Cacti are one of the most interesting plants in the world, in my opinion. They are certainly one of the most unusual plants, that we know of. They grow without any leaves but still do Photosynthesis but in a different way than other plants, they grow in parts of the world where almost no other plant can survive, and have thorns to keep potential predators away.
Cacti don´t have leaves because they evolved to grow in very hot environments where traditional leaves would be burned by the sun. Cacti do a different Kind of Photosynthesis called Crassulacean Acid Metabolism (CAM) that allows them to survive in deserts and doesn´t require any leaves. They also don´t need as much water as regular plants do.
If you want to know how Crassulacean Acid Metabolism (CAM) Photosynthesis works then check out this article where I explain how cacti do photosynthesis and why.
Why Would Leaves Be Burned By The Sun?
Traditional plants have pores, do called Stomata, that are located throughout the plant but most of them are on the leaves. These stomata are open during the day to allow Photosynthesis to take place.
During the process of Photosynthesis CO2 is being taken in by the plant through the Stomata.
Then a chemical process will use water, the sun, and the CO2 to create sugar for the plant.
Then finally the byproduct, Oxygen, is being released through the Stomata.
This is, extremely simplified, the process of traditional photosynthesis through the leaves of the plant.
But this would not work if the environment is very hot and dry, like in the desert, because almost all of the water would evaporate through these open Stomata and the leaves would be burned in no time.
So Cacti had to adapt to these harsh environmental conditions by creating another way of photosynthesis where the stomata are not located on leaves but on the stems of the plant itself and the stomata are only open at night to keep the plant from drying out.
How Did Cacti Adapt to The Hot and Dry Environment of the Desert?
First and most obvious, the Cacti lost their leaves and instead build bigger and sturdier stems.
These stems were also able to hold and store more water over a long period of time.
The Stomata, pores of the plant, are mostly located at the stems and they are shut during the day, opposed to most regular plants where the stomata would open during the day to do photosynthesis.
The cactus instead only does a limited amount of photosynthesis during the day with all the CO2 it stored during the night time. Then during the night, the Stomata open up to release the oxygen that the cactus created as a byproduct of photosynthesis during the day.
Then the cactus absorbs as much CO2 as it possibly can before closing the Stomata again before sunrise.
And then the whole process repeats. This is called Crassulacean Acid Metabolism (CAM) Photosynthesis. As I mentioned above, you can read more about that in another article of mine.
The cactus also developed thorns or, more accurately, spines to keep predators away.
In the desert, water is rare. So thirsty rodents and other smaller animals are after the reservoir of water that cacti store within their stems.
This is why they needed a way to defend themselves from those animals. Their Solution were sharp and dangerous-looking spines that would keep these predators far away from their precious water.
So there you have it, this is why and how the cactus has evolved and why the cactus does not have leaves but big stems and thorns instead.