The farina is mostly composed of pentacyclic triterpenoids. Other members, such as D. viscida and D. anomala, have a clear, sticky layer on their foliage. 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. 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