The Lady Slipper newsletter, and now blog, of the Kentucky Native Plant Society has been published since the Society’s founding in 1986. We occasionally feature an article from a past issue. In this article from 2005, Robert Paratley, Curator, University of Kentucky Herbarium, takes an in-depth look at the dogbane family, Apocynaceae. This article ran in Vol. 20, No. 1. If you would like to see these and other past issues, visit the Lady Slipper Archives, where all issues from Vol. 1, February 1986 to Vol. 39, 2024, can be found.
The world of the dogbane family Apocynaceae
Robert Paratley
Curator, University of Kentucky Herbarium
Indian-hemp or dogbane, Apocynum cannabinum, is a tall, branching perennial found in most parts of Kentucky. It is very common in old fields, roadsides and other disturbed habitats, but is not particularly conspicuous in flower, with small greenish-white flowers whose petals are fused into a small urns hape. Indian-hemp is more conspicuous in fruit, where the two parts of the pistil separate at maturity and elongate into narrow, dark, long-pointed follicles (pods). These are filled with numerous light, tufted seeds adapted to catch the wind, making it an effective seed disperser. Break the plant and the sap is milky white. Another species, the spreading dogbane (Apocynum androsaemifolium), is similar but smaller than Indian-hemp and is widespread in northern U.S. It is listed as rare in Kentucky. Both species are on any serious list of poisonous plants. Numerous cases of livestock poisoning have been recorded, although apparently no human cases are known. (Turner and Szczawinski, 1991).
Both are members of the Apocynaceae, a large, mostly tropical family of mostly woody members. This article will highlight the features of this complex, diverse family that barely gets a toehold in the temperate zone. The Latin genus name Apocynum was coined by Linnaeus. He combined two Greek roots, apo meaning “away” or “off”, and cyn meaning “dog” – a dog repellent. The common name echoes this idea. (Perhaps dogs found the scent of the European dogbane Apocynum venetum unpleasant and kept away from it.) Apocynum became the namesake genus for the family Apocynaceae, whose name is credited to the French botanist Antoine-Laurent de Jussieu in the early 1800s. Until recently, the concept of the family remained fairly consistent, including about 200 genera and 2,000 species (Cronquist, 1981). Recent research has expanded the concept of the family to include the milkweeds, which have traditionally been taught as a distinct but closely related family. Following older fashion, I will not discuss milkweeds here.
Look up the Apocynaceae in the Gleason and Cronquist Manual of vascular plants of Northeastern U.S (1991), and you’ll discover just how poorly represented the family is in eastern U.S. Only four genera are included, covering less than two pages. Briefly rounding out the listing for our state, there is blue star, Amsonia tabernaemontana, a perennial herb common in moist woods and stream banks, particularly in western Kentucky. Clusters of pale blue flowers make it conspicuous in spring and a popular item in perennial gardens. The final native is the hardest to find— a rare climbing vine of wet woods and stream banks named Trachelospermum difforme, the climbing-dogbane. It is scattered in southern and western Kentucky and is mostly a plant of the southern Coastal Plain. Its small yellowish flowers appear in the summer.
A close look at flower structure and pollination in periwinkle
Probably the most familiar member of the dogbane family is the common garden periwinkle or myrtle, Vinca minor, an evergreen ground cover from Europe with handsome blue flowers in early spring. The creeping stems root and send up leafy shoots with one or two attractive blue flowers. It is most often seen in older gardens, and is sometimes discovered persisting for many years in overgrown old homesites, having naturalized in many places throughout our state. [Note: As of 2013, Vinca minor is classed as an invasive species in KY by the Kentucky Invasive Plant Council.] Less commonly seen is Vinca major, the greater periwinkle, a Eurasian cousin that is more robust in all respects. It rarely naturalizes.
Exploring the periwinkle in some detail gives us a chance to understand many characters of a typical member of the Apocynaceae. Vinca bears opposite, simple leaves with smooth margins. While Amsonia and a small minority of Apocynaceae bear alternate leaves, the vast majority of the family bears opposite leaves very much like Vinca. The large,showy and tubular Vinca flower is more typical of the Apocynaceae than the flower of Indian-hemp. Like Apocynum, the petals are fused. But here a narrow tube is formed for most of the length of the corolla. The lobes of the petals flare out, perpendicular to the tube, as a bold signal to pollinators. This shape is called salverform, a very common form in the family. The corolla lobes of periwinkle show a slight bend even when the flower is completely open, because they were tightly twisted around each other in the closed bud. This is consistently seen throughout the family. Some members possess little glands or outgrowths, called colleters, on parts of the flowers. (Other members have these on leaf bases or leaf stalks.)
A look inside the periwinkle flower reveals more about its construction. The narrow corolla tube is nearly blocked by a large stigma, the upper portion of the pistil. It often resembles an engine piston in shape and is composed of three distinct regions with different functions in pollination. The five stamens are attached to the inside of the narrow corolla tube by very short filaments. The longer anthers, the actual pollen producing organs, are in close proximity to the stigma. In many members of the family, the anthers are actually fused directly to the stigma. Pollination in the Apocynaceae is usually a specialized affair, and the stamens and pistil are modified to facilitate the transfer of pollen from one flower to another. Visiting insects come in search of nectar, offered at the base of the corolla tube by secretion of two glands. In periwinkle (and most Apocynaceae), the tube portion of the flower is fairly long as well as narrow. For an insect to successfully feed, it must have a lengthy feeding part. This long feeding tube or tongue is guided to the base of the flower through a very tight space between the anthers. When the insect finishes feeding, it withdraws, depositing any pollen it obtained visiting a previous flower at the base of the stigma, which is shaped like a thin inverted cone. It is called the scraper, and does just that to the pollen on the visitor’s mouth parts on its way up and out. The interior of the scraper— the lower region of the stigma— is the only area where pollen can germinate. As the insect withdraws further,it comes in contact with a midsection of the stigma that secretes a sticky substance. This adhesive comes into play as the insect withdraws to the top of the stigma. Pollen from the flower’s anthers has been deposited on the brushy surface here, sticking to the insect, which then carries away the new pollen to the next flower. Cross pollination is ensured. (The periwinkle will not self pollinate;it is self-sterile.) If successfully fertilized, periwinkle and most of the family produce fleshy fruit produce many-seeded follicles, often paired (Zomlefer,1994; de Wit, 1965).
A family of warm climates- examples from Southern gardens and conservatories
To experience the range of diversity in the family,we must explore the tropics and subtropics of the New World, Africa, and Asia. Its species range from tall, buttressed canopy trees in the rain forests of Malaysia and Indonesia, to cactus like succulents of dry southern Africa and Madagascar. A great many are shrubs or small trees in the understory of closed rainforests or in seasonally dry open areas. The Mediterranean and Southeast Asian coastal dunes are particularly rich in shrubby Apocynaceae. Central and South American rainforests harbor many lianas, woody vines that use canopy trees for support. West African rainforests also contain many lianas and small trees
But before heading for exotic parts, we can make quick a stop in our temperate gardens to find the rosy periwinkle, Catharanthus roseus, originally from Madagascar. The white, pink to rosy red flowers are quite similar to those of the periwinkle. A very popular temperate zone bedding or container annual, its real significance is as an important pharmaceutical plant (see below).
Most of us will not experience first hand a Javanese rainforest or Madagascar scrub, but we can travel to the Gulf States and lower Southeastern gardens to experience the beauty and diversity of the exotic Apocynaceae. California gardens and roadsides will also do nicely, as will the always-warm conservatories further north. Among the most popular and well-known outdoor shrubs in the South is the oleander, Nerium oleander. This Mediterranean shrub is also a popular houseplant in cooler climates, sporting pinkish flowers (varieties are white, yellow, or even deep red) showing the typical features of the family. The corolla lobes have a striking fringe growing out of the top of the tubular part of the flower. Oleander, though tender to frost, is a very tough plant, tolerant of both drought and salt. It is one of the woody plants of choice in sandy areas near the salt spray of the ocean. One often sees it in moveable tubs in areas with too long or cold a winter, away from the modifying influence of the Gulf or Atlantic. I noticed that it is a very popular highway beautification shrub in California, somehow enduring the scorching Central Valley summer heat. Oleander is notoriously poisonous, and can be a dangerous houseplant if accidentally ingested. There are oleander-related fatalities yearly, and it is said that even the smoke from its burning wood is poisonous (Riffle, 1998).
Also grown outdoors in the South are two species of East Asian climbers and ground covers of the genus Trachelospermum, the star-jasmines. The small white very fragrant flowers are produced in profusion in the spring.One sees star-jasmines as ground-covers, trellis and wall climbers, and as hedges. It will thrive near the Atlantic coast as far as Delaware (Dirr, 2002). Another genus with a “jasmine” common name is Tabernaemontana, the pinwheel jasmine and wild orange jasmine, fragrant-flowering shrubs growing in nearly frost-free areas in the coast. These shrubs are not related to the true jasmines, which are in the olive family(forsythias, lilacs, and privets).
As we head south into subtropical Florida, we find the most popular and widespread is frangipani, Plumeria rubra. Native to seasonally dry coastal hillsides of Central America and the West Indies, it was brought to southern Europe where it was introduced into cultivation under glass. The common name comes from Italian, referring to the fragrance of the flower oil distilled into perfume. It was soon adopted throughout the world. The plant drops its leaves in climates with a dry season, its few-branched habit looking barren until the rainy season returns. In areas of consistent rainfall it is evergreen and blooms periodically throughout the year. (Riffle, 1998). Called the Temple flower in Buddhist Asia, frangipani is chosen for its consistent blooming for planting around sacred sites. “Its singular ability to burst into leaf—and even into flower— when lifted from the ground not unreasonably suggests immortality, so it is frequently planted near graves.” (Perry, 1972). Like oleander, frangipani does well in the spray of salty ocean air.
Two commonly grown vines, Allamanda and Mandevilla,both hail from the South American tropics and bear large salverform flowers typical of the family. Allamanda, also called golden trumpet, is typically bright yellow, while Mandevilla (a large genus of often weedy vines, often confusingly called all amanda) is white, pink or red. Mandevilla splendens, the commonly grown species, is seen even here in Kentucky as a summer flowering potted plant that must be moved indoors before fall. These climbers too are notoriously toxic, as discovered by peoples in their native Brazil (see below).
Other important horticultural genera of southern gardens and streets are Alstonia, the scholar tree, an important timber and medicinal plant in India, and Carissa macrocarpa, the Natal plum, with edible berries. Edible fruit is rare in the Apocynaceae, but the dense spines of the Natal plum are not. Referring to its dense armament of spines, Kirsten Llamas (2004) claims “Clambering varieties can infiltrate tree canopies and become difficult to control. Be forewarned that climbing forms are used to fence out elephants in Africa.” Unfortunately, a number of these plants are invasive in Florida.
From Africa and the arid portions of Madagascar come a pair of horticultural curiosities: Adenium obesum, the desert-rose, and Pachypodium, the clubfoot. Both exhibit pachycauly— a short, thick and usually swollen trunk bearing whorls of leathery leaves. Unrelated to cacti, Pachypodium especially bears an uncanny resemblance to that mostly New World family. Many species have stems that are low, ground hugging mounds; some are trained into odd bonsai forms. Only when the large, salverform flowers with slightly twisted petals come out do Pachypodium and Adenium betray kinship to oleander and periwinkle.
Toxic plants, plant medicines, latex
It is the potent and complex chemistry of the family that makes plants like oleander so toxic, and has made several others so important as medicinal plants of great benefit. The Apocynaceae commonly produce two types of metabolites important to humans. Alkaloids, nitrogen based compounds that very often affect our nervous system, are widespread in the family. Their effects, usually disruptive, are varied depending on the particular alkaloids present. These include raising or lowering blood pressure or heart rate, sweating, or paralysis. Alkaloids can also alter mood, perception, stimulate or depress. Some alkaloids disrupt cell division and growth, and it is for this reason that two alkaloids isolated from the rosy periwinkle (Catharanthus roseus) have helped modern medicine fight childhood leukemia and Hodgkin’s disease. An indigenous remedy for diabetes, the rosy periwinkle failed screenings as a glucose regulator but was noted to shrink or arrest cancerous tumors. Success in clinical trials has put its most active alkaloids on the market as vincristine and vinblastine. Rosy periwinkle is now grown commercially for its active principles by the Lilly Company in southern Texas. (Sheldon, et. al., 1997).
An alkaloid named reserpine, collected from the root bark of Rauvolfia, the serpent-wood, is the active principle in medicines that control hypertension (high blood pressure). Members of Rauvolfia are found in the Old World from West Africa to Indonesia, as well as in Central America and the West Indies. They have been important medicinals in local cultures throughout this range. Before serpent-wood was saving lives in modern times, its Indian species Rauvolfia serpentina had a 3,000 plus year history in Ayurvedic medicine. Its powerful alkaloids have been used to stimulate contractions in childbirth, sooth spasms, allay snakebite (hence the name) and to bring down fever. Its tranquilizing effects have been used as an aid in meditation. Various species elsewhere have been used as powerful emetics, to combat digestive parasites, and to alleviate symptoms in syphilis, leprosy, and dysentery. The active principle resperpine was isolated by Indian doctors in the 1930s and found to relax blood vessels and reduce heart output, lowering blood pressure. It came to attention of Western doctors in the 1950s. For a time, it was tried as a treatment for schizophrenia. Demand led to over collection, which created serious pressure on wild populations. The Indian government has at times restricted export and made a not yet successful effort to cultivate the plant on a large scale
The second group of active principles in the family is the cardiac glycosides. As the name suggests, these chemicals affect the heart in mammals including humans. They are specialized metabolites related to steroids, and are dangerous because they very difficult to regulate as effective medicines. The correct dose is an effective stimulant to the heart muscle; a small overdose leads to severe complications and possible death. From West Africa and Indonesia comes Strophanthus, whose wild-looking flowers are conservatory curiosities. Heart stimulants and respiratory aids have been developed from species of Strophanthus, the most notable heart medicine stimulant named strophanthin. The seeds of several Strophanthus have been used as the natural starting point to the synthesis of the anti-inflammatory cortisone
All of the plants mentioned in this article are poisonous, some markedly so. People who grow oleander or frangipani must keep pets and children away from any part of these plants, which may contain alkaloids, cardiac glycosides, or both. The ability of cardiac glycosides to kill through heart or respiratory failure has been exploited by many indigenous tribes, which have tipped their blowgun darts with serum prepared from Mandevilla (Brazil) or Strophanthus (Africa). A similar glycoside is found in oleander! Also in West Africa is a plant the natives call iboga, Tabernanthe iboga. Chemists have isolated twelve psychoactive alkaloids from the root of the shrub. The power of the plant as a hallucinogen has led to a cult centered around rituals where it is ingested. Shamanistic practice in Gabon and other West African countries employ iboga to communicate with spirits, ancestors. The tribal name iboga means guide to the ancestors. Out-of-body experiences and trances are parts of these complex rites. One such ritual is a rite of passage for young males. The plant is quite toxic—the active and lethal doses are similar— and the ritual is essentially a test of survival that not every participant finishes alive (Schultes, et. al., 2001).
Lastly, the latex from a number of Apocynaceae has been commercially developed into rubber. None are important in worldwide commerce, but, historically, several were important. The Allies in World War II brought several into play when supplies of hevea rubber in Southeast Asian plantations were cut off by the occupation of the Japanese. A cousin of our blue star, Amsonia palmeri of southwestern North America, has been tapped as a substitute source for rubber, but on a small scale. The latex of several species in different parts of the tropics has been developed into a gum-like chew, and a number of tropical Apocynaceae are used for timber
Although the Apocynaceae is barely present in the Kentucky flora, our few representatives are just the smallest hint of a large, interesting, and very useful plant family. From beautiful shrubs and climbers of conservatories and southern gardens, to medicines and poisons, the Apocynaceae have delighted, intoxicated, healed, and hurt us.
Sources
Arthur Cronquist, 1981. An integrated system of classification of flowering plants. Columbia University Press.
H.C.D. de Wit, 1965. Plants of the world. The higher plants v.2. Thames & Hudson.
Michael A. Dirr, 2002. Dirr’s trees and shrubs for warmer climates. Timber Press.
Henry A. Gleason and Arthur Cronquist, 1991. Manual of the flora of northeastern United States and adjacent Canada. New York Botanical Garden.
V. H. Heywood, 1985. Flowering plants of the world. Prentice Hall.
Kirsten Albrecht Llamas, 2004. Tropical flowering plants. A guide to identification and cultivation. Timber Press.
Julia F. Morton, 1977. Major medicinal plants: Botany, culture and uses. Charles C. Thomas Publisher.
Frances Perry, 1972. Flowers of the world. Bonanza Books.
Robert Lee Riffle, 1998. The tropical look: An encyclopedia of dramatic landscape plants. Timber Press.
Jennie W. Sheldon, Michael J. Balick & Sarah A. Laird, 1997. Medicinal plants: Can utilization and conservation coexist? New York Botanical Garden.
Richard Evans Schultes, Albert Hoffmann, and Christian Ratsch, 2001. Plants of the gods. Revised edition. Healing Arts Press.
Robert Lee Riffle, 1998. The tropical look: An encyclopedia of dramatic landscape plants. Timber Press.
Turner, Nancy J. and Adam F. Szczawinski, 1991. Common poisonous plants and mushrooms of North America. Timber Press.
Wendy Zomlefer, 1994. Guide to flowering plant families. University of North Carolina Press.