Cactus

A cactus is a special kind of plant in the Cactaceae family. There are about 127 groups of cacti, with around 1,750 known types. Cacti grow in many shapes and sizes and are found naturally in the Americas, from Patagonia to parts of western Canada. One type, Rhipsalis baccifera, can also be found in Africa and Sri Lanka.

Cacti live well in very dry places, like the Atacama Desert, one of the driest areas on Earth. To survive with little water, cacti have thick stems that store water. Most cacti do not have normal leaves; instead, they have sharp spines that are changed leaves. These spines protect the plant from animals and help save water by giving shade and slowing air movement.

Cacti usually bloom with lovely flowers and make fruits that can be eaten. Some people grow cacti for decoration, while others use them for food or animal feed. Even though many spiny plants look similar, only plants in the Cactaceae family are true cacti. Their special ways of storing and saving water make them interesting and important, especially in dry and desert areas.

Morphology

Cacti come in many shapes and sizes. Most cacti have thick stems that store water and make food through photosynthesis. They usually do not have leaves, or their leaves are very small. Instead, their stems have special areas called areoles that can produce spines, flowers, and sometimes hairs.

Cacti can grow in different ways. Some grow like trees with one trunk and branches. Others grow like shrubs with many stems close to the ground. There are tall, column-shaped cacti and smaller, round cacti that grow in clusters. In tropical areas, some cacti grow on trees or as climbing plants with flat, leaf-like stems.

The stems of most cacti are green and make food for the plant. They may have bumps, ridges, or spines. Areoles are special spots on the stem where spines and flowers grow. In some cacti, such as tree-like types, leaves are present and help with making food, but in most, the stems do this job instead.

Cacti often have spines that come from their areoles. These spines help protect the plant. Most cacti have flowers with many parts. The colors of cactus flowers can be white, yellow, red, or even magenta.

Adaptations for water conservation

All cacti have special ways to use water wisely. Most cacti live in hot, dry places, but even their earliest ancestors were good at surviving when water was hard to find. A few cactus types have adapted to grow as climbers or in tropical forests, where saving water is less important.

One big change in cacti is that many don’t have normal leaves. Instead, they have spines, which are changed leaves. Spines protect the cactus from animals and help save water by trapping air and providing shade. They can even collect water from fog and mist, which then flows to the roots.

For most cacti, the stem is where they store water. These stems can hold up to 90% water and change shape to save water. When there’s little water, the stem shrinks; when rain comes, it swells. The outside of the stem has a waxy layer that stops water from leaving.

Cacti also have special ways to make food without losing too much water. Usually, plants open tiny holes in their leaves during the day to take in air for making food, but this makes them lose water. Cacti mostly open these holes at night when it’s cooler and there’s less water loss. They store air inside and use it during the day to make food with much less water loss.

Taxonomy and classification

Naming and grouping cacti has been tricky. The Swedish scientist Carl Linnaeus first tried to sort them in 1737. He later changed his mind and put all cacti into one group called Cactus. The word "cactus" comes from an old Greek word, κάκτος (kaktos), used by a scientist named Theophrastus.

Other scientists later split cacti into many groups. By the early 1900s, the name Cactus was confusing, so scientists decided to use another name. They chose Mammillaria as the main name, but kept the family name Cactaceae.

In 1984, scientists formed the International Cactaceae Systematics Group to help sort out cactus family groups. They divided cacti into about 125–130 groups with 1,400–1,500 species. They organized these into four main subgroups, called subfamilies. These subfamilies are:

• Subfamily Pereskioideae: Includes the genus Pereskia. These plants can be trees or shrubs with leaves.

• Subfamily Opuntioideae: Contains about 15 groups. These plants often lose their leaves and have stems split into parts, called "joints" or "pads."

• Subfamily Maihuenioideae: Has just one group, Maihuenia, with two species. These plants grow in low mats and keep their leaves.

• Subfamily Cactoideae: The largest group, with nine tribes. These are the "typical" cacti, coming in many shapes. Their stems are usually ribbed and not split into parts.

Scientists are still learning more about cacti, so their grouping may change in the future.

Phylogeny and evolution

Phylogeny

Studies looked at how different types of cacti are related. They found that some cacti, like Pereskia, are quite different from others. Some cacti, such as Leuenbergeria, do not have the same stem features as most others. These plants start forming bark early and do not use their stems to make food through photosynthesis.

Early cacti were small shrubs or trees with leaves that made food. They lived where it was dry at times. These first cacti could save water well when making food. Later, some cacti started using their stems to make food instead of leaves. This helped them survive better in dry places.

Evolutionary history

We do not have fossils of the first cacti to learn about their history. But where cacti grow today gives us clues. Cacti mostly grow in South America and southern North America. This suggests they evolved after the ancient land of Gondwana split into South America and Africa about 143 to 101 million years ago.

Older ideas thought cacti began around 90 to 66 million years ago. Newer studies think they may have started much later, about 35 to 30 million years ago. The first cacti may have grown in Central America and northern South America. The cacti with thick, juicy stems evolved later in southern South America and then moved north. These thick-stemmed cacti likely began about 25 million years ago. The many different kinds of cacti we see today probably developed in the last 10 to 5 million years as the world became drier.

Distribution

Cacti grow in many places, from flat coastal areas to tall mountains. Except for one type, they naturally live in the Americas, from Patagonia up to British Columbia and Alberta in western Canada. Main areas with dry cacti include the deserts of North America, parts of the southwestern Andes in Peru, Bolivia, Chile, and Argentina, and the Caatinga in eastern Brazil. Cacti that climb trees grow mostly in wetter places, like coastal mountains and forests in southeastern Brazil, parts of Bolivia, and forested areas of Central America.

One type of cactus, called Rhipsalis baccifera, grows naturally in both the Americas and the Old World, including tropical Africa, Madagascar, and Sri Lanka.

Naturalized species

Many cacti have been taken to places outside the Americas by people and now grow wild there, especially in Australia, Hawaii, and the Mediterranean region. In Australia, some cacti like Opuntia stricta were brought in the 1800s to make fences. They spread quickly and became a problem, but special moths helped control them. Some cacti in the Arabian Peninsula have also been introduced and are growing more common.

Reproductive ecology

Cactus flowers are pollinated by insects, birds, and bats. Bees are the most common pollinators. Butterflies usually pollinate brightly colored flowers that open during the day, while moths pollinate flowers that open at night. Hummingbirds also help pollinate cacti by visiting flowers that are red and have a special shape.

After flowering, cacti produce fruits with many seeds inside. Some fruits are fleshy and sweet, which attracts birds that eat them and help spread the seeds. Other fruits may be dry and spiny, which can stick to animals or be carried by the wind to new places.

Uses

Early history

People have used cacti for thousands of years. Evidence shows that people in Chile and Brazil used cacti around 15,000 years ago. In Mexico and Peru, seeds from cacti have been found in very old waste dumps, showing they were eaten long ago. The Aztecs in Mexico used a type of cactus called opuntia for many things, including food and symbols in their stories. When Europeans first arrived in the New World, they found cacti and brought some back to Europe.

Food

The opuntia cactus, also called the Indian fig cactus, has been an important food for a very long time. Its fruit, called tuna, and its pads, called nopal, are eaten. In Mexico, the nopal industry was worth a lot of money. This cactus grew in places like Sicily and Algeria after people brought it there. Many types of cacti have edible fruit, such as the fruit from the saguaro cactus, which people in the southwestern United States and northwestern Mexico have used for years.

Ornamental plants

Cacti have been grown for beauty since they were first found in the New World. By the 1800s, people in Europe had big collections of them. Today, cacti are grown in greenhouses and as houseplants, especially in places where it is too cold outside for them. They are also used in gardens that need little water, like in California.

Other uses

Cacti are used in many ways besides food and decoration. Their spines can be removed to feed animals, and some cacti are used in medicines. Cacti are also used to build things, like fences and furniture, and some of their fine hairs are used to make fabric.

Conservation

All cacti are protected by special rules to make sure they don’t disappear. These rules are part of an agreement called CITES. It helps control how cacti are bought and sold around the world. Most cacti can only be sold with special permits.

There are three big problems that hurt cacti in the wild: building things, animals eating them, and people taking them for sale. Building projects like dams, cities, and farms can destroy where cacti live. Animals like goats can damage cacti by eating them. Sometimes people take cacti from their homes to sell.

To help protect cacti, there are two main ways: keeping them in their natural homes and growing them in special places. Protecting their homes means using laws and making parks where cacti can grow safely. Examples of these parks include Big Bend National Park in Texas, Joshua Tree National Park in California, and Saguaro National Park in Arizona. Another way to help cacti is by keeping their seeds and plants in botanical gardens, like the Desert Botanical Garden in Arizona.

Cultivation

Cacti are popular plants, and many books teach people how to grow them. Because cacti grow in many places, it is hard to copy their exact home conditions. This section talks about growing semidesert cacti in pots inside a greenhouse or at home.

When growing cacti in pots, the soil needs to let air in and store water and nutrients. There are many ways to mix the soil, but it is important to let the air in. Cacti need careful watering, and the amount depends on where they are kept and the type of soil. In summer, they should dry out between waterings. In winter, some people give them a little water, while others stop watering for a few months.

Cacti grow best with plenty of light, but in a greenhouse they might need some shade from the strongest sunlight. It is best not to let the temperature go above 32 °C (90 °F). In winter, temperatures between 5 °C (41 °F) and 10 °C (50 °F) work well for many types.

Cacti can grow from seeds, pieces of plants, or by grafting. Seeds are planted in moist soil and kept warm until they sprout. Pieces of plants can be cut and allowed to heal before being placed in soil to grow roots. Grafting is used for cacti that are hard to grow on their own. One strong plant is used as a base, and a piece of the desired cactus is attached to it.

Cacti can sometimes get pests like mealybugs, thrips, scale insects, whiteflies, and red spider mites. They can also get diseases from fungi, bacteria, or viruses, especially if they are over-watered. There are ways to control pests and some diseases.