The Crassulaceae (, from Latin crassus, thick), also known as the crassulas, the stonecrops or the orpine family, are a diverse family of flowering plants primarily characterized by succulent leaves and a form of photosynthesis known as crassulacean acid metabolism (CAM), in which plants photosynthesize in the daytime and exchange gases during the cooler temperatures of the night. The blossoms of crassulas generally have five floral parts. Crassulaceae are usually herbaceous, though there are some subshrubs, and relatively few trees or aquatic plants.
The Crassulaceae is a medium-sized family in the core eudicots clade, along with the order Saxifragales, whose diversity has made infrafamilial classification very difficult. The family includes over 1,800 species and 34–40 genera—depending on the circumscription of the genus Sedum—distributed over three subfamilies. Members of the Crassulaceae are found worldwide, though are mostly concentrated in the Northern Hemisphere and Southern Africa (where the most species are found). They are adapted to thrive in typically dry, arid (hot or cold) areas where water may be scarce, and richer, organic substrates may be at a minimum or nonexistent. The Sempervivum (houseleeks) of Central and Southern Europe, or the Orostachys (dunce-caps) of Asia, for example, grow in rocky crevices at higher elevations, where soil is at a minimum but precipitation, sun exposure, and winds may be intense; these plants have thus adapted to absorb water by having succulent leaves, despite living often on slopes or near-vertical cliff faces, from which water typically drains quickly. Their roots are adhesive and grip to any rock, dirt, wood or other surface they come into contact with, while also gleaning minerals from said material. Seedlings that may sprout near more permanent sources of water, such as pooling rainwater or seeps, may experience rot or discoloration over time, and not survive.
Crassulaceae are mainly perennial, and have huge global economic importance as collectible specimens, and as indoor and outdoor garden plants. Many species in the family, especially of the African genera, have highly unusual (for plants) and otherworldly appearances, often with interesting textures (fuzzy, hairy, spiky, scaly) or curious nodules or growths, all of which are typically the result of environmental adaptations, such as "fuzzy" Kalanchoe tomentosa utilizing its hairs as a sunscreen. Apart from some sensitive species, most are quite hardy, typically needing only minimal care. For every alien-looking species, there are just as many others with a more "conventional", rosette growth habit, something reflected in many common names, such as 'Black Rose', another name for the common Aeonium arboreum var. 'Swartzkopf'.
Well-known genera and species include the many forms of Crassula ovata ('Jade', 'Money Plant' or 'Friendship Tree'), Kalanchoe blossfeldiana (florists' or supermarket-kalanchoe); Cotyledon, such as 'Chalk Fingers' and 'Pig's Ear', Sempervivum such as cobweb houseleek (or hen-and-chicks) and S. calcareum, and Aeonium such as A. haworthii (and its popular variegate 'Kiwi'), A. arboreum, A. canariense, A. urbicum; Monanthes, Umbilicus (pennywort), Bryophyllum, Echeveria, Sedum and Dudleya.
Description
General: Crassulaceae is a monophyletic family of morphologically diverse terrestrial perennial, rarely annual or hapaxanthic (flowering once in a lifetime), flowering plants that demonstrate xerophytic adaptations, with thick succulent leaves, a thick waxy cuticle and Crassulacean acid metabolism. Crassulaceae are generally herbaceous but there are some subshrubs, and relatively few treelike, epiphytic (growing on surface of plants), scandent (vine like) or aquatic plants. Most species are herbaceous leaf succulents, with regular 5 part (pentamerous or fivemerous) flowers, isomerous free carpels and one or two whorls of stamens.
Vegetative: stems are sometimes succulent, as may also be the underground caudices (rootstock), and may form rhizomes or corms. Bulbils may form along the stem or leaf margins. The leaf arrangement is opposite and decussate or alternate and spiral, and they are frequently aggregated into rosettes. The leaf shape is simple (rarely pinnate) and usually entire, or crenate to broadly lobed, sometimes dentate or more deeply incised, glabrous (smooth) or tomentose. In cross section the leaf blades are flat or round. They may be sessile or petiolate. Stipules are absent. New plants often form easily from vegetative parts that fall off the parent plant.
Reproductive: The inflorescence is usually terminal to lateral with many-flowered thyrses of cymes, less commonly spikes, racemes or panicles, rarely few to single flowered and axillary. The inflorescence is often many-branched and bracteate. The flower clusters are red, yellow, or white.
The flowers are often apopetalous (separate corolla segments), pentamerous (five-parted), actinomorphic (radially symmetrical), except for the zygomorphic Tylecodon grandiflorus, with one to two whorls of 4–20 sepals that are usually as many as or twice as many as the number of petals and two whorls of stamens, five in each whorl (i.e. as many as or twice the number of petals), with their filaments either free or fused to the petals at the base and sometimes unequal. Anthers are basifixed and open lengthwise. The flowers are bisexual, less commonly unisexual (more or less dioecious). Ovaries superior to partially inferior, with carpels equal to the number of petals, each forming a single locule, superior, free or almost so, basally with a small to conspicuous basal nectary scale, gradually tapering to a short to long style with few to many ovules. The fruit is usually capsular with dehiscent follicles, opening along the carpal suture and many seeded. The seeds are small (1.5–3 mm), smooth, elongate, papillate to longitudinally ridged, and generally brownish.
However, a number of genera (e.g. Sempervivum, Aeonium) are polymerous (3-32), have basally fused or partially fused corolla segments, where the petals may form a corolla tube of varying length (e.g. Kalanchoe, Cotyledon), or have only a single whorl of 5 stamens (e.g. Crassula, Tillaea), while Sedum includes much of the morphological diversity within the family as a whole. Although the typical number of floral parts is four or five, a number of genera, such as Sempervivum and Jovibarba, demonstrate polymery (at least ten or greater parts).
Genome
The genome of Kalanchoë fedtschenkoi has been sequenced as a model to study the evolution of Crassulacean acid metabolism (CAM).
Chromosome numbers are highly variable. The original base chromosome number is x=8, decreasing to 7 in Crassula. In Sedoideae, the base number increases to 9 in the Kalanchoe clade, but Kalanchoe have x=17 or 18 (or a multiple), and is probably of polyploid origin, derived from a tetraploid Cotyledon like taxon. In the Telephium sensu Hart clade the base number has increased to 12 and higher. Of the subclades within Telephium, the first (Hylotelephium sensu Thiede & Eggli: Hylotelphium, Orostachys and Sinocrassula) has x=12, and of the Rhodiola clade Phedimus has x=16 and Umbilcus x=24, representing another episode of polyploidy. Within Sempervivum, Sedum series Rupestre (Petrosedum) has x=28. Within the Leucosedum, most taxa are diploid, with 2x=12, 14 but includes two subclades, one with x=6 or 7, the other x=6, but a few have 14 or 16. Acre includes a wide variety of chromosome numbers from x=6 to 270, and occasionally larger and displays widespread polyploidy. Aeonium includes some Sedum species with x=8, while the remaining taxa are x=18. The core of this clade is probably polyploid from an ancestor with x=8.
Phytochemistry
Crassulacean acid metabolism (CAM photosynthesis) is named after the family, because the pathway was first discovered in crassulacean plants. It is one of the few families that still has CAM as an active, photosynthetic pathway, and is unique in which all its members are known to possess CAM.
Taxonomy
Originally described by Saint-Hilaire (1805) as Crassuleae, and therefore has his name as the botanical authority. Authority has also, at times, been given to De Candolle (DC), who first used the term "Crassulaceae" in 1815. He later placed the family among the Dicotyledons.
One of the most complete treatments was Alwin Berger's revision in 1930. At that time the family comprised about 1,500 species, distributed over six subfamilies and 33 genera. Circumscription of the family has remained relatively stable, with the exception of the placement of the genus Penthorum and Tetracarpaea, which has at times been placed either in their own monogeneric family, Penthoraceae and Tetracarpaeaceae, or in the Saxifragaceae. When Penthorum and Tetracarpaea were separated from Crassulaceae, they became a natural monophyletic group. Some later authors, such as Cronquist, included only 900 species. Thiede and Eggli (2007), in their treatment of the family, describe 34 genera with about 1,410 species. The size of the genera varies considerably, from Sedum, the largest with 300–500 species, to the smallest, which are monotypic. Estimates of the number of species has varied between 1500 (Berger 1930) and 900 (Cronquist 1981).
Molecular phylogenetics has shown that morphological characters and chromosome numbers are so labile in the family, with rampant polyploidy and aneuploidy, that they cannot be used reliably to infer evolution, even at low taxonomic levels, with few exceptions. For instance Prometheum and Rosularia have been segregated from Sedum by their basic chromosome numbers.
Crassulaceae is a medium size monophyletic grouping within the core eudicots. Originally considered a primitive member of the Rosidae, in the order Saxifragales, it is now placed, with that order as a superrosid under the classification system of the Angiosperm Phylogeny Group. There, the Saxifragales are a sister group to the rosids. Classification within the family is difficult and complex because many of the species hybridize readily, both in the wild and in cultivation, and the family is morphologically, cytologically and geographically diverse. As a result, generic boundaries have been considered unclear with frequent intergradation of characteristics between taxa, which may represent recurrent adaptation to xeric habitats.
Genera 39 genera are accepted.
Adromischus Aeonium Afrovivella Aichryson Chaloupkaea Chazaroa Chiastophyllum Cotyledon Crassula Cremnophila Dudleya Echeveria Graptopetalum Hylotelephium Hypagophytum Jeronimoa Kalanchoe Kungia Lenophyllum Meterostachys Monanthes Orostachys Pachyphytum Perrierosedum Petrosedum Phedimus Pistorinia Prometheum Pseudosedum Quetzalcoatlia Rhodiola Rosularia Sedum Sempervivum Sinocrassula Thompsonella Tylecodon Umbilicus Villadia
Phylogeny
Crassulaceae has been considered a part of the order Saxifragales by most modern authors, including Cronquist (1981), Takhtajan (1987), and Thorne (1992), based on phenotypic features, but subsequently confirmed by molecular methods. The place of Crassulaceae within Saxifragales has varied over time, as molecular data accumulates. The number of families within Saxifragales varies depending on the delimitation of individual families. Here, 14 families are shown in a cladogram, according to the Angiosperm Phylogeny Website, situating Crassulaceae as sister to the Haloragaceae sensu lato, and thus forming one of two subclades of the core Saxifragales.
Biogeography and evolution