What is succulence?
The term "succulence" comes from the Latin word succus = juice. Succulents ("sap plants") are at home where water is in short supply at times. They store it so that they can grow even in dry seasons and be able to bloom if necessary. To this end, the plants have developed amazing adaptations in their physique.
Depending on the location of water storage, we refer to the plants as stem succulents, leaf succulents or root succulents.
Stem Succulence
Typical stem succulents, such as cacti or spurge (Euphorbiaceae) have no leaves. The "leaf green" necessary for photosynthesis (assimilation), also known as chlorophyll, is located in the shoots. There are also stem succulent species without green stems that sprout normal leaves but shed them in the dry season.
Leaf succulence
The water is stored in the leaves, either in the whole leaf or only in certain parts.
Among leaf succulents, there are many species whose leaves are arranged in a rosette. Often the leaves are so densely packed that they partially overlap each other. This reduces the evaporated surface, which reduces the water loss of the plant. In most rosettes, the leaves are arranged in such a way that one leaf is never exactly on top of another leaf.
Water storage as a dynamic process
During the dry season, the plant consumes part of the stored water. This must be replaced again during the growing season. In order to enable the alternating shrinking and swelling of their storage tissue, many stem succulents have developed ribs, which fill and empty over the course of the year like the folds of a bellows.
Distribution
Succulents are native to the deserts, semi-deserts, grassy steppes and mountains of the Old and New Worlds. These drylands have a lot in common: strong sunlight, high daytime temperatures and strong nighttime cooling. The associated regular dew formation in the morning is an important factor for the survival of the plants in the long periods of drought. After months without rain, heavy rainfall and flooding can occur. Now the succulent has to fill all its water reservoirs. Growth, flowering and fruit set take place during this time.
The definition of succulence that is generally valid today was co-developed with the participation of the Succulent Collection Zurich. It states that succulent plants can survive prolonged water shortages by storing water in their tissues and thus maintaining physiological processes such as flower or fruit formation.
Succulents in Switzerland?
Yes, there are – and they are even local! No less than 24 species from the Crassulaceae family grow in stony or rocky places, from dry grasslands in the lowlands to the boulder scree slopes in the Alps. These include our native species from the genera houseleek (Sempervivum) and wall pepper (Sedum), such as the mountain houseleek (Sempervivum montanum) or the hot wall pepper (Sedum acre). At the locations of these plants, the water drains quickly due to the permeable soil and succulent leaves are therefore a survival advantage.
Great diversity – the most important families
There are around 260,000 species of flowering plants (angiospermae) worldwide. Of these, around 12,500 are succulents – that's almost every 20th plant species. In the Zurich Succulent Collection, we cultivate about 4440 species, i.e. a little more than a third of all succulents in the world.
Succulent species occur in 79 plant families. However, the majority comes from just nine families, whose representatives have adapted particularly strongly to arid habitats. Here we introduce the survivors:
The approximately 1890 species of cacti, all of which are succulent, all live in North and South America, from sea level to the highest Andean peaks, with one exception. The tiny creatures among them measure 1–2 cm, the giants over 20 meters.
Cacti are mostly stem succulents: their thickened green shoots take over photosynthesis, while the leaves are almost always transformed into thorns. The inside of the cactus shoots consists mainly of storage tissue, the "skeleton" is formed from a network of woody vascular bundle strands.
The majority of the approximately 440 species are native to North America, especially Mexico. Tough, rosette-shaped leaves are typical. It often takes several decades for the inflorescence to grow out of the rosette, which dies after flowering and fruiting.
Agaves are prized crops in Mexico. The sweet plant juice is drunk fresh as "aguamiel" (honey water) or fermented as "pulque" (agave beer). The fermented and distilled juice of boiled agave hearts is used to make the tequila schnapps. The leaf fibres of the sisal agaves are processed into ropes, cords and carpets.
The family counts about 1430 species, practically all of which are succulent. They are native to the northern hemisphere, southern Africa and Madagascar. According to their name (Latin crassus = thick), the plants have thick storage leaves, which are often arranged in compact rosettes (as in houseleek). Some species also have succulent shoots or rhizomes. The Crassula genus includes the smallest succulents of all: tiny, short-lived herbs such as Crassula connata from America, which only live for about eight weeks.
The asphodelaceae family includes around 1050 species, of which about 825 are succulent. The genus Aloe with a good 615 species is mainly at home in southern Africa and Madagascar and shows a great variety of flowers. Aloe are perennial rosette plants with succulent roots. The smallest species grow only a few centimeters in size, while others grow up to 10 meters high. Aloe leaves are fleshy, usually smooth at the edge and only rarely prickly and armed without a stinging terminal thorn.
The globally distributed family includes a good 6500 species, including around 940 succulents, mainly from the genus Euphorbia. Their growth habits range from cactus-like leafless, thorny shapes to thornless stem succulents with deciduous leaves to species with underground storage roots. All spurge plants have a corrosive and poisonous milky sap. The (apparent) flower of the euphorbia is an extremely smaller, complex inflorescence (called cyathium), which is surrounded by conspicuous bracts or nectar glands.
The second largest succulent family with around 1950 succulent species is mainly at home on the west coast of South Africa and Namibia. Most ice plants are low shrubs with thin woody shoots and paired succulent leaves. In addition, there are also specialized forms such as the "living stones" (genus Lithops). The flowers of many ice plants only open at the highest position of the sun - hence their name. The capsule fruits only open when it rains, with the seeds being hurled away by up to 1.5 m by the impacting raindrops.
This family is common in the tropics and subtropics. Most of the approximately 1150 succulent species are stem succulent. They pursue various strategies for pollination: The flowers of the carrion flowers simulate carrion or dung in their colouring and fragrance and thus attract blowflies; a phenomenon known in biology as mimicry. The cauldron trap flowers of the candlestick flowers (Ceropegia) temporarily trap the pollinators – small flies and mosquitoes. The flowers of the Malagasy Pachypodium species are probably pollinated by butterflies.
With around 22,000 species in about 730 genera, orchids are probably the largest plant family of all. They are distributed worldwide with a focus on the tropics and subtropics. In about 4500 mostly epiphytic species, succulent adaptations such as more or less thickened leaves or storable roots can be found. In the stem succulent species, spherical to club-like thickened shoots or stem pieces, so-called pseudobulbs, often occur. Typical of orchids are their two-sided symmetrical (zygomorphic) flowers, pollen in packets (pole lines) and capsule fruits with dusty seeds.
Of the approximately 3500 species of pineapple or bromeliad family, about 500 are succulent. The family owes its German trivial name to the probably best-known representative, the pineapple (Ananas comosus). With the exception of one species, the entire family is native to the American double continent from Florida to central Argentina.
In the succulent species, it is primarily the more or less fleshy leaves that are responsible for water storage. Such leaf succulence occurs both in ground-dwelling and rock-dwelling epiphytes as well as in many epiphytes from the arid regions. Typical for the family are scale hairs (so-called trichomes) on the leaves, which allow the plants to absorb water and nutrients directly, i.e. without a detour via the roots. At the same time, the trichomes serve to protect against excessive evaporation and sunlight.