Why does a cactus do not have leaves?

Cacti have adapted by being leafless and growing spines but not thorns. There is a big difference between the two. Thorns are a type of stem while spines are a form of leaf modification. read more

Cacti do have leaves. Cactus spines are modified bud scales, which are leaves. Cacti in the Opuntia subfamily have in addition small succulent cylindrical leaves at each node. These fall off as the pad or joint reaches maturity. And Pereskia and some related genera have flat, “ordinary” looking leaves. But it's true that most cacti do not produce or retain green, photosynthetic leaves. read more

Cactus thorns are the highly modified leaves of plant lineages that evolved to compete in desert conditions. Plant leaves are typically wide and thin and have a big surface area to volume ratio and thus can easily heat up and are the main route for evaporation and water loss from a plant. read more