Cactus

A cactus is a special kind of plant that belongs to the Cactaceae family. There are about 127 different groups of cacti, with around 1,750 known species. These plants come in many shapes and sizes, and they grow naturally in the Americas, from places as far south as Patagonia to parts of western Canada. One type, Rhipsalis baccifera, can even be found in Africa and Sri Lanka.

Cacti are perfectly suited to live in very dry places, like the Atacama Desert, which is one of the driest areas on Earth. To survive with so little water, cacti have thick, fleshy parts called stems that store water. Most cacti do not have regular leaves; instead, they have sharp spines that are actually modified leaves. These spines protect the plant from animals that might eat it and also help reduce water loss by providing shade and slowing down air movement around the cactus.

Cacti usually bloom with beautiful flowers and produce fruits that can be eaten. Some people grow cacti as decorative plants, while others use them for food or even as feed for animals. Even though many spiny plants around the world look similar, only those in the Cactaceae family are true cacti. Their unique ways of storing water and conserving it make them fascinating and important plants, especially in dry and desert regions.

Morphology

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

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

The stems of most cacti are green and can make food for the plant. They may have bumps, ridges, or spines coming out of them. 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 can be straight, curved, or even flat, and they help protect the plant. Some cacti also have tiny, easily broken spines called glochids that can cause irritation if they get into the skin.

Most cacti have flowers with many parts that can look similar, making it hard to tell them apart. These flowers usually have many stamens and a single style that may split into several stigmas at the end. The colors of cactus flowers can range from white to yellow, red, and even magenta.

Adaptations for water conservation

All cacti have special ways to use water efficiently. 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 and argued about since Europeans first saw them. The Swedish scientist Carl Linnaeus first tried to sort them in 1737, putting them into two groups. Later, in 1753, he changed his mind and put all cacti into just one group, which he called Cactus. The word "cactus" comes from an old Greek word, κάκτος (kaktos), used by a scientist named Theophrastus for a spiky plant.

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

In 1984, a group of scientists formed the International Cactaceae Systematics Group to help sort out cactus family groups. Their work led to dividing 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, which has features similar to the earliest cacti. These plants can be trees or shrubs with leaves, and their stems are smooth and round.

• Subfamily Opuntioideae: Contains about 15 groups. These plants often lose their leaves when they grow up and have stems that are split into parts, called "joints" or "pads." They range in size from small cushions to tall tree-like plants.

• Subfamily Maihuenioideae: Has just one group, Maihuenia, with two species. These plants grow in low mats and keep their leaves, but they don’t use a special water-saving process that many cacti have.

• Subfamily Cactoideae: The largest group, with nine tribes. These are the "typical" cacti, coming in many shapes from tree-like to plants that grow on other plants. Their stems are usually ribbed and not split into parts. Some tribes have flat, segmented stems that climb or grow on other plants.

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

Phylogeny and evolution

Phylogeny

Studies from 2005 and 2011 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 by using it very 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 different places, from flat coastal areas to tall mountain regions. Except for one type, they naturally live in the Americas, from places as far south as Patagonia up to British Columbia and Alberta in western Canada. Some main areas where dry cacti are common 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 or live on other plants grow mostly in wetter places, like the 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. Scientists think birds may have carried its seeds long ago, or that it may have traveled on old ships.

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 and for other uses. They spread quickly and became a big problem, but special moths were used to help control them. Some cacti in the Arabian Peninsula have also been introduced and are growing more common over time.

Reproductive ecology

Cactus flowers are pollinated by insects, birds, and bats. Bees are the most common pollinators, and they were likely the first to pollinate cacti. Butterflies usually pollinate brightly colored flowers that open during the day, while moths pollinate flowers that are white or pale and open at night. Hummingbirds also help pollinate cacti by visiting flowers that are red, have a special shape, and produce lots of nectar.

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 a 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. A red dye from a small bug that lives on opuntia cacti was used for a long time before people made synthetic dyes. 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 under special rules to make sure they don't disappear. These rules are part of an agreement called CITES, which helps control how these plants are bought and sold around the world. Most cacti can only be sold with special permits to protect them.

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 also damage cacti by eating them. Sometimes people take cacti from their homes to sell, which can nearly remove some plants from certain areas.

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, so they can be protected and maybe brought back to the wild later.

Cultivation

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

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, but some can handle colder temperatures.

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, but viruses have no cure.