat of the day and instead do all their gas exchange at night when it is cooler and more humid. This little trick greatly reduced water loss and gives CAM photosynthesis plants an advantage in very dry environments.