Family of flowering plants
The Crassulaceae (, from Latin "crassus", thick), also known as the stonecrop family or the orpine family, are a diverse family of dicotyledon flowering plants characterized by succulent leaves and a form of photosynthesis, known as Crassulacean acid metabolism (CAM). Flowers generally have five floral parts. Crassulaceae are usually herbaceous but there are some subshrubs, and relatively few treelike or aquatic plants. Crassulaceae are a medium-sized monophyletic family in the core eudicots, among the order Saxifragales, whose diversity has made infrafamilial classification very difficult. The family includes approximately 1,400 species and 34–35 genera, depending on the circumscription of the genus "Sedum", and distributed over three subfamilies. Members of the Crassulaceae are found worldwide, but mostly in the Northern Hemisphere and southern Africa, typically in dry and/or cold areas where water may be scarce, although a few are aquatic.
Crassulaceae are mainly perennial and have huge economic importance, internationally, as collectible, indoor and garden plants. Many species in the family have a bizarre, alien and intriguing appearance, with interesting textures, growths or even "furry" coverings, and are quite hardy, typically needing only minimal care. Still, many others have a more typical, "conventional" rosette form, something reflected in many common names, such as 'black rose' often referring to "Aeonium arboreum" var. 'Swartzkopf' or var. 'Merlot'. Well-known genera and species include the many forms of "Crassula ovata" ('jade plant', '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, canariense, urbicum"; "Monanthes", "Umbilicus" (pennywort), "Bryophyllum", "Echeveria", "Sedum" and "Dudleya".
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Description.
General: Crassulaceae is a 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 &amp; 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.
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.
Crassulaceae evolved approximately 100–60 million years ago in southern Africa with the two most basal phylogenetic branches (Crassula, Kalanchoe) representing the predominantly southern African members. Other sources suggest that Crassulaceae evolved approximately 70 million years ago together with Haloragaceae "sensu lato" (Penthoraceae, Haloragaceae). The family is considered to have had a gradual evolution, with a basal split between Crassuloideae and the rest of the family (Kalanchoideae, Sempervivoideae). The Sempervivoideae subsequently dispersed north to the Mediterranean region, and from there to Eastern Europe and Asia (Sempervivum and Leucosedum clades), with multiple groups spreading over the three continents of the Northern Hemisphere. Two lineages from the European Crassulaceae eventually dispersed to North America and underwent subsequent diversification. The Aeonium clade dispersed from northern Africa to adjacent Macaronesia.
Distinct centers of speciation developed in Macaronesia (Aeonium clade), Mexico ("Sedum" and Echeverioideae in clade 7), and southeastern Asia ("Sedum sarmentosum", and "S. morissonensis" in Acre clade). On arrival in the Northern hemisphere the Sempervivoideae reached its greatest diversity. Conversely, few representatives of the Crassulaceae occur in South America and Australia. "Sedum" species are found in most of these regions, generally grouped with genera endemic to that region. For instance the North African "S. jaccardianum" and "S. modestum" (Aeonium) are a sister group to the endemic Macaronesian species in that clade.
The Macaronesian archipelago appears to have been reached by Crassulaceae at least three times. Once by the ancestor
of "Aeonium" and "Monanthes", most likely from the Western Mediterranean region, with the closest extant relatives of these two genera ("Sedum caeruleum", "S. pubescens"), coming from this region (Aeonium clade). The second migration was by an ancestor of a clade of three "Sedum" species ("S. nudum", "S. lancerotense" and "S. fusiforme" (Acre clade)), which appear to have originated in Mexico. The third occurrence likely involved the ancestor of a lineage within the genus "Umbilicus" (Rhodiola clade). The Macronesian flora include three genera from the Sempervivoideae, "Aeonium", "Aichryson" and "Monanthes" (Aeonium clade), together with several "Sedum" spp. and one species of "Umbilicus" (Rhodiola). North America was reached at least twice, once by an ancestor of "Parvisedum" and "Dudleya", and once by a subclade of Acre. For a mapping of morphological features and biogeography on the phylogenetic tree, see Fig. 3.
Chromosome numbers have played a limited role in elucidating evolution, but suggest a core of x=8, with subsequent polyploidy. For a mapping of chromosome numbers on the phylogenetic tree, see Fig. 4.
Subdivision.
History.
When Carl Linnaeus published his "Species Plantarum" in 1753 only a few genera, included in the modern circumscription of Crassulaceae were described; the type genus "Crassula" (10 species), "Tillaea" (3), "Cotyledon" (6), "Sempervivum" (6), "Rhodiola" (1) and "Sedum" (15). By 1777, "Rhodiola" had been submerged into "Sedum", only to be separated again in the twentieth century.
While the family can fairly easily be recognised, identifying its constituent genera has been far more problematic. For an extensive history of subfamily Sedoideae, see . Saint-Hilaire's original description in 1805 included seven genera, as did De Candolle (1815). In a much more extensive treatment in 1828, he divided the Crassulaceae into the two groups, Isostemonae and Diplostemonae (i.e. haplostemony vs. obdiplostemony) on the basis of the number of staminal whorls. The former corresponded to the modern Crassuloideae. 
Two lineages, six subfamilies, and 33 genera of Crassulaceae were described by Berger in 1930: 
Lineages, subfamilies, biogeography, No. genera, "type genus" (No. species in genus)
Each of these contained one of the largest genera. Though various revisions since have proposed simpler schemes, such as Borisova (1939, revised 1969). Berger's classification has proven practical and been the most widely used, although some of the subfamilies are