Why do cacti have thorns?

When you think of cactus, what comes to mind first? Is it the prickly "thorns"? Or the "leaves" that look like palms? I still remember the first time I saw a cactus when I was a child. Out of curiosity, I reached out to touch the thick "leaves" of the cactus, and suddenly I felt a sharp pain. I pulled my hand back and found that many thorns were piercing my fingers. This "curiosity journey" is still fresh in my memory.

After many years, I read about cactus in a book and realized that my understanding of cactus was a little distorted. It turned out that what I pricked myself with was actually a cactus leaf! The harsh environment did not limit its growth. The cactus even used its wisdom to build a "cool armor" for itself.

Cactus image source: nwf.org

Part 1

The "leaf" is not a leaf, and the "thorn" is not a thorn

The "cactus" we often talk about is actually a large family with many members. However, the topic we want to talk about today is not how many species of cacti there are, but a very special morphological trait in this type of plant - spines. If you look closely at the "leaves" of cacti, you will see sparsely distributed small nests (kē) on them; the nests are mostly round, with a diameter of about 3-5 mm, and often clustered with many small spines.

The small nests and spikes on the cactus. Image source: popsci.com

In fact, these spikes are the real leaves of the cactus, but they are extremely degraded and do not look like the leaves of common plants. These highly specialized leaves are a weapon for the cactus to defend against herbivores and can also help prevent water loss. What we often think of as leaves are actually just the fleshy stems of the cactus. In the absence of true leaves, the cactus relies on these fleshy flat stems for photosynthesis.

Why do the leaves of cacti degenerate into this appearance that is completely different from leaves? This has to do with the environment in which the cacti live. Cacti are native to the Americas and are widely distributed in deserts and arid areas in Brazil, Paraguay, Uruguay and Argentina. These areas are hot and dry all year round with little rainfall. In order to reduce the transpiration and water loss of the leaves, the cacti grow into this strange appearance. To sum it up in one sentence, the leaves of cacti are not leaves, and the thorns are not thorns. The thorns are the degeneration of leaves, but the stems grow into the appearance of leaves.

Cacti grow in dry and hot environments. Image credit: vermontpublic.org

Part 2

Although the thorns are small, they have a great effect.

Although the leaves of the cactus no longer have the ability to photosynthesize and have degenerated into thorns, as the saying goes, existence is reasonable, and these thorns also have functions that people may not expect.

As mentioned earlier, the thorns of cacti are a good armor for them and can be used to defend against herbivores, but their functions are not limited to this. Small thorns can also have big uses.

Guess what the picture below is? Does it look like a snowflake?

The thorn of the white star looks like a snowflake. Image source: popsci.com

In fact, these are the thorns of a cactus called White Star (Mammillaria plumosa). These thorns act like a photographic umbrella, a light modulator that diffuses light, meaning they can spread light over a larger area, ensuring that even if the cactus grows in a shady place, it can still get enough light for photosynthesis.

In addition, the thorns of the cactus also have the function of collecting water. If we observe the thorns of the cactus under a microscope, we will find that the seemingly smooth thorns actually have many small spikes. These spikes help the condensation of water vapor. When the water vapor condenses into water droplets, it will drip into the soil along the thorns of the cactus. This can be regarded as a measure for the cactus to adapt to the arid environment.

Cactus thorns Image source: popsci.com

In addition to defense and water collection, the thorns of cacti have other uses. Friends who have planted cacti may know that each stem of a cactus has the ability to take root in the soil and grow into a new individual. The stems of some cacti are easily broken, and they rely on the thorns on their fleshy stems to hang on passing animals, thereby spreading to further places.

Hey, the tiny thorns contain the great wisdom of plants.

Part 3

Pull out that annoying thorn

Although the thorns of cactus have various functions for cacti, for us humans, these small thorns are too annoying. When I was a child, I touched a cactus out of curiosity, and the process of dealing with these small thorns afterwards made me exclaim, "Why can't I control my hands?"

Of course, the attacks of these thorns are not fatal, but the tips of the thorns of many cacti have barbs, which can irritate the skin or cause infection when they penetrate the skin. If not handled properly and the tip of the thorn is not removed, it can cause long-lasting pain.

Barbs at the tip of a cactus thorn Image source: Reference 2

When pricked by a cactus thorn, it is best to first remove the fleshy stem connected to the thorn, and then decide how to deal with it based on the type of thorn.

The straight thorns of cactus are the easiest to deal with. Just pull them out and everything will be fine. However, if the aforementioned barbed and spear-like thorns pierce the skin, more delicate treatment is required. We can try to pick out the remaining thorns with a sterilized needle or tweezers, or we can try to pull the barbs out of the skin with sticky objects such as tape. Don't try to do this with your mouth and teeth. If a small barb enters your mouth, it may lead to a "tragedy".

However, isn't it enough to just look at a cactus from a distance? After all, it has already told us with its actual actions that it is dangerous and you will become unfortunate if you don't get close to it.

References:

[1]Mauseth, JD (2006). Structure–function relationships in highly modified shoots of Cactaceae. Annals of Botany, 98(5), 901-926.

[2]Liu, H., Liu, S., Jiao, J., Lu, TJ, & Xu, F. (2017). Trichomes as a natural biophysical barrier for plants and their bioinspired applications. Soft Matter, 13(30), 5096-5106.

Produced by: Science Popularization China

Author: EVEE (School of Life Sciences, Peking University)

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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