Plants that belong to the cactus family are stem succulents, perform photosynthesis, have specialized skin surfaces and have spines and unique flowers. Succulents are plants that absorb large amounts of water in their tissues. Cacti are stem succulents, which means they store their water in thick stems, rather than in leaves. Plants such as aloe vera are sometimes confused with cacti because their long, pointed leaves hold water.
Since cacti have no leaves, the chlorophyll needed for photosynthesis appears in the stems. Photosynthesis allows plants to make their own food using carbon dioxide, nutrients and water. Using the sun's heat, they turn these items into sugar and oxygen. Most plants take in their carbon dioxide during the day and perform photosynthesis at the same time. Cacti take in carbon dioxide at night and store it until daylight, when photosynthesis occurs. The cooler night temperatures cause less evaporation.
The skin surfaces of cacti are often ribbed, which allows the skin to expand without splitting, similar to the bellows on an accordion. The skin is also thick and waxy, which protects the plant from evaporation and insect damage. Sharp spines also protect that skin from foraging animals.
All cacti produce flowers. Special organs called areoles form along the stem ribs or on the top of the plant. Unlike in other plants, cacti flower petals and sepals look much the same. The male parts, the stamens and anthers, and the female part, the stigma, are close together. A good example of this is the hedgehog cactus, which has bright purple flowers, yellow stamens and a bright green, many-lobed stigma right in the center.