It is generally accepted that cacti evolved from a common ancestor with large leaves and a woody stem, so at some point could have been considered a tree.
However, they have adapted to their environment to the point where they have their own distinct properties that now set them apart.
Where are they found?
Cactus plants were originally native to North and South America, the West Indies, and the tropical regions of Africa.
The migration patterns of birds carrying cactus seeds and the introduction by humans to new regions mean that cactus plants have also become naturalized to many other areas of the world.
Cacti can become a pest in areas that they have invaded an example is a Prickly Pear, introduced to Australia by settlers from Brazil in the late 1700s. Cacti’s abilities to survive extreme conditions make them difficult to kill, which can be a problem.
What Kind of Plant is Cactus?
Cacti are a member of the Cactaceae family, which includes nearly 2,000 distinct species predominantly found throughout the New World.
The most widely known and recognized plants of the family Cactaceae include the Prickly Pear Cactus, Organ Pipe Cactus, Fishhook Cactus, Saguaro Cacti, Chin Cactus, Pachycereus Pringlei, and Barrel Cactus species.
Over millions of years, cacti have adapted to survive the harsh and unwelcoming conditions that they are associated with today.
Dry, arid, hot stretches of sand and soil don’t make the most hospitable environment for most flora. However, members of the cactus family have claimed these conditions as their own.
Adaptations to dry environments
Plants that are able to survive such severe conditions actually have the name xerophytes. They do so with adaptations over generations of plants mutating to better suit their environment.
In the case of cactus plants, the small surface area of their leaves reduces the space that liquid evaporation can occur from, increasing the cacti’s ability to retain water.
When talking about surface area, we can also reference the sphere or short and wide shape of most cactus plants that increases volume while reducing surface area.
Another useful adaptation is the ribbed outer texture of a cactus, allow it to expand to hold and store more moisture.
What Distinguishes a Cactus as A Plant Rather than A Tree?
There are a number of features of the cacti species that class them as plants as opposed to trees.
While trees use their leaves to collect energy, the Cactus can Photosynthesise through its stem.
They use a unique form of photosynthesis where they take in carbon dioxide collected at night when stomata open (staying closed throughout the heat of the day reduces water loss) and store it until they have access to sunlight (the other crucial component of energy synthesis in plants) during the day.
Although cactus stems can grow as tall as tree trunks (take a look at the whopping height of saguaro cactus!), they have very different properties.
Cacti have succulent stems as opposed to the woody trunks seen in trees, with the hydrophobic waxy coating preventing the escape of moisture.
Again increasing water retention. This waxy coating is much more effective at keeping moisture in than porous and hydrophilic wooden tree bark.
Cacti have spines to protect them from predators, a factor contributing to their lengthy survival as a plant family.
Prickly pear cactus is one of the many species with densely packed spikes, effective in reducing predation. In contrast, a tree will generally rely on thick bark or bitter sap to ward off those wanting to take a bite.
Cacti grow areoles – which are small, lumpy growths from which spikes emerge. These can look similar (but are generally smaller) than knots or burls that grow on trees, with the difference being that areoles do not grow in response to stress but instead as a form of protection.
These areoles and spikes not only distinguish cacti from trees but also from succulents, which do share some of the same properties and are a broad category that Cactaceae members fall into.
Can a Cactus Grow on a Tree?
So we’ve distinguished trees and cacti but do they ever coexist? A plant that grows on another plant or tree is known as an epiphyte, and although this relationship is uncommon in cacti, they can and do grow on branches in some cases.
An epiphyte is not a parasitic species where a parasite will take water and nutrients from its host, and epiphytic cacti rely on pockets of moisture pooling on tree branches to grow. Because of this relationship, epiphyte cacti are most commonly found in rainforests.
Cacti are a collection of unique plant species in the world. Over millions of years, they have adapted to dry and harsh conditions that many plants would perish in.
The Cactus has evolved from a common ancestor that is believed to have been a tree, but these days has its own distinct characteristics that set it apart from other species of flora worldwide.