Why do cacti have thorns? – A PUZZLE

I’ve always assumed that cacti only had thorns for protection, but it turns out they have a lot more functions and are truly remarkable adaptations. They help cacti overcome the difficulties of living in desert conditions in several interesting ways.

Why do cacti have thorns? Thorns help cacti thrive in desert environments by providing multiple functions. They protect the plant from predators, provide shade, help regulate temperature, reduce water loss, and even help the plant spread and reproduce.

Read on and I’ll explain exactly how cacti use their spines to survive and thrive in the harsh conditions they live in.

Why do cacti have thorns?

There are 6 main reasons cacti have thorns. I’ll explain each reason in detail, and I’m sure you’ll be fascinated by the interesting uses of these tricky adaptations.

1. Cactus spines trap air and regulate plant temperature

Although cactus spines are small, they are usually very numerous and cover the surface of the plant. In the same way that wearing a sweater traps an insulating layer of air near your skin, the spines of a cactus trap a layer of air near the surface of the plant, reducing ventilation.

This layer of air insulates the cactus and prevents large temperature fluctuations. This is useful in very hot desert conditions, but also protects them from large temperature fluctuations between day and night, and during different seasons, where it can get quite cold at night in some desert climates.

2. Provide shade

Why do cacti have thorns?

Cactus spines are small and thin and not what you would normally expect from sunscreen. Hold a single cactus thorn and the shadow it casts is small and seemingly insignificant.

However, because cactus thorns are so numerous, the combined effect of all the thorns provides a surprising amount of shade. There aren’t many places to find shade in a desert, and the sun can easily damage a cactus with no way to prevent anything from reaching the surface of the cactus.

In addition to reducing heat, the shade from cactus spines also reduces evaporation of water from the surface of the cactus, reducing water loss and allowing the cactus to survive a dry spell more efficiently.

3. Thorns protect cacti from predators

Cactus spines are extremely sharp and can be very painful to the touch unless you are careful. I’m always amazed at how sharp they are when repotting a cactus, and it reminds me why special care is needed around these plants.

Cacti are often one of the most numerous plants in a desert environment and for the many animals that live here a cactus would be a welcome meal, but the thorns prevent most animals from feasting on these plants.

Unfortunately for cacti, there are a number of animals that don’t care about the sharp spines of cacti at all and eat them anyway. Javelins, camels, and many rodents and insects eat cacti, despite their thorns.

4. Mobility and reproduction

Some types of cacti have barbed spines that easily stick to animals (or even people) who get too close to them. This is a clever reproductive mechanism, exhibited mainly by Cholla cacti (Cylindropuntia).

If something gets too close, the barbed thorns will get stuck in the animal’s fur or skin. Part of the cactus will be separated from the parent plant and transported to another location.

When these cactus parts eventually detach from the animal, a new plant will grow where they fall.

This behavior is not exclusive to cacti, since many plants rely on the help of animals to reproduce and propagate, but some cacti have attributed this function to their spines, with excellent results.

Interestingly, this means that when you see hundreds of Cholla cacti in the desert, they are likely clones of each other and genetically identical, due to the efficiency of this breeding technique.

5. Cactus spines help collect water

The deserts where many cacti live are notoriously arid and rainfall is unpredictable. It can be a long time before the next rain falls, so cacti and other desert plants must use adaptations to collect, store, and retain water.

Cactus spines play an important role in capturing water from the air and can do so very effectively due to the large surface area in contact with the air.

Cacti do not lose water from these spines because they are metabolically inactive, but they do allow water vapor from the air to condense on the spines and flow to the surface of the plant and into the soil to be absorbed by the roots.

Even if it hasn’t rained for a long time, a lot of moisture can still be transported through the air. Warm air holds more moisture, but when it cools it cannot carry all the moisture and some condenses on surfaces. Thus, at dusk, dew forms and the cacti are ready to draw this source of water from the sky.

6. Cactus thorns are modified leaves

Cactus spines are actually highly modified leaves, although they don’t have the same structure as normal leaves and are completely inert. Cacti have adapted to the harsh conditions in which they live by developing areoles instead of branches and spines instead of leaves.

The problem with leaves is that the large surface area causes a lot of water loss in a process called transpiration. Cacti adapted by transferring functions from the leaves to the cactus body. This means that photosynthesis takes place in the trunk of the cactus, rather than where we normally think it happens in other plants.

The adaptation and development of spines developed gradually, as the hot, inhospitable conditions of the deserts led to survival advantages through smaller leaves. Gradually, leaf functions shifted to the central stem, and the remaining leaves hardened and developed the other spiny adaptations we see today.

What is the difference between Thorns and Thorns?

Many plants have thorns, but only a few types of plants have thorns, and yet they often look very similar. So what’s the difference? Thorns are modified leaves, while thorns are modified branches.

All prickly plants also have leaves with a certain description. Plants with thorns have no leaves, because the thorns ARE the leaves.

Interestingly, many plants that we normally associate with thorns do not. The thorns of roses are actually thorns. Thorns are pointed extensions of the epidermis of the stem or leaf and differ from both prickles and quills.

Are cactus thorns poisonous?

Cactus thorns are not poisonous. However, they can cause significant skin damage, especially in species with barbed spines. Barbed spines are designed to detach and lodge in the skin of the animal or human they come in contact with. These spines are difficult to remove and can cause localized dermatitis or a granulomatous reaction if left untreated.

Opuntia cacti have very short spines that dig into the skin and are very difficult to remove. They irritate the skin and are very uncomfortable. I spent many frustrating hours trying to remove the prickly pear bones from the skin with tweezers. This is not a situation I would like to find myself in.

Why do cacti have thorns that grow new thorns?New cactus thorns grow

Do cactus thorns grow back?

Cactus spines emerge from the areoles as the plant grows. Mature thorns are inert and will not regrow if damaged or removed. New spines will only grow from new areoles as the cactus grows.

If you are growing cacti as an ornamental, handle them with care as cosmetic damage from damaged or broken thorns is permanent.

I hope you enjoyed learning why cacti have thorns. This article is part of a series examining some of the interesting characteristics of cacti and succulents. Check out some of the other articles in this series below.

  • What soil do cacti need?
  • How fast do cacti grow?
  • How long do cacti live?
  • What is the difference between succulents and cacti?

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