Crassulaceae

Botanical characteristics

The Crassulaceae are a family of succulent plants characterized by the ability to accumulate water in fleshy and thickened leaves, an adaptation that allows them to survive in arid or semi-arid environments. The habit of plants in this family varies from herbaceous to shrubby, with some species even reaching a tree-like form. The leaves often have a thick and waxy texture, reducing water loss.

The flowers are generally small, with structures that vary depending on the genus, but often feature a corolla formed by free or partially fused petals. The fruit is usually a capsule that, at maturity, opens to release the seeds. A distinctive feature of the family is CAM photosynthesis (Crassulacean Acid Metabolism), a particular metabolic mechanism that minimizes water loss by closing stomata during the day and opening them at night, a name derived precisely from this family in which it was first discovered.

Distribution and diversity

The family has a cosmopolitan distribution, present on all continents except Antarctica. It includes about 1400 species divided into 36 genera, organized into three main subfamilies: Crassuloideae, Kalanchoideae, and Sempervivoideae. The areas with the greatest diversity and concentration of species are:

Mexico and the southwestern United States, with about 300 species and endemic genera such as Cremnophila, Dudleya, Echeveria, Graptopetalum, Pachyphytum, Thompsonella, and Villadia.
South Africa, where about 250 species are found, many endemic, especially in the genera Adromischus, Crassula, and Tylecodon.
Macaronesia, with about 200 species, many belonging to the genera Aeonium and Monanthes.
Himalayan range, with about 200 species and numerous endemics among Rosularia, Pseudosedum, and Sinocrassula.
Mediterranean Basin, with about 170 species and largely endemic genera such as Chaloupkaea, Petrosedum, Pistorinia, and Sempervivum.

These plants evolved between 100 and 60 million years ago in southern Africa, with subsequent dispersion across various continents. Most species are xerophytes, adapted to conditions of low water availability, but some are also found in temperate, mountainous, or semi-aquatic environments.

Main genera and representative species

Among the best-known and widespread genera are:

Crassula, with species such as Crassula ovata, highly appreciated as an ornamental indoor plant worldwide.
Kalanchoe, which includes species often cultivated for their decorative flowers.
Sedum, a large and diverse genus, also present in many European and Mediterranean areas.
Sempervivum, typical of European mountainous areas, such as the Alps, known for its rosette shape and cold resistance.
Aeonium, mainly distributed in Macaronesia, with species of great ornamental interest.
Echeveria and Dudleya, highly appreciated in Mexico and the southwestern United States for their succulent forms and vivid flowers.

In Italy, species mainly belonging to the genera Sempervivum, Sedum, and Petrosedum are present, found in mountainous and rocky environments.

Etymology

The family name derives from the type genus Crassula, a Latin name referring to the "thick" or "fat" texture of the succulent leaves, a typical characteristic of these plants. The family was formally classified in 1805.

Curiosities and uses

Crassulaceae have no significant agricultural uses but are highly appreciated in the ornamental sector for their hardiness, ease of cultivation, and often bizarre and fascinating appearance. Some species have become very popular houseplants, such as Crassula ovata. From an ecological point of view, their CAM photosynthesis capacity represents a key adaptation for survival in arid environments, contributing to the biodiversity of xeric and mountainous habitats.