You’ve certainly seen personally or in a movie and photos some giant cactus, meters tall standing as a tree. Have you wondered if cactus are actually trees? What is a cactus?
Generally, cactus is not a tree. Cactus is a subcategory of succulents from the Cactaceae family. Cactus differs from a tree by its stem tissues, photosynthesis process, water retention capacity, and presence of areoles. However, some species of trees and cactus are similar in shape, and they are both perennial plants.
Cactus can grow as tall as trees and look like a tree only without a leaf. How can you tell if you’re looking at a tree or a cactus? Stick around and see what makes them similar yet different.
Table of Contents
Cactus is a succulent plant under the Cactaceae category that does not include trees and has more than 1750 species. Cactus is a Greek word that refers to a spiky plant that grew in ancient Greece. They have unique morphological and physiological abilities that can adapt to dry environments.
Cactus are perennial plants with thick, fleshy stems and sharp spines (mostly) used to store water.
Cactus are xerophytes, which means plants may adjust to a dry environment by using processes to limit water loss or store available water. Their distinguishing feature is the areoles corresponding to the nodes of other plants where leaves, flowers, and shoots grow.
Some cactus species can live more than a hundred years and grow as tall as the trees.
Due to the lack of documented cactus fossils, determining the time and location of origin requires some guesswork. According to modern genetic analysis and population radiation studies, the Cactaceae family initially appeared about 30-35 million years ago, most likely in southern South America as claimed by the University of Arkansas’ Division of Agriculture.
In 1789, French botanist Antoine Laurent de Jussie grouped the cacti under a family name Cactaceae. Cacti are native in North and South America, but Mexico has the most variety and species.
A tree is a permanently woody perennial plant with a single stem, leaves, and branches. It comes in different sizes. But some tree species grow to a significant height and live up to several years.
A tree is not a taxonomic group but a term used in biology for perennial plants that develop a woody trunk, branches, and leaves. A plant with rigid stems and branches is considered a tree, no matter its size. Trees, as they grow, develop a woody tissue through the process of secondary plant growth.
Cactus is not a tree. Some cactus species can be called a tree in terms of its shape and height. Those species are called arborescent (tree-like) because they grow as tall and massive as a tree.
But in terms of other physiological and morphological abilities, a cactus is far different from a tree. Cactus is a succulent and classified under the Cactaceae family.
Cactus are called trees because of some similarities they both have. However, they also have several distinct characteristics which make them completely different.
Cactus and tree can be confused due to the fact that often giant cactus might have shape, size and lifespan similar to trees. However, this should not fool you.
Most trees grow multiple stems and several branches extending in all dimensions. These stems support the leaves, which are responsible for photosynthesis.
Some trees grow so big that they can reach up to 300 ft like Giant Sequoia trees (tallest tree recorded). Trees are often used as shade because of their lush leaves and branches.
Like trees, some species of cactus grow arms like branches of a tree.
Other species grow tall, such as Saguaro (Carnegiea gigantea) and Mexican Cardon (Pachycereus pringlei). They are the tallest cactus species that can grow to more than 60 feet.
Both trees and cacti are considered perennial plants, which means they can live for several seasons through the years.
Perennials can be short-lived, lasting only three years, or they can thrive for decades. Trees and cacti can live for several hundreds of years. The giant Sequoia tree is the oldest tree living more than 2000 years old. The humongous Saguaro and Pacycereus species are among the oldest cactus that lives for over 300 years old.
Stem tissues, photosynthesis, water retention, and areolas are the main differences between trees and cacti.
Cactus and trees have trunks that provide structure on the plant. Stems are used in transporting and storing essential elements such as water and nutrients needed for growth.
Trees have permanently woody stem tissues called the cambium. Every year, the cambium adds a new layer of growth to both the outside and inside of the tree. This fundamental characteristic of trees makes them sturdy and provides structure and support.
Cactus have sponge-like succulent stem tissues that hold a massive amount of water. Their characteristic to retain moisture inside their stems enables them to survive hot, dry conditions. Old cactus such as Saguaro have woody trunks but become porous and fibrous when dry out.
Although cacti and trees use the same component to manufacture food, they photosynthesize in different ways. Photosynthesis is a cellular process in plants to produce food using water, carbon dioxide, and light energy absorbed by chlorophyll.
For photosynthesis to occur in trees, the roots need to take in water and transport them to the leaves, where the chlorophylls and carbon dioxide are. In other words, photosynthesis in trees takes place in their leaves.
Cacti, however, photosynthesize in their epidermis. They use a unique process of making food called Crassulacean Acid Metabolism (CAM).
In CAM photosynthesis, a cactus stomata open only during nighttime to let carbon dioxide and moisture in. Cactus cells store the carbon dioxide they absorb at night in the form of malic acid. The acid will be converted back to carbon dioxide during the day for photosynthesis to take place. Stomata, even when closed, allows sunlight to come in.
Why are cactus stomata open only at night?
When plants open their stomata to get carbon dioxide for photosynthesis, they lose a lot of water, especially in hot, arid deserts. Cactus open their stomata only at night to avoid excessive water loss due to evaporation. Because the night is colder and less dry, less water evaporates from the cactus.
The stems of cactus and trees can retain water on their stems to keep the system hydrated. But trees cannot take up and store much water as cactus does.
Cactus stems are spongy and waxy, so water absorption and retention are possible. Evaporation is slow, so water loss is unlikely. Cactus in the desert can quickly absorb water during an outpour.
Aside from their roots, cactus can take in water through their skin and spines. During long periods of no rain, the cactus rely on this water to survive. A giant Saguaro cactus can take in 4,800 pounds of water, according to Washington State University.
Trees can absorb water from their surroundings through their leaves. However, it is not a very efficient method of water uptake. Trees absorb the majority of their water through their roots.
Trees retain water but not for long periods. Water absorbed from the roots is used up quickly due to transpiration on the leaves. Transpiration occurs when leaves open their stomata to take in carbon dioxide, and water starts to evaporate. Water uptake increases during hot conditions. This explains why most trees cannot survive prolonged periods of drought.
Cacti have a unique feature on their body that other plants don’t have. They have tiny bumps called areoles. This distinct physiological characteristic of a cactus makes them easier to identify from the rest of the plants.
Areoles are present in all cacti, with or without spines. Because the structure of these areoles varies among cacti, this is one way to recognize one cactus plant from another. The areoles are usually filled with felt, wool, bristles, hair, or spines, although they can also be bare. Cactus flowers and shoots emerge from the areoles.
Trees do not have areoles but nodes. The nodes of a tree produce leaves, branches, and often flowers.
Not all trees grow big and tall. There are trees smaller than a cactus. You cannot just call a plant a tree because it is huge and tall.
There are miniature trees that can be grown on your desk. They are called Bonsai.
The term “bonsai” refers to both the plant and the technique used to miniaturize it. The technique began about 1200 years ago in China, where woody plants are grown in containers. The plants got more valuable as they became dwarf in the containers. The Japanese perfected the craft as we know it today.
Bonsai trees are often seen in Japanese zen gardens, and it has a lot of advantages.
They are well suited to the small garden where space is limited for regular-sized trees for ornamental purposes. Dwarf trees are ideal for home orchards since they take up less space, are easy to spray and trim, and give enough fruit for the average household throughout the season.
1) Some cacti species can undoubtedly be tree-like because of their shape, but most species are by far similar to trees.
2) Cactus stems are spongy that can store water, while trees have permanent woody stems that cannot hold water over a long period.
3) Some cactus species can grow so tall and big that sometimes they can be mistakenly referred to as trees.
“Cactus Evolution,” Division of Agriculture, University of Arkansas
“Cactaceae Juss.,” Global Biodiversity Information Facility
“Living in a Carbon World,” Carleton College
“Cacti/Desert Succulent,” National Park Service, U.S. Department of Interior
“How Do Cacti Survive in such hot and dry environment,” Washington State University.
“Growing Cactus,” Aggie Horticulture, Texas A&M University
“Bonsai,” Texas A&M University
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