Have you ever wondered why cacti have spines instead of leaves?
Cacti have spines instead of leaves beacause are better at conserving water in a desert environment
But how do spines turn cacti into water-conserving machines? Turns out, it’s by being multi-taskers!
Spines Help Cacti Conserve Water
Spines, scientifically speaking, are different from thorns. Where thorns are modified stems, and are usually found alongside leaves, spines are actually highly modified leaves.
This shows just how good cacti have gotten at desert survival! Regular leafy plants lose a lot of moisture through their leaves, and in the desert, where every drop counts, cacti needed something a little different.
Over time, their leaves adapted to be smaller and tougher until they stopped carrying water altogether.
Limited water circulation means limited water loss, making tough, tiny spines the perfect alternative to typical leaves.
It's All in the Stem
But what about photosynthesis?
Most plants use their leaves to turn water, carbon dioxide, and light energy into the sugars their cells need to function.
As cactus leaves turned into cactus spines and lost their ability to photosynthesize, the plants had to find a new way to produce food.
Since cacti have thick fleshy stems, these took over the job of photosynthesis.
In most plants, stems would be too inefficient to produce enough sugar, since they don’t get much light, but once again, a desert environment and spines make it work.
Deserts get a lot of light, and dry, dead surfaces like rock and sand let all that light bounce around instead of just absorbing it.
This means that cacti can get light coming from lots of directions instead of just overhead, which gives their stems better access.
And, unlike regular leaves, spines are slender enough to allow plenty of that light through, meaning cacti have no problem synthesizing the food they need.
Made In The Shade
Deserts are hot! In fact, the one of the hottest recorded surface temperatures on record was in California’s Mojave Desert, which hit 134 degrees Fahrenheit!
So how do you beat the heat when you can’t put out a beach umbrella?
Believe it or not, the dense spines of a cactus actually do help to shade the plant’s stem.
Despite thick layers of a waxy coating, cactus stems will still lose water in extreme heat. The mottled shade of a cactus’ spines can help keep the whole plant from overheating and letting all that water leach away,
but without blocking so much light the plant can’t photosynthesize.
A Built-In Water Collection System
Spines do more than just keep cacti from losing water. They can also help collect it!
The delicate, hairlike spines of a cactus are great at catching even the slightest bit of moisture, giving dew a place to condense.
It’s like in the summer, when dew settles on each blade of grass in the lawn—except in this case, the blades of grass are cactus spines, and each one is perfectly angled.
Either the dew will drop straight to the ground at the base of the cactus or the spine will direct smaller droplets of water toward the stem of the cactus so it can collect and fall when it’s heavy enough.
At the base of the cactus, the plant’s shallow root system is ready and waiting. Cacti are very efficient at collecting any water they come across, and these tiny drops of dew are just what the plant needs to make it through dry times.
For some plants, the moisture collected from dew might its only source of water for yearsbetween rain storms!
Thorns Keep Thirsty Animals At Bay
Cacti are great at storing water, and in a desert, that’s a precious commodity!
So there’s a very good reason why cacti have turned their leaves into defenses that give Fort Knox a run for their money.
Though spines don’t keep every animal at bay—especially pests small enough to creep beneath the spines—they do help limit the damage.
The threat of animals snacking on a cactus isn’t just limited to the bites they take, either.
Any cut into a cactus’ waxy flesh will make it leak water until the affected area dries out and seals off, so anything the plant can do to prevent the loss is crucial.
This might be why some cacti have so many spines!
What About Spineless Cacti?
So spines are a pretty good adaptation for a desert-dwelling plant.
Why would some cacti not have any at all?
Once again, there are some pretty good reasons, with one of the most common being one of the least obvious: human intervention.
Several species of cactus, such as the prickly pear, are used for food, while others are grown decoratively.
No one harvesting a field of prickly pear or potting up a cute little cactus for a windowsill wants to brave the thorns, so thornless varieties were selectively bred by horticulturalists and plant enthusiasts.
They crossed plants with smaller and smaller thorns until at last they developed “thornless” varieties.
Of course, some of these varieties might still have vestigial thorns, and some individual plants might be thornier than others, but for all intents and purposes, humans have bred the thorns away.
In The Wild
In the wild, similar things can happen.
Some cacti grow in areas where spines aren’t as important, like relatively wet, sub-tropical or tropical areas with limited predators.
The Hawaiian Islands are a good example. Some areas have very few herbivores that might chew on spineless cacti, so when a normally spiny plant ends up with a genetic mutation for spinelessness, it survives.
The spineless genes stay in the gene pool, and before long, there’s a population of spineless cacti, often growing right alongside its spiny neighbors.
Of course, there’s another, simpler reason for some cacti to be spineless.
They might not really be cacti at all! The growing popularity (no pun intended) of succulent plants means that all kinds of new and unusual plant species are now readily available at local growers, and though almost all cacti are succulents, not all succulents are cacti.
To know if your spineless “cactus” is really a cactus at all, check for areoles, which are the spots on the stem where spines, hairs, or flowers would grow.
These are a hallmark of all cactus species, and are a highly modified branch structure.
Wait. There are Leafy Cacti?!
As if spineless cacti weren’t weird enough, there are a couple varieties that completely buck the trend for spines.
They stuck with leaves!
The most common “leafy” cacti are in the plant genus Pereskia, which includes several different species, like the beautiful Rose Cactus above ❤️.
These plants, which live in moist, tropical environments, can look almost like trees, with leaves and bark, but they don’t have a typical branching structure.
In fact, if you look closely, you’ll see that Pereskia cacti have the same growth pattern as their spiny cousins, with their leaves and blossoms emerging from areoles.
Scientists think that this might be what all cacti looked like in prehistoric times, before changing climates forced most types of cacti to adapt to drier climes.
So why did most cacti lose their leaves?
Turns out, they didn’t! They just opted for a highly specialized leaf that does exactly what they need it to—help the plant gain and keep water in a tough desert environment.
I hope you enjoyed reading this post as much as I enjoyed writing it 🙂
What’s your favorite type of cactus?
Got any tips and tricks for avoiding those spines?
Share in the comments below!