Dahlia
Dahlia is a genus of bushy, tuberous, herbaceous perennial plants native to Mexico and Central America. Dahlias are members of the Asteraceae family of dicotyledonous plants, its relatives include the sunflower, daisy, chrysanthemum, and zinnia. There are 49 species of dahlia, with flowers in almost every hue, with hybrids commonly grown as garden plants.
Dahlias were known only to the Aztecs and other southern North American peoples until the Spanish conquest, after which the plants were brought to Europe. The tubers of some varieties are of medicinal and dietary value to humans because, in common with species of Inula and many other flowering plants, they use inulin, a polymer of the fruit sugar fructose, instead of starch as a storage polysaccharide.
Description
Dahlias are perennial plants with tuberous roots. They are not frost hardy, and require protection from frost if grown in regions with cold winters. While some have herbaceous stems, others have stems which lignify in the absence of secondary tissue and resprout following winter dormancy, allowing further seasons of growth. As members of the family Asteraceae, dahlias have composite flower heads called capitula that are composed of multiple florets arranged in a central disc with surrounding petal-like rays. Each floret is a flower in its own right. The modern name Asteraceae comes from the type genus Aster and the Ancient Greek word for "star", referring to the appearance of a star with surrounding rays.The stems are leafy, ranging in height from as low as to more than. Flower forms are variable, with one head per stem; these can be as small as in diameter or up to . The majority of species do not produce scented flowers. Like most plants that do not attract pollinating insects through scent, they are brightly colored, displaying most hues, with the exception of blue.
The great variety in species results from garden dahlias being octoploids, having eight sets of homologous chromosomes. In addition, dahlias also contain many transposons—genetic pieces that move from place to place upon an allele—which contributes to their manifesting such great diversity.
Taxonomy
Taxonomic history
Early history
Spaniards reported finding the plants growing in Mexico in 1525, but the earliest known description is by Francisco Hernández, physician to Philip II, who was ordered to visit Mexico in 1570 to study the "natural products of that country". They were used as a source of food by the indigenous peoples, who both gathered wild specimens and cultivated crops. The indigenous peoples variously identified the plants as "Chichipatl" and "Acocotle" or "Cocoxochitl". From Hernández's perception of Nahuatl to Spanish the word is "water cane", "water pipe", "water pipe flower", "hollow stem flower", or "cane flower", all referring to the hollow plant stems.Hernandez described two varieties of dahlias as well as other medicinal plants of New Spain. Francisco Domínguez, an Hidalgo gentleman who accompanied Hernandez on part of his seven-year study, made a series of drawings to supplement the four volume report. Three of his drawings showed plants with flowers: two resembled the modern bedding dahlia, and one resembled the species Dahlia merckii; all displayed a high degree of doubleness. In 1578, a manuscript titled Nova Plantarum, Animalium et Mineralium Mexicanorum Historia, was sent back to the Escorial in Madrid. It was translated into Latin by Francisco Ximenes in 1615. In 1640, Francisco Cesi, President of the Academia dei Lincei of Rome, bought the Ximenes translation and, after annotating it, published it in 1649–1651 as two volumes, Rerum Medicarum Novae Hispaniae Thesaurus Seu Nova Plantarium, Animalium et Mineralium Mexicanorum Historia. The original manuscripts were destroyed in a fire in the mid-1600s.
European introduction
In 1787, the French botanist Nicolas-Joseph Thiéry de Menonville, sent to Mexico to steal the cochineal insect valued for its scarlet dye, reported the strangely beautiful flowers he had seen growing in a garden in Oaxaca. In 1789, Vicente Cervantes, director of the Botanical Garden at Mexico City, sent "plant parts" to Abbe Antonio José Cavanilles, director of the Royal Gardens of Madrid. Cavanilles flowered one plant that same year, then the second one a year later. In 1791 he called the new growths "Dahlia" for Anders Dahl. The first plant was called Dahlia pinnata after its pinnate foliage; the second, Dahlia rosea for its rose-purple color. In 1796, from the parts sent by Cervantes, Cavanilles flowered a third plant, which he named Dahlia coccinea for its scarlet color.In 1798, Cavanilles sent D. pinnata seeds to Parma, Italy. That year, the Marchioness of Bute, wife of the Earl of Bute, the English Ambassador to Spain, obtained a few seeds from Cavanilles and sent them to Kew Gardens, where they flowered but were lost after two to three years.
In the following years Madrid sent seeds to Berlin and Dresden in Germany, and to Turin and Thiene in Italy. In 1802, Cavanilles sent tubers of "these three" to Swiss botanist Augustin Pyramus de Candolle at University of Montpelier in France, Andre Thouin at the Jardin des Plantes in Paris and Scottish botanist William Aiton at Kew Gardens. That same year, John Fraser, English nurseryman and later botanical collector to the Czar of Russia, brought D. coccinea seeds from Paris to the Apothecaries Gardens in England, where they flowered in his greenhouse a year later, providing Botanical Magazine with an illustration.
In 1804, a new species, Dahlia sambucifolia, was successfully grown at Holland House, Kensington. Whilst in Madrid in 1804, Lady Holland was given either dahlia seeds or tubers by Cavanilles. She sent them back to England, to Lord Holland's librarian at Holland House, who successfully raised the plants and produced two double flowers a year later. The plants raised in 1804 did not survive; new stock was brought from France in 1815. In 1824, Lord Holland sent his wife a note containing the following verse:
The dahlia you brought to our isle
Your praises for ever shall speak;
Mid gardens as sweet as your smile,
And in colour as bright as your cheek.
In 1805, German naturalist Alexander von Humboldt sent more seeds from Mexico to Aiton in England, Thouin in Paris, and Christoph Friedrich Otto, director of the Berlin Botanical Garden. More significantly, he sent seeds to botanist Carl Ludwig Willdenow in Germany. Willdenow now reclassified the rapidly growing number of species, changing the genus from Dahlia to Georgina; after naturalist Johann Gottlieb Georgi. He combined the Cavanilles species D. pinnata and D. rosea under the name of Georgina variabilis; D. coccinea was still held to be a separate species, which he renamed Georgina coccinea.
Classification
Since 1789 when Cavanilles first flowered the dahlia in Europe, there has been an ongoing effort by many growers, botanists and taxonomists, to determine the development of the dahlia to modern times. At least 85 species have been reported: approximately 25 of these were first reported from the wild; the remainder appeared in gardens in Europe. They were considered hybrids, the results of crossing between previously reported species, or developed from the seeds sent by Humboldt from Mexico in 1805, or perhaps from some other undocumented seeds that had found their way to Europe. Several of these were soon discovered to be identical with earlier reported species, but the greatest number are new varieties. Morphological variation is highly pronounced in the dahlia. William John Cooper Lawrence, who hybridized hundreds of families of dahlias in the 1920s, stated: "I have not yet seen any two plants in the families I have raised which were not to be distinguished one from the other. Constant reclassification of the 85 reported species has resulted in a considerably smaller number of distinct species, as there is a great deal of disagreement today between systematists over classification.In 1829, all species growing in Europe were reclassified under an all-encompassing name of D. variabilis, Desf., though this is not an accepted name. Through the interspecies cross of the Humboldt seeds and the Cavanilles species, 22 new species were reported by that year, all of which had been classified in different ways by several different taxonomists, creating considerable confusion as to which species was which. As of now Dahlias are classified into 15 different species by botanist Liberty Hyde Bailey.
In 1830 William Smith suggested that all dahlia species could be divided into two groups for color, red-tinged and purple-tinged. In investigating this idea Lawrence determined that with the exception of D. variabilis, all dahlia species may be assigned to one of two groups for flower-colour: Group I or Group II.
Modern classification
The genus Dahlia is situated in the Asteroideae subfamily of the Asteraceae, in the Coreopsideae tribe. Within that tribe it is the second largest genus, after Coreopsis, and appears as a well defined clade within the Coreopsideae.Subdivision
Infrageneric subdivision
Sherff, in the first modern taxonomy described three sections for the 18 species he recognised, Pseudodendron, Epiphytum and Dahlia. By 1969 Sørensen recognised 29 species and four sections by splitting off Entemophyllon from section Dahlia. By contrast Giannasi using a phytochemical analysis based on flavonoids, reduced the genus to just two sections, Entemophyllon and Dahlia, the latter having three subsections, Pseudodendron, Dahlia, and Merckii. Sørensen then issued a further revision in 1980, incorporating subsection Merckii in his original section Dahlia.When he described two new species in the 1980s, he placed them within his existing sections. A further species, Dahlia sorensenii was added by Hansen and Hjerting in.
At the same time they demonstrated that Dahlia pinnata should more properly be designated D. x pinnata. D. x pinnata was shown to actually be a variant of D. sorensenii that had acquired hybrid qualities before it was introduced to Europe in the sixteenth century and formally named by Cavanilles. The original wild D. pinnata is presumed extinct. Further species continue to be described, Saar describing 35 species. However separation of the sections on morphological, cytological, and biochemical criteria has not been entirely satisfactory.
To date these sectional divisions have not been fully supported phylogenetically, which demonstrate only section Entemophyllon as a distinct sectional clade. The other major grouping is the core Dahlia clade, which includes most of the section Dahlia. The remainder of the species occupy what has been described as the variable root clade which includes the small section Pseudodendron but also the monotypic section Epiphytum and a number of species from within section Dahlia. Outside of these three clades lie D. tubulata and D. merckii as a polytomy.
Horticulturally the sections retain some usage, section Pseudodendron being referred to as 'Tree Dahlias', Epiphytum as the 'Vine Dahlia'. The remaining two herbaceous sections being distinguished by their pinnules, opposing or alternating.