Cacti and other succulents. Relationships and a shared strategy

A family and a functional trait

Succulence is the capacity to store water in living tissues and subsequently draw on these reserves. It is expressed in the structure of a leaf, stem or underground organ. Cactaceae, the cactus family, is a botanical family. The first term describes a functional strategy, whereas the second identifies a position in plant classification. Cacti are a prominent group of succulents, and comparable water-storage strategies also occur in many other families. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]

A fleshy aloe leaf and an enlarged cactus stem store water in different organs. Their comparable function evolved in separate plant lineages. The independent appearance of similar traits is termed convergence. This makes it possible to compare water-use mechanisms while retaining the separate identities of families, genera and species. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]

Structure helps identify the plant

Areoles are characteristic structures on cactus shoots. They can produce spines, hairs, branches and flowers. They help identify the family even when spines are inconspicuous. The resemblance between a green, ribbed succulent spurge stem and a cactus concerns overall form, whereas identification relies on organ structure. [2-WWW]

The diversity of succulents is already apparent in their leaves and stems. An anatomical review compared leaf succulents such as Lithops, Crassula and Aloe with stem succulents such as Carnegiea and Hoodia. Their fleshy organs differ in the proportions of photosynthetic and storage cells. The functional group thus encompasses plants that share an aspect of water management but have different body plans. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]

[Fradera-Soler et al. 2022, p. 2292, ryc. 1]

Water reserves in a particular habitat

Stored water extends the period during which a plant can function between rainfall events. Tissues gradually release water and decrease in volume. Replenishment requires water from the surroundings. High succulent diversity therefore occurs, among other places, in semiarid habitats with seasonal rainfall. Adaptation to drought involves both the timing of reserve replenishment and the rate at which reserves are used. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]

Succulence also occurs in small, rapidly drying microsites within a wetter environment. An epiphyte growing on a tree accesses a different water supply from a plant rooted in soil. Rock-dwelling plants and some alpine species can also benefit from fleshy organs. A succulent appearance therefore does not specify a single temperature requirement or overwintering regime. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]

Anatomy and metabolism

Water storage and CAM photosynthesis are associated in many plants. Large vacuoles can accommodate both water and the organic acids involved in the daily carbon cycle. The strength of CAM is nevertheless established through physiological measurements. Fleshiness describes anatomy, whereas daily gas exchange describes metabolism. These two kinds of information complement each other in a species account. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]

Cacti remain the main focus of this atlas. Other succulents provide a basis for comparing water-storage organs, tissue organisation and responses to dehydration. Classification presents plant relationships, while the shared biology section explains mechanisms. This structure allows readers to move from a species name to a specific feature of its anatomy. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2290–2293]