Definition of a Succulent
So, What Exactly Is A Succulent?
by L. Maynard Moe
I am a field botanist (taxonomy and ecology) by training and only recently became interested in cacti and (other) succulents. After reading popular gardening and collecting books, and talking with other collectors, I realized that my notion of a succulent plant and the collector's notion are not the same. Maybe others in our club share this confusion, so here is an article that may be informative.
Many are quite familiar with the term 'succulent," but misconceptions abound. Comments such as "This aloe plant is in the succulent family," or "I don't grow cacti, I only grow succulents,' reveal some of these misunderstandings. Of course, our own society's name, "Bakersfield Cactus and Succulent Society," as well as some books such as 'Succulents: the Illustrated Dictionary," which do not include cacti, contribute to this confusion by implying that cacti are somehow different from other succulents. The basis for this confusion appears to be a misunderstanding of the term 'succulent," which is a descriptive term, not a scientific classification. Generally a succulent is a plant that has developed (through evolutionary adaptation to water stress) swollen, water-storing tissues which protect it from desiccation. This includes plants such as pickleweed (Salicornia) adapted to high salinity, as well as cacti and aloes which are adapted to hot, dry climates.
Note that the definition of succulent does not specify what organ of the plant has the water-storing tissue. There are three broad categories of succulents - leaf succulents, stem succulents, and caudiciforms - depending on the part of the plant that is succulent. Most species in the stonecrop family (Crassulaceae) are leaf succulents. These include such common plants as Sedum (stonecrop), Sempervivum (hen-and-chickens), Aeonium (aeonium) Crassula (Jade plant), Dudleya (live forever), and Echeveria (hen-and-chicks). Very interesting plants that display highly specialized succulent leaves are members of the ice plant family (Aizoaceae). These include such genera as Argyroderma, Cheiridopsis, Conophytum, Faucaria, Lithops, Titanopsis, and Trichodiadema. Some of these, such as Lithops and Conophytum, have no stem and consist of a pair of succulent leaves. Some are called "living stones."
Most (but not all) species in the cactus family (Cactaceae) are stem succulents as are many species in the genus Euphorbia (Euphorbiaceae) and some species in the milkweed family (Asclepiadaceae). These plants often have reduced or absent leaves and the stem is photosynthetic. Caudiciforms are also stem succulents but differ in that the stems are not photosynthetic. Instead they have a swollen base, or caudex, which is usually the lower part of the stem but may also be the exposed upper part of the root. Several plant families have species that are caudiciform such as ponytail palm (Beaucarnia - Liliaceae), dorstenia (Dorstenia - Moraceae, the mulberry family, elephant-foot (Dioscorea - Dioscoreaceae, the yam family), cissus (Cissus - Vitaceae, the grape family), pachypodium (Pachypodium - Apocynaceae, the dogbane or oleander family), adenia (Adenia - Passifloraceae, the passionflower family), oxalis (Oxalis - Oxalidaceae), geranium (Pelargonium - Geraniaceae), and othona (Othona - Asteraceae, the sunflower family).
Some plants cross boundaries and are succulent in more than one way. For example, some Cassulas and Anacampseros (Portulacaceae, the purslane family) have both succulent stems and leaves, and some Anacampseros and Trichodiadema (Aizoaceae, the ice plant family) are caudiciform with succulent leaves.
By convention some plants that display succulence are not considered succulents, such as impatiens and some orchids even though they have fleshy stems and leaves. Plants with fleshy underground bulbs (lilies, daffodils, tulips), corms (gladiolus, freesia, crocus), rhizomes (iris, calla) or tubers (dahlia, begonia, potato) are also not considered succulents. An exception is pregnant onion (Bowiea) which has a large, green, above-ground bulb and is often found in succulent collections.
It appears that this confusion stems from the collector's interest in unusual plants adapted to warm dry conditions as opposed to interest in plants merely having succulent tissues. So, collectors have one notion of what a succulent is, and botanists have another notion of what succulence is.
A comment on distributions: Most succulents are from tropical and subtropical dry regions and do not tolerate cold. In addition, one would expect succulents to occur in all of the world's deserts. However, desert regions of Asia and Australia have few species, whereas the Americas and Africa have many. In fact, the region with the greatest diversity of succulents is South Africa which contains roughly half of the world's succulent species. Mexico probably has the second greatest concentration, most of which are cacti (there are no native cacti in Africa).
