Succulents. Almost everyone is familiar with the term. Yet questions such as “is that Aloe a member of the succulent family?” reveal basic misconceptions on the part of many people as to just what a succulent is. The core of the problem here is that the term “succulent” is merely a descriptive term and not a scientific classification. Thus any plant that has evolved swollen water-storing tissues as a protection against desiccation is succulent by definition, regardless of what family it belongs to. A few families, such as the Crassulaceae (the stonecrop family, which includes plants such as the Hen-and-Chicks and jade trees) are composed entirely of succulents. But in many other cases, succulence occurs only in certain groups of a family of largely non-succulent plants.
Some confusion is created by terms such as “Cactus and Succulent Society” or “Cactus and Succulent Show”, which seem to imply that a cactus is not a succulent. In fact, cacti (members of the family Cactaceae) are a family of succulent plants, separated out only because of their great popularity among growers. Although the cactus family is largely composed of succulents, there are a few types (not often seen in collections) that have retained ordinary leaves and are non-succulent.
Succulents may be grouped in three broad categories, according to which part of the plant has a capacity for water storage. Thus we have “leaf succulents”, such as the century plant; “stem succulents”, such as the saguaro; and “caudiciforms”, such as the pony-tail palm.
LEAF SUCCULENTS: The unusual appearance of the leaves may cause confusion. Thus the large spiky leaves of a century plant or tree aloe may be called “arms” or “branches” by someone whose notion of a leaf is a little too conventional. Or the tight rosettes of leaves in a Hen-and-Chicks plant may be incorrectly called “petals”.
STEM SUCCULENTS: Here the plants utilize juicy swollen stems for water storage. In many instances, they have reduced or eliminated the use of leaves for photosynthesis and respiration, transferring these functions to the stem, which usually takes on a green color. In some cases, such as the dangling flattened stem of an orchid cactus or the pad of a prickly-pear, the stem may be mistakenly referred to as a “leaf”. Cacti as a group are prominent examples of stem succulents.
CAUDICIFORM SUCCULENTS: This group of succulents consists of plants with a swollen base, or caudex, which may actually be the lower portion of the stem or the exposed upper part of the root. Plants of this sort may range from tiny fat-stemmed fig relatives (Dorstenia) to huge trees, such as the baobabs of Africa and Madagascar (Adansonia). Caudiciforms normally have leaves and cannot use their stems for photosynthesis.
Of course, some plants may cross boundaries and be succulent in more than one way, as a jade tree with both succulent stems and succulent leaves. Also the boundaries of what is included as a succulent are flexible and subject to interpretation, and by convention, certain plants that display succulence are not counted as succulents – think of Impatiens with its succulent stems or Cattleya orchids with their thick leaves and swollen pseudobulbs. Conversely, some non-succulent plants which are drought-adapted and grow alongside succulents are often counted in with them. Examples of this would include Yuccas (although some species do have leaves thick enough to qualify as succulents) and Puyas.
While many plant families have some succulent members, succulence predominates in only a few. Three such families stand out: the cactus family (Cactaceae), the ice plant family (Mesembryanthemaceae or Aizoaceae), and the stonecrop family (Crassulaceae). All are large families with many genera and hundreds of species, nearly all succulent. The Agave family (Agavaceae) has been split up and now includes only Agaves, Yuccas, and their closest relatives. These plants are prominent in the deserts of Mexico and the Southwestern United States, and staples of succulent gardens here in the Bay Area. The lily family (Liliaceae) has also been split up, but one of its splinters (the Aloeaceae) includes the Aloes and their relatives – prominent plants in many African landscapes. Another important group of African succulents are the Euphorbias and their relatives (Euphorbiaceae). The genus Euphorbia is widespread around the world, but a large majority of the succulent species is from Africa, where they range from little globular leafless stems (E. obesa) up to large well-branched spiny trees. Some are quite cactus-like in appearance, though their flowers are very different. This is an example of convergent evolution, where groups of differing ancestry have solved similar survival problems in similar ways, and come out looking a lot alike.
Plants have acquired succulence as a means of storing water. The need for this is obvious in desert regions, which of course often contain succulent plants. But plants may be succulent because of the particular ecological niches they occupy as well. Thus a plant growing on a rocky outcrop or a sandy bank may have a need to conserve water due to the extreme drainage where it grows, even if the area is not particularly dry. A majority of succulents come from tropical and subtropical areas, and this is the reason why so many cannot be grown outdoors in regions which experience freezing temperatures in winter. There are, however, species that can take extremes of cold. One example would be the prickly pears (cacti of the genus Opuntia) inhabiting our northern states and southern Canada. Another would be the Spiral Aloe (A. polyphylla), which grows at an altitude of 10,000 ft. in Southern Africa’s Drakensberg mountains.
Although succulents come from many countries and inhabit many environments, they are very unevenly distributed geographically. For reasons that are not clear, the desert regions of Asia and Australia have very few succulents, while those of Africa and the Americas are very rich in them. Even within Africa, West Africa and North Africa have few succulent plants, while the eastern and southern parts have many. Of all the world’s succulent “hot spots”, none surpasses South Africa. This one country contains roughly half of the world’s succulent species. Another area with a heavy concentration is Mexico, along with adjacent parts of the United States. A third “hot spot” is south-central South America, with many species of cacti and succulent bromeliads (members of the pineapple family, the Bromeliaceae). In addition to these continental regions, some islands and island groups are well known for their succulent flora. Madagascar, the Canary Islands, and the Galapagos are prominent examples.