Cactus

Cacti are specialized members of the plant family Cactaceae. This family belongs to the order Caryophyllales. There are approximately 127 different genera within this group. Scientists have identified about 1,750 known species of cacti. These plants are famous for their ability to survive in extremely dry environments. They are native to the Americas, stretching from Patagonia to western Canada. One exception is Rhipsalis baccifera, which grows in Africa and Sri Lanka.

Most cacti are succulents, meaning they have fleshy parts to store water. In most species, the stem is the only part used for this storage. To prevent water loss, many cacti have lost their true leaves. They have replaced them with spines, which are highly modified leaves. Spines protect the plant from herbivores. They also reduce airflow near the plant and provide shade.

Spines grow from unique structures called areoles. Areoles are highly reduced, specialized branches. They are the primary identifying feature of all cacti. Areoles can also produce flowers, which are often tubular and multipetaled.

Cacti use a unique process called crassulacean acid metabolism, or CAM. This is a specific type of photosynthesis used to conserve moisture. In most plants, transpiration occurs during the day. Transpiration is when carbon dioxide enters the plant and water escapes. Cacti avoid this by performing gas exchange at night. They take in carbon dioxide during the cooler, more humid night hours. The plant then stores this carbon dioxide as malic acid. When daylight returns, the plant uses the stored acid for photosynthesis. This method significantly reduces water loss.

The physical structure of a cactus stem helps it manage water. Many stems are ribbed or fluted. The number of ribs often follows Fibonacci numbers, such as 2, 3, 5, or 8. These ribs allow the stem to expand and contract. When it rains, the cactus expands to absorb water. During a drought, the stem shrinks as water is used. A large saguaro can absorb as much as 150 gallons of water during a single rainstorm.

Smaller, globe-shaped cacti use this shape to maximize water volume. This shape also provides the lowest possible surface area for water loss.

Cacti come in many different growth habits. Some are arborescent, which means they grow like trees with woody trunks. Others are shrubby, with several stems growing from the ground. Some are columnar, consisting of tall, cylinder-shaped stems. There are also globular cacti, which are shaped like balls. In tropical regions, some cacti act as epiphytes. These are plants that grow on other trees. These climbing cacti often have flattened, leaf-like stems and fewer spines.

There is a wide range of sizes among cactus species. The tallest free-standing cactus is Pachycereus pringlei. It can reach a maximum recorded height of 49 feet. In contrast, the smallest cactus is Blossfeldia liliputiana. This tiny plant is only about 0.4 inches in diameter at maturity.

Some species look very different from the typical spiny cactus. Genera like Leuenbergeria and Pereskia have persistent leaves and bark-covered stems. Leuenbergeria is considered close to the ancestral species of all cacti. These plants resemble the trees and shrubs growing around them.

Humans use cacti for many different purposes. Many species serve as ornamental plants in gardens. Others are grown as fodder or forage for animals. Some are harvested for food, particularly their fruit.

An insect called cochineal lives on some cacti and is used for other products. It is important to note that some other plants look like cacti. For example, some members of the Euphorbiaceae family are also spiny succulents. However, these are not true cacti. Scientists use the presence of areoles to tell them apart.