A leaf’s job moved to the stem
In most plants, leaves capture sunlight and exchange gases needed for photosynthesis. In cacti, leaves are reduced to spines, while the green stem takes over the work of making sugars. This is more than a change in shape: it shifts photosynthesis to a thick, water-storing part of the plant that can keep functioning through long dry spells.
The stem’s surface is built for that role. It contains chlorophyll, the light-absorbing pigment normally concentrated in leaves, and its fleshy interior stores water. A waxy outer layer helps slow evaporation. Because the stem is thick rather than broad and papery, it can gather sunlight without exposing as much vulnerable tissue to dry air.
Spines help manage the cost of sunlight
Spines are modified leaves, but they no longer serve as substantial solar panels. Their small surface area means less water can escape from them. They can also cast a little shade over the stem and make it harder for animals to eat the plant—useful benefits in harsh habitats.
Cacti further limit water loss through a photosynthetic strategy called CAM. Instead of opening their pores, or stomata, mainly during the hot daytime, they generally take in carbon dioxide at night, when cooler air slows evaporation. They store it temporarily and use it to make sugars during the day, when sunlight is available and the stomata can stay mostly closed.
The result is a division of labor: the stem collects light and stores water, while spines provide protection with little surface to lose moisture from. A cactus can keep photosynthesizing without carrying the broad, thirsty leaves that would make surviving a dry season much harder.
