By Marion Blois Lobstein
Botany Chair, Prince William Wildflower Society (Article adapted from articles published in PWWS’s Wild News); Professor Emeritus, Northern Virginia Community College
Most of the 200 species ofAsclepias are native to the New World. The history of taxonomy or scientific names and classification of this interesting group of plants is complicated and convoluted. Dioscorides, the Greek physician who wrote hisMateria Medica in the first century A.D., first used Asclepias,but he was describing a plant other than milkweed, which does not grow in Europe. Possibly he was describing European dogbane (Vincetoxicum hirundinarie) which is an Apocynum species. True milkweed orAsclepias species were first collected and sent to Europe by French and English explorers in the 1500s and 1600s. In 1585, John White, the English artist who was part of the ill-fated Roanoke Lost Colony, illustrated Asclepias syriaca. The herbalist Gerarde included a description and illustration of Indian swallowwort that may have been a dogbane, but by the 1633 the illustration was that of Asclepias syriaca that was also call Apocynum syriacum. Around 1620, Louis Hebert, a French colonist and pharmacist in New France (Eastern Canada) sent seeds ofAsclepias syriacum to Paris to investigate its medicinal properties. By 1635, Philip Cornut, a doctor and botanist in New France, described both Asclepias syriaca and Asclepias incarnada in Canadensium plantarum historia. He used the genus Apocynum for both species, but was confused about the identification Asclepias syriaca, and used the name syriacum referring to a dogbane for the Mid-East. Linnaeus in 1753 used the genusAsclepias and still used the species epitaph syriaca. Linnaeus is thought to have used a specimen of this species collected by John Clayton and sent to Gronovius, who shared the specimen with Linnaeus. In the Flora Virginica (2nd ed.,1762), the use of both Asclepiasand Apocynum epitaphs seem to be linked closely together.
Since many of the characteristics of Apocynum species are similar to those of Asclepiasspecies, it is easy to see why the two genera were often confused, and the two were in the same family by the end of the 1700s. Species of both genera usually have milky sap with latex and opposite leaves (Butterfly-weed [Asclepias tuberosa] is an exception to both of these characters) as well as similar flower structure in five sepals, five petals, two carpels fused at the top by with the two ovaries free, and similar fruits-follicles or pods. Only the Asclepias species (as well as other genera in formerly in the Asclepidaceae), however, have the stigma of the pistil and stamens forming the gynostegium—described in the general article on butterfly-weed and other milkweeds in our area—and the pollen in pollinia. There are gradations of the pollina formation in some members of the former narrowly defined Apocynaceae.
Michel Adanson in 1763 proposed “Apocyna” as a family that included Apocynum and Asclepias. The accepted family name of Apocynaceae was based on A.L. de Jussieu’s “Apocineae” in 1789. In 1810, however, Robert Brown split Apocynaceae into two families, Asclepiadaceae and Apocynaceae, based on whether or not the pollen is packaged in pollina (only in Asclepiadaceae). During the 1800s, there were various treatments of these two families. In the 1990s and 2000s, the molecular investigation of the DNA of species in these two families has led to combining the two families once again into the Apocynaceae. Members of the former Asclepidaceae are now in the subfamily Asclepiadoideae. It seems molecular data has brought us full circle in re-combining these two families into Apocynaceae.
Some interesting websites dealing with taxonomic change in Apocynaceae and former Asclepidaveae:
“Apocynaceae: Brown and now,” by Mary Endress, Royal Botanic Gardens & Domain Trust
“The history and use of milkweed (Asclepias Syriaca L.),” by Erika E. Gaertner, inEconomic Botany (April/June 1979: 33, no. 2): pp 119-123.
“Nomenclature and Iconography of Common Milkweed,” by Jules Janick and Winthrop B. Phippen, in Chronica Horticulturae, 53, no. 2 (2013)
[References: “The history and use of milkweed (Asclepias Syriaca L.),” by Erika E. Gaertner, in Economic Botany 33, no. 2 (April/June 1979), 119-123; “Nomenclature and Iconography of Common Milkweed,” by Jules Janick and Winthrop B. Phippen, in Chronica Horticulturae, 53, no. 2 (2013).]
Selected glossary of botanical terms
[Source: Alan S. Weakley, J. Christopher Ludwig, and John F. Townsend. Bland Crowder, ed. Flora of Virginia. (Fort Worth: BRIT Press, 2012)]
Androecium Collectively, the stamens of a flower.
Anther The expanded, apical, pollen-bearing portion of the stamen, comprising one or, usually, two pollen sacs and a connecting layer.
Calyx (plural calyxes) The outer whorl of the perianth; collectively, all the sepals of a flower.
Carpel A unit of the gynoecium with a simple pistil formed from on emodified leaf, or that part of a compound pistil formed from one modified leaf; megasporophyll.
Corolla Collectively, all the petals of a flower, whether distinct or connate; the inner whorl of a perianth.
Corona A set of petal-like or crown-like structures between the corolla and the androecium in some flowers, derived by modification of the corolla or androecium.
Gynostemium A compound structure resulting from the union of stamens and pistil.
Perianth Collectively, the calyx and corolla of a flower, especially when they are similar in appearance.
Pollinium (plural pollinia) I n many Orchidaceae and Asclepiadaceae, a coherent cluster of many waxy pollen grains, transported as a unit during pollination.
Sepal A segment of a calyx.
Stigma The part of a pistil adapted for the reception of pollen.
[Images: Milkweed flower structure accessed at “A NeoTropical Savanna,” by Mary Farmer, http://ntsavanna.com/the-tropical-milkweed/; Asclepias syriaca, Deanna LaValle High]