Aristolochia is a genus of about 120 species of shrubs or twining vines native to Europe, temperate and tropical America and Asia. Distinctive flowers and the presence of aristolochic acid characterize Aristolochia and the few other genera of the family (Aristolochiaceae) including Asarum, the genus of wild ginger found in temperate zone forests of North America and Europe. Within Aristolochia is a group of robust vines or lianas cultivated in warm climates for their spectacular flowers but also often escaped into natural areas where they can become problems when they climb high into trees and overwhelm other plants. Flowers of several tropical Aristolochias are very large, reaching or exceeding 10 cm (4 inches) in length. One of these species, A. littoralis, depicted in the photographs, annually flowers in my yard in July.
The flowers lack petals but feature an expanded garish-colored calyx. The calyx includes an expanded pouch that encloses the relatively small reproductive structures. The calyx including both the pouch and the flaring, purple and yellow lobes, in side view resemble (to some), a “Dutchman’s pipe”, one popular name for the plant.
The reproductive structures consist of anthers (without filaments) fused to styles thus forming a short column called a gynostemium. The pouch must be cut open to see the gynostemium (see photograph). The calyx has a central opening that is bright yellow and leads to the inflated pouch. Its bright color plus a distinctly foul odor emitted from the flower are reported to attract pollinators to whom the odor of decay is alluring! As an aside I note that to me, although foul, the odor is not similar to the rotting flesh smell of Stapelia (Asclepidaceae) or Amorphophallus (Araceae), both of which, unlike Aristolochia, I have observed attracting flies in my yard.
The wasps and flies attracted to the flower enter the calyx pouch aided by downward pointing plant hairs (see photograph). According to the Kew Gardens (U.K.) website (kew.org), in the closely related A. grandiflora (and I presume, in A. littoralis), the first day the flower is open, the odor is emitted, the stigmatic surfaces (female organ) on the top of the gynostemium are pollen-receptive and wasps and flies reach the gynostemium after entering and traversing the pouch. Presumably in some cases the insects bear pollen from a previously visited flower. The pouch hairs prevent the visitors from leaving. On day two, however, the odor disappears, the plant hairs wither and the pollinators are able to escape, carrying newly acquired pollen with them.
In the U.S., Aristolochia vines seldom set fruit, perhaps because essential pollinators are absent outside the tropics. Indeed I have never seen an insect visit A. littoralis flowers here. Nevertheless cross-pollination must sometimes occur, or there is some self-compatibility, given that two years ago a single fruit, with seeds, matured in my backyard plant (see photograph) . Aristolochia includes a number of temperate zone species with small flowers and less robust growth found in forests of North America and the Old World. In the U.S., Aristolochias are the required food plant of both the Pipewort Swallowtail and the Polydamus Swallowtail. The range of the former is predominately temperate in distribution (although extending into Mexico) and thus dependent primarily on the temperate zone Aristolochias while the latter is found throughout warm areas of South America and the West Indies and thus is primarily dependent on the tropical Aristolochias. Either the aristolochic acid content of these species is low or the butterflies tolerate or are otherwise adapted to the toxic acid. Both butterflies successfully utilize the tropical Aristolochias introduced into Florida (see photograph). In fact, the Pipewort Butterfly has expanded its range in response to the presence of cultivated Aristolochia vines and its caterpillars store the acid thereby gaining protection from bird predators.
Aristolochic acid is of further interest. This compound is diagnostic of the Aristolochiaceae and is derived (after several steps) from the amino acid tyrosine. Despite the historical uses of several species of Aristolochia in Chinese medicine concoctions, the U.S. Food and Drug Administration (FDA) has issued a strong warning to consumers to avoid ingesting Aristolochia plant material in any form as it is known to cause cancer in humans. The evidence that plants containing aristolochic acid cause human cancer is strong. That aristolochic acid extracted from Aristolochia plants causes cancer in experimental animals also is well established. Consuming wild Aristolochia plants has recently been verified as a cause of many often fatal, kidney disorders in horses and in humans resident in certain areas of rural Croatia. Balkan endemic nephropathy (BEN), as the disease was originally known in the 1960s when reports first appeared in western European outlets, was seen to be concentrated in agricultural areas near tributaries of the Danube River starting in the late 1950s. The cause of the diseases were completely unknown for decades. Many possible agents like heavy metals, allergies, and mycotoxins were eventually dismissed. The mycotoxin, a common food contaminant which grows in stored grains and is known to damage kidneys got the most attention. In 1969 another agent was proposed in a Serbian journal – seeds of a Balkan weed occurring near fields of wheat used to bake bread. But it wasn’t till the 1990s that the toxic compound involved was definitively identified. At that time, first in Belgium then in other countries, women taking Chinese herbs for weight control contracted kidney problems identical to BEN. “Chinese herb nephropathy” as it was called in western Europe, was traced to the ingestion of plant material from Aristolochia clematitus, the same species as first proposed in the 1969 report as causing kidney failure in the Balkans. A. clematitus is a twining herbaceous vine with heart-shaped leaves, native to Europe and bearing tubular yellow flowers, much smaller than those of the tropical Aristolochias. The toxic ingredient is, however, the same aristolochic acid found throughout the genus. It is now recognized as having been a cause of the kidney failure in thousands of residents of the Balkans who had consumed contaminated flour, and of the kidney failure of the Belgian victims. “Aristolochic acid nephropathy” is the increasingly preferred medical name for the disorder as manifested in both the Balkans and in Belgium, replacing the two original names. IARC Working Group (200_). Plants Containing Aristolochic Acid ( www. monographs.iarc.fr/ENG/Monographs/vol100A-23.pdf.) Science 344: 146, 11 April 2014. (www.sciencemag.org)