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Types of succulence

Succulent plants, also called succulent plants, are plants that in the course of their evolution have assumed particular forms to survive in arid, hot and often with high percentages of salt; most plants on the globe could not survive in these areas, especially due to the lack of water.
Succulent plants have evolved to the point that they are not only able to store water, but also lose very little through evaporation. In nature there are two large groups of succulents: on the one hand we have succulent plants originating in Central and South America, most of these succulents are part of cacti, and they have evolved turning their foliage into thorns; on the other we have succulents originating from Africa, none of which belongs to cacti and practically none of which has evolved its leaves into thorns.
In order to absorb all the water they receive with precipitation, most succulents develop a shallow root system near the surface of the soil; this is because sporadic rains, or dew, present in arid areas, often evaporate quickly as soon as the temperatures of the day rise; It is therefore essential that the plants that live in these climates can have access to every water resource present, even ephemeral ones, such as the morning dew.
There are different types of succulence, and there are several devices that the various succulent plants have evolved to survive in hostile environments

The cacti

In order to vegetate, bloom, produce seeds, in the arid areas of the American continent, cacti have evolved various expedients; the first of all, what we first notice, is the absence of leaves, which have turned into thorns. Through the leaf page, a common garden plant, constantly evaporates a large amount of water; everyday. The succulents often have no leaves, have very few stomata, or the cells of the epidermis of plants that take care of exchanging oxygen and water with the environment.
To be able to practice photosynthesis, cacti have evolved by developing chlorophyll directly on the stems; these stems are also hardly woody, and remain fleshy and rich in water even in old plants, to better store them.
In addition to these devices, cacti generally have a spherical or columnar shape, without branches, so as not to expose large quantities of epidermis to heat and evaporation; the epidermis of cactus stems is then often, sometimes covered with wax or hair, to reduce transpiration and the heat received.

Plants with succulent foliage

Most of the plants with succulent foliage are native to Africa, but they also exist in America, and in most of the globe, even in Europe; these are succulent plants, or plants with succulent stems and leaves, or plants without stems and with leaves arranged in fleshy rosettes (like i sempervivum), or even perennials, which dry up during the winter, such as sedum, or even large fleshy rosettes and short semi-woody stems (for example aloe).
All these plants have retained chlorophyll in the foliage, which has also developed fleshy tissue that stores water.
Generally these plants are without thorns, since they have kept their foliage.

Plants with succulent stems

There are plants with succulent stems that are not cacti, such as euphorbias, succulent plants, stapelias; they differ from cacti in that they are generally well branched, they hardly have sharp thorns, sometimes they produce some leaves. For example, the euphorbias can produce some leaves in the wet period of the year, which will fade when the dry season arrives.
Also in this case the stems practice most of the photosynthesis, so they are green; and collect most of the moisture, so they are well fleshy.

Plants with succulent roots

There are plants, generally of African origin, that have evolved fleshy roots, in which they collect large amounts of water; when the climate is too hot and dry, these plants can even completely lose the aerial part, to avoid perishing excessively; when the climate returns to be less hot or less dry, the plants will sprout again.

Plants with caudex

Plants with caudex are succulent stem plants, but semi-woody; they are very particular plants, which can remain even months without water, months during which they lose most of the foliage. When the rains arrive, they begin to vegetate and produce innumerable leaves, generally not succulent.

The acid metabolism of succulent plants

This metabolism is often indicated with the acronym CAM, and although it has been noticed for the first time in a crassula plant, it does not belong exclusively to this genus of plants, but also to many other succulents.
Plants generally have a metabolism that allows them to breathe through the stomata, small "pores" of the leaves; as we humans do, plants also breathe in oxygen during the night, and emit carbon dioxide. During the day they take the opposite path: they inhale carbon dioxide, and they exhale oxygen.
Most succulent plants close their stomata during the day, to avoid evaporation, which would be very harmful in arid areas.
These plants open their stomata at night, storing carbon dioxide in the form of malic acid; this carbon dioxide is then used during the day, with closed stomata.