The word cactus reminds everyone of spines. There are numerous kinds of species that grow around the world. Though very few species do not have thorns, most of the species have stems covered with sharp spines that come in different shapes. Cactus is not only restricted to hot and dry regions. Certain species even grow during winter and some others can grow in high humidity of the tropics.
Sharp spines help stop animals from eating their fleshy stems. Another important effect is to keep the plant cool by screening sun’s rays. Spines trap an insulating layer of air close to the plant. This helps to reduce evaporation of water by breaking up drying winds. They also help collect rain drops dropping the water under the plant, where it soaks into the soil and is absorbed.
Normally leaves of the plants make food. They contain chlorophyll and in the presence of sunlight, water and nutrients of soil, leaves make food for plants. In the case of cacti, they store water in their stems. In different species, spines work the way the leaves do. Unlike other plants, cacti do not make food in leaves. Instead they manufacture food in their thin green outer layer of stem.
Plants with thick, water filled stems or leaves are called succulents. Cacti are more recognizable type of succulent as most kinds have clusters of spines and store water in their stems. Most other succulents store water in their leaves and their spines are not clustered. A cactus may store enough water to serve its needs for two years or longer. As the supply gets used up, the plant wilts.