PLANTS

I collect and cultivate cacti. And some other succulents sometimes, but mostly cacti.

When you read that, you know exactly what plants I'm talking about, right? But... what is a cactus?

Let’s look at the following definition: “Cacti are the group of closely related plants that have clusters of spines and that are native to North and South America.” (MAUSETH et al., 2002).

Alright, “closely related plants”, as are all plants placed within the same family, so it doesn't make a cactus different from any other plant. Neither does “ntive to north and south america", as there are other plants native to there as well. So, what's important is "cluster of spines".

What defines a cacti, and makes it different from other plants, is the presence of a structure called the areole, from which clusters of spines form from. Those spines are modified, highly-specialized leaves that had its surface area reduced to prevent water loss. They are not capable of photosynthesis, however. In cacti, it’s the stem (meaning the body) that is responsible for the photosynthesis and for the gas exchanges.

Ok, but how did they become like that?

What made an ordinary plant evolve to be what we call today a cactus?

First, we have the reduction of the size of leaves, and even the loss of them, to reduce the loss of water, which led to the development of succulent tissue that was able to store that much needed water, so the plant could survive in environments with not a lot of rain throughout the year. Spines also serve as a way to protect themselves from animals trying to get to that water stored.

Then, they evolved another four important characteristics: the development of bark late in life, essential since the photosynthesis is made by the stem; the loosening of the cortex cells, which allowed oxygen to leave the cactus more quickly, necessary since oxygen is what’s toxic to the plant; the evolution of flexible ribs, so it can adapt its body as it gains and as it loses water; and the creation of cortical bundles, which are tissues that carry water and transport sugars more quickly.

So with all that, we have a cactus.