Dudleya, commonly known as liveforevers, is a genus of rosette-forming succulent plants in the stonecrop family, Crassulaceae, consisting of about ~70 taxa found in southwestern North America and Guadalupe Island. The genus is very morphologically diverse, with species ranging from large evergreen plants to tiny deciduous geophytic plants. The flowers of Dudleya have parts numbered in fives, with the petals arranged in tubular, star-shaped, or bell-shaped forms and, when fruiting, are filled with tiny, crescent-shaped seeds.
The genus evolved as neoendemics, from ancestors in the stonecrop genus, Sedum. The ancestors probably radiated southward from Sedum during the creation of the dry summer climate, in the California region, five million years ago. Early botanists classified the larger species as Echeveria and Cotyledon, while the geophytic species were placed under Sedum. Taxonomic efforts, started by Joseph Nelson Rose and Nathaniel Lord Britton, created three genera, including Dudleya. These genera were all eventually subsumed into Dudleya proper with the work of Reid Moran. Phylogenetic research is still at an early stage in the genus, and is complicated by the fact that many species are becoming endangered and over-harvested (poached).
Dudleya is a relatively obscure genus, in comparison to other, more widely cultivated succulents; converging interests, by succulent collectors, native plant enthusiasts and gardeners alike, have led to the wider cultivation of many species as ornamental plants. In the wild, many species of Dudleya are vulnerable, as land development and poachers often threaten particularly niche populations of plants. Poached plants are often shipped to East Asia, especially South Korea, where hybridisation and cultivation of succulents is very popular. Conservationists, nurseries and governments combat Dudleya poaching through propagation programs and protection laws.
Description
Characteristics and subgenera This genus is the only taxon of the Sedeae tribe to have evolved sympodial branching.
The genus is traditionally divided into three subgenera, two of which were formerly their own genera. The subgenera consist of Dudleya, Stylophyllum, and Hasseanthus. +SubgeneraDudleyaStylophyllumHasseanthusThe subgenus Dudleya, or the Eududleya, is characterized by broad, flattened leaves, and tight petals on the flower that form a tube (connation). When the flowers go to fruit, they retain the tight and tubular character. The most recognizable plants of this first group would be the chalk dudleya and giant chalk dudleya.Formerly segregated as the genus Stylophyllum, the plants of this group usually have narrow leaves, often elliptic to round in cross-section, resembling fingers; hence common names like fingertips (Dudleya edulis, also the type species for this subgenus). The flowers have the petals spreading at the middle, and are not arranged connately like in the subgenus Dudleya. However, some plants in this subgenera have flat-leaves, like Dudleya traskiae, and some have campanulate flowers that appear intermediate between Dudleya and Stylophyllum, like Dudleya campanulata. The flowers and fruit of the Stylophyllum are more spreading than in the subgenus Dudleya, but still remain tight towards the base.Formerly known as the genus Hasseanthus, this grouping is typically distinguished by underground corm-like stems, with small, inconspicuous leaves that usually disappear before flowering, and widely spreading flowers and fruit. The spreading flowers, which resemble those of Sedum, may be a plesiomorphic feature. The seeds are among the largest in the genus. The type species is Hasseanthus variegatus, which is a synonym for Dudleya variegata. Perhaps the most widely known members of the Hasseanthus group are the critically endangered short-leaved dudleya, and Hendrix's dudleya, famous for its name.
Epicuticular wax
One of the most famous features of some Dudleya is their waxy coating. Numerous species have their foliage covered in an epicuticular wax, or farina. This waxy coating is usually white, chalky, and mealy, and is the namesake in many epithets, like chalk dudleya, powdery liveforever, and giant chalk dudleya. The wax on the giant chalk dudleya is a surface with some of the highest ultraviolet reflectivity ever discovered in plants. When exposed to water, the wax may coat drops on leaves, preventing their evaporation.Spectral Properties of Heavily Glaucous and Non-Glaucous Leaves of a Succulent Rosette-Plant, Thomas W. Mulroy, Oecologia, 1979, The farina is mostly composed of pentacyclic triterpenoids.Manheim Jr, B. S., & Mulroy, T. W. (1978). Triterpenoids in epicuticular waxes of Dudleya species. Phytochemistry, 17(10), 1799-1800.
Other members, such as D. viscida and D. anomala, have a clear, sticky layer on their foliage.Dudleya anomala (Davidson) Moran. Desert Plant Life. 14: 191. 1942. This adhesive layer ultimately helps the plant protect its leaves from the sun, much like ones with "white" wax. It accomplishes this solar protection by allowing dust to adhere to the sticky layer on the leaves, which coats the foliage with dust and debris, blocking solar radiation.
Taxonomy Taxonomic history
Early history The earliest species of the genus described was Cotyledon caespitosa, by Adrian Hardy Haworth in 1803, which would later be known as Dudleya caespitosa. The same species would later be described again as Sedum cotyledon by Joseph Franz von Jacquin in 1811, and in 1840 Thomas Nuttall described both Echeveria pulverulenta (Dudleya pulverulenta) and Echeveria lanceolata (Dudleya lanceolata). The differing generic placements of these early descriptions suggest that taxonomic disagreements over the genus had an early start, with some of these generic placements persisting even after the proper description of the genus by Britton and Rose.
Nathaniel Lord Britton and Joseph Nelson Rose's revision of the North American Crassulaceae upended many of the early taxonomic classifications, with the newly minted genus Dudleya containing 60 species, of which 41 were newly described by Britton and Rose. The two also defined the related genera Stylophyllum, which contained 12 species, and Hasseanthus, with 4 species. The primary differences between Dudleya and Stylophyllum were between the leaf shape, floral structure and petal orientation, whilst Hasseanthus was characterized by a different vegetative structure and more niche adaptations, primarily corm-like stems and deciduous leaves.Uhl, C. H., & Moran, R. (1953). The cytotaxonomy of Dudleya and Hasseanthus. American Journal of Botany, 492-502.
In the 1930s, Alwin Berger revised the status of Dudleya and Stylophyllum into a sectional ranking within Echeveria, while merging the Hasseanthus genera into Sedum. This was in part due to a belief that Dudleya had evolved from Echeveria radiating northward from Mexico, evidenced by the similar tubular corollas, while Hasseanthus possessed aesthetic similarities to Sedum, primarily flowers with broadly spreading petals. Stylophyllum was believed to occupy a transitory position between Dudleya and Hasseanthus. These conclusions were widely accepted by botanists for the first half of the 20th century, until proper molecular and phylogenetic analyses began to appear.
Reid Moran, Charles H. Uhl and early phylogenetics In 1942, Reid Moran, a botanist with a longstanding interest in the genera, offered a new revision of the taxa. Moran recognized Dudleya as being distinct from Echeveria, and merged Stylophyllum as a subgenera along with Eududleya (which was changed to just subgenus Dudleya). Moran still recognized Hasseanthus as a separate genus, but realized that it was closely related to Dudleya. With assistance from cytologist Charles H. Uhl, Moran came to the conclusion that Dudleya and Hasseanthus were related closer to each other than they were to Echeveria or Sedum.
As Moran and Uhl conducted more cytological and taxonomic research on the genus, it became clear that Hasseanthus was also a subgenera of Dudleya, citing the karyological uniformity and the formation of hybrids between the genera. In contrast, while Echeveria and Sedum freely hybridize within themselves and each other, all attempts at hybridizing Dudleya with Echeveria have failed. Uhl came to the conclusion that there are probably no intergeneric hybrids with Dudleya, believing that Dudleya had split from Echeveria far enough that intergeneric reproductive success was unlikely. Despite Uhl's research, he still believed that Dudleya had evolved northward from Mexico with Echeveria, probably some time before the formation of the summer-dry climate of California, making Dudleya a paleoendemic.
Modern classifications and phylogenetic research Around 1993, one of the only literary treatments of the genus was written, Paul H. Thomson's Dudleya and Hasseanthus Handbook. In the book, Thompson made numerous changes and adjustments to species, and described several new species. The book included a large number of photographs, extensive descriptions, and horticultural practices for the care of Dudleya, derived from a long interest growing, visiting and collecting the plants. However, he failed to follow the International Code of Botanical Nomenclature guidelines for describing new taxa, forgoing designated type specimens, which invalidated his treatment. His treatment also contains extensive pseudoscientific material, including references to the lost continents of Atlantis, Lemuria and Mu, a belief that cosmic rays and divine intervention directly induced the chromosome counts and evolution of Hasseanthus and Dudleya, and criticism of paleontologists and the scientifically accepted age of the Earth.
Later research challenged the notion that Dudleya and Echeveria formed two distinct lineages within Echeverioideae. Joachim Thiede instead proposed that Dudleya evolved completely independently from Echeveria, evolving from relatives within the Leucosedum clade like Sedum moranii. This places Dudleya as a neoendemic that evolved recently with the formation of the summer-dry climate in California. This was evidenced by the similarities of seed-surface ornamentation between Dudleya and Sedum, which is unlike Echeveria, and the reproductive isolation between Echeveria and Dudleya.Thiede, J., & Eggli, U. (2007). Crassulaceae. In Flowering Plants· Eudicots (pp. 83-118). Springer, Berlin, Heidelberg.
ChicagoHart, H. T., & Eggli, U. (1995). Evolution and systematics of the Crassulaceae. Backhuys.
Research in the 21st century has supported the hypothesis of Dudleya's relation to Sedum. Phylogenetic analysis has given weight to this hypothesis, concluding that Dudleya is much closer to North American members of Sedoideae than to Echeverioideae. The plant Sedum spathulifolium, which co-exists with Dudleya in Oregon and California, has been shown in multiple phylogenetic reconstructions to be a sister taxon to Dudleya, although it may not be the true sister taxon, as larger samplings are needed. Berger's hypothesis of a relationship between Hasseanthus and Sedum may still prove true, as in addition to morphological similarities, the Hasseanthus-type taxa could be basal to the genus. However, no truly conclusive evidence of significant divergence within Dudleya exists, and it remains unclear which groups are basal or derived.
Selected species
Dudleya abramsii Dudleya abramsii subsp. abramsii – Abrams' liveforever Dudleya abramsii subsp. affinis – San Bernardino Mountains liveforever Dudleya abramsii subsp. bettinae – Betty's dudleya Dudleya abramsii subsp. calcicola – limestone dudleya Dudleya abramsii subsp. murina – Mouse-gray dudleya Dudleya abramsii subsp. setchellii – Santa Clara Valley dudleya Dudleya acuminata – Vizcaino liveforever Dudleya albiflora– White-flower liveforever Dudleya anomala – Todos Santos liveforever Dudleya anthonyi – Anthony's liveforever