What Are Succulent Plants?

Succulents are a widely diverse group of plants consisting of thousands of species in 300 genera. They are not limited to desert plants booby-trapped with spines, and they include a variety of plants with different sizes, shapes, flesh tones and flower colors. In home gardens, succulents are particularly suitable for Xeriscapes. They are also grown indoors as houseplants and terrarium specimens.

Succulent plants are like sponges. They absorb water, using their leaves, stems or roots as storage reservoirs. That trait is why they typically appear to be swollen or are described as "fleshy." Some of the best known succulents are cactuses. Succulent plants have evolved to the point that their leaves have been condensed to areoles, small bumps from which flowers emerge and sharp spines protrude. Succulents, however, are in 60 plant families. About only six of those families are dedicated exclusively to desert plants.

Succulence is a story of survival. Desert plants have evolved into succulent plants simply because their natural habitat receives relatively little annual rainfall. Collectively, succulent plants also represent an example of convergent evolution, a phenomenon in which unrelated species develop similar traits in response to similar environments, even though separated by distance. For example, the agave plant known as the hardy century plant (Agave parryi ssp. truncata), native to Mexico, bears a close resemblance to African aloe (Aloe saponaria), yet they are unrelated and evolved on different continents. Hardy century plant grows outdoors all year in U.S. Department of Agriculture plant hardiness zones 6 or 7 through 9, and African aloe grows as a perennial outdoors in USDA zones 8 through 11.

Succulents generally fall into one of three categories: leaf, stem or root succulent. It’s usually simple to distinguish the plants of those categories from each other just by looking at how they are structured.

Leaf succulents store water in their leaves that spiral outward from a rosette base at their center. Although it may be difficult to detect, each of leaf succulent has a small stem. The leaves are plump and each is typically covered with a tough, waxy skin. The outer layer of this skin, however, contains assimilating tissue, which consists of spongy cells that absorb water. The waxy coating also helps to prevent the loss of moisture to evaporation by deflecting the sun’s rays

Stem succulents either have no leaves or only a few small ones that are short-lived. The lack or low number of leaves helps to reduce moisture loss and dictates that these plants grow in a columnar or spherical form.

Also referred to as geophytes, root succulents have the distinguishing characteristic of being able to survive long periods of drought. Unlike many other succulents, root succulents draw and store water in their underground tubers. Above the ground, root succulents usually have stems and leaves that fall off for winter. Even though their stems and leaves also store water, these plants tend to shed them during dry periods, which helps the plants survive foraging animals and fires.