Introduction to the Aristolochiaceae or Birthwort Family

The Aristolochiaceae are a family with bizarre-looking flowers, and each genus has its own unique and often very bizarre morphology. Since I always believe the weirder the better, the first time I saw an Asarum and an Aristolochia, I was instantly in love with their unique beauty.

And the bizarre got even more bizarre when the Angiosperm Phylogeny Group (IV) recently combined two very small families into the Aristolochiaceae so that it would not be paraphyletic. Since these new subfamilies are themselves very unique and different morphologically from the traditional Aristolochiaceae (which only included Asaroideae and Aristolochioideae), I am describing the subfamilies separately from the traditional Aristolochiaceae.

Beginners Guide to the Birthwort (Arisolochiaceae) Family

If you’re new to plant morphology, this guide is a perfect beginner’s description to help you learn how to identify the Aristolochiaceae family, with no need to know any scientific jargon. Below this are morphology photos and pictures of some species found in North America to further help you with identification. But for researchers or those wanting to learn a more in-depth version, check out the Scientific Botanical Description below the images in addition to genus-level distribution data.

Leaves and Stems of the Birthwort Family

These are mostly woody vines, shrubs, or herbs with simple (never compound) leaves on stalks (petioles) that are usually arranged in a spiral pattern and are often heart-shaped (cordate) or kidney-shaped (reniform). Most have aromatic leaves that may have gland dots. The Hydnoroideae are leafless parasitic herbs that live almost entirely below the soil.

Flowers of the Birthwort Family

The flowers are unusual and highly characteristic of the family, often known for their S-shaped floral tubes, but they are also highly variable. They are very small to often large and showy and often emit strong, foul odors. They also often have unique and specialized hairs designed to trap flies. They are held there until the hairs wither, releasing them to escape and pollinate other flowers.

Reproductive Features of the Birthwort Family

The reproductive features are highly variable in the family, but they typically have 3-36 stamens, which are often fused to the female parts into a specialized central column called a gynostegium but are also often free. The ovary may be inferior (attached below the point of attachment of the floral parts) or superior (attached above the floral parts).

Fruits of the Birthwort Family

Fruits are mostly dry capsules that split open when ripe to release their seeds. They may sometimes be a fleshy berry-like fruit.

Morphology of Aristolochiaceae in North America

Some Species of Aristolochiaceae Found in North America

Aristolochioideae Subfamily

Aristolochia taliscana – Mexican Birthwort

This is a lush medium green vine with heart-shaped (cordate) leaves with a deep cleft and a rounded to notched tip. Flowers are solitary in leaf axils with an S-shaped petal-like calyx in burgundy, purple, or brown, usually with yellow markings. The limb is covered with conspicuous purple hairs that trap pollinating insects. Native to western Mexico.

Aristolochia watsonii – Watson’s Dutchman’s Pipe

This one has very deeply arrow-shaped (sagittate) green leaves with purple markings. The calyx tube is straight or S-shaped, and the limb has maroon spots and no elaborate hairs. It is native to the southwestern USA and northwestern Mexico.

Asaroideae Subfamily

Asarum caudatum – Western Wild Ginger

This long favorite of mine has classic heart-shaped leaves with very long stalks from the ground (no plant stem). Flowers are solitary and burgundy or greenish-yellow, with a 3-part calyx with very long lobes that taper to a fine point. It is native to the Pacific Northwest, from BC, Canada, south to California, and east to Idaho and Montana, USA.

Asarum (Hexastylis) arifolia Little Brown Jug

This low herbaceous perennial has triangular, arrow-shaped, heart-shaped, or diamond-shaped leaves that are often variegated (this one shows slight variegation). Flowers are jug-shaped (urceolate) and brown, purple, or reddish with three lobes that may or may not be flared (visible at the base of the leaf stalk). Native to the southeastern USA.

Asarum (Hexastylis) shuttleworthii – Shuttleworth Ginger

This one has round to heart-shaped & usually variegated leaves. Its flowers are similar to A. arifolia but much larger. It’s a narrow endemic of the southern Appalachians & the surrounding area, eastern USA.

Uses of the Aristolochiaceae Family 

Several species of Aristolochia (e.g., A. grandiflora, A. gigantea, A. littoralis) are cultivated globally as ornamental greenhouse and garden vines for their bizarre, showy perianths. Many Asarum (Hexastylis) species are grown as shade-tolerant, evergreen groundcovers.

While many were used in traditional medicine, they are no longer recommended since Aristolochia and some Asarum contain a toxin known to be carcinogenic to humans and rats. However, historically, the fruits of the Hydnoroideae were collected, roasted, and eaten by indigenous human populations.

Ecosystem and Wildlife Values of the Aristolochiaceae Family 

Caterpillars, including the pipevine swallowtail, Battus philenor, and tropical birdwing butterflies, are specialized herbivores on Aristolochia. Rather than succumbing to the toxins, larvae sequester aristolochic acids within their bodies, rendering both the caterpillars and adult butterflies unpalatable to birds and other predators.

The fleshy, geocarpic, baccate fruits of Hydnora (commonly known as “jackal food” in southern Africa) develop a fragrant, astringent-to-sweet pulp packed with starch and tannins. They serve as an important food source for mammals like baboons, jackals, porcupines, and rhinos.

Taxonomy of Aristolochiaceae

The Aristolochiaceae family has 682 species in 7-9 genera of the Piperales order in the Magnoliids clade of mesangiosperms. This clade is technically not part of either the monocots or the dicots but instead is one of the sister groups to monocots and dicots, having diverged early on.

APG IV (2016) has combined the former Hydnoraceae and Lactoridaceae, as their exclusion would render the Aristolochiaceae paraphyletic. Molecular phylogenies have confirmed that Lactoris and the holoparasitic hydnoras are nested deeply within the traditional aristolochioid lineage.

Aristolochioideae Subfamily

Plants are mostly lianas, but occasionally shrubs or herbs. Inflorescences are usually axillary, and the flowers, or at least the floral buds, are zygomorphic, though Thottea is actinomorphic or weakly zygomorphic. The gynoecium has 4-6(2-3) carpels, which are constricted apically. One of the defining synapomorphies is the stamens, which in Aristolochia (6, rarely 4-5 stamens) are fused with the style into a 3- to 6-lobed gynostemium, while in Thottea the 6-40 stamens are arranged in whorls around the lower part of a multi-lobed style column. Stigmas are wet or dry and are commissural in Aristolochia. Fruits are mostly 6-locular dehiscent capsules that dehisce septicidally, but Thottea forms 4-angled, 4-valved siliquiforms, and Pararistolochia (if separated from Aristolochia) forms indehiscent baccate fruits.

Genera of the Aristolochioideae Subfamily

Aristolochia (569 s.l.), Thottea (46).

Asaroideae Subfamily

Plants are rhizomatous perennial herbs with actinomorphic and trimerous flowers that are solitary and terminal. Flowers mostly are apetalous, made of 3 connate sepals forming a fleshy calyx tube, but have 3 distinct sepals and petals in Saruma. The gynoecium is inferior or half-inferior, and the stigma has multicellular papillae, and a key defining feature is that unlike the Aristolochioideae, the stamens are distinct or only weakly adnate to the style column and do not form a gynostemium. The fruit of the Asarum is an irregularly dehiscent capsule and a follicular schizocarp in Saruma. 

Genera of the Asaroideae Subfamily

Asarum (138 s.l.), Saruma (1).

Hydnoroideae Subfamily

Unusual obligate root holoparasitic herbs that resemble fungi with warted, cylindrical haustorial roots that make contact with host roots. The plants lack leaves, stems, and chlorophyll and live mostly underground with only fleshy, foul-smelling flowers emerging above the soil.

Genera of the Hydnoroideae Subfamily

Hydnora (10), Prosopanche (7).

Lactoridoideae Subfamily

The Lactoridoideae is a monospecific subfamily consisting of a narrowly endemic plant found only in the cloud forests of Robinson Crusoe Island (Juan Fernández Archipelago, Chile). It is a small, glabrous perennial subshrub with small alternate leaves prominently dotted with pellucid oil cells, with very small flowers and dry fruits that split into pod-like segments.

Genera of the Lactoridoideae Subfamily

Lactoris monospecific (Lactoris fernandeziana).

Key Differences From Similar Families

The unique flowers of most members of the Aristolochiaceae make them difficult to confuse with other families. The subfamilies Aristolochioideae and Asaroideae are so unique that they are rarely confused with other families, and the Lactoridoideae is a rare microendemic from the Juan Fernández Islands. Still, the Piperaceae is a sister lineage within Piperales, and climbing species share cordate/palmate-veined alternate leaves, swollen nodes, and aromatic pellucid oil cells, but the Piperaceae differs by bearing spicate, minute, ebracteate flowers lacking a perianth, whereas the Aristolochiaceae bears large, showy, synsepalous perianths.

The Balanophoraceae are often confused with Hydnoroideae due to the leafless, achlorophyllous root-holoparasite habit and fungus-like subterranean emergence. However, the Balanophoraceae differ by their dense, club-like inflorescences with hundreds of minute unisexual flowers, compared to the solitary, massive, 3–4-merous trap flowers in the Hydnoroideae.

The Orobanchaceae are the most common root-parasite family that produces fleshy, leafless/scaly flowering stems, but the Orobanchaceae flowers are sympetalous, bilabiate corollas arranged in erect racemes or spikes with superior ovaries and four didynamous stamens.  

Scientific Botanical Description of the Aristolochiaceae

Note that, with the addition of the two very unique subfamilies, I have combined the Lacoridoideae in the traditional Aristolochiaceae description since they share many features but added descriptions of the Hydnoroideae separately simply because they are so different from the rest.

Habit & Leaf Form of Aristolochiaceae

Most Aristolochiaceae are perennial lianas, twiners, rhizomatous herbs, or small shrubs bearing essential oils. Most plants are mesophytic, except the xerophytic Hydnoroideae.

Leaves are simple, alternate, spiral (distichous in Lactoridoideae), flat, and either herbaceous or herbaceous and membranous. They are petiolate, non-sheathing (sheathing in Lactoridoideae), simple, aromatic, and may be gland-dotted, pellucid, or punctate.

The lamina is usually entire or sometimes dissected, palmatifid, or trilobed and is often cordate, reniform, or auriculate. They are either palmately or pinnately veined and are cross-venulate. Leaves are mostly exstipulate, except in the Lactoridoideae, which possess intrapetiolar stipules, and occasionally the first 1–2 leaves of suppressed branches simulate stipules in Aristolochia.  

The Hydnoroideae are holoparasitic herbs with a very peculiar vegetative form that lacks leaves, stems, and chlorophyll. The vegetative component consists of a coarse, rhizome-like pilot root with many slender, unbranched haustorial roots that parasitize the roots of host plants.  

Flowers of Aristolochiaceae

Plants are hermaphrodites or polygamomonoecious to gynomonoecious in Lactoridoideae. Pollination in Aristolochia is entomophilous and occurs via Diptera, with a conspicuously specialized mechanism that uses articulated hairs to trap flies within the perianth tube; these hairs wither to release the flies, allowing them to visit another flower. In Asaraoideae, pollination is often via fungus gnats or beetles; in Hydnoroideae, it is exclusively pollinated by carrion beetles, and the Lactoridoideae are primarily anemophilous.

Flowers can be solitary or aggregated in axillary or occasionally terminal cymes, racemes, or spikes. Flowers are very unusual in shape and can be minimal to very large. They are often malodorous and smelling of carrion but may also be odorless. They are actinomorphic to zygomorphic (Aristolochia) and tricyclic to pentacyclic.

The perianth usually has parts joined in one whorl, which is often petaline, with 3 sepals connated into a campanulate, rotate, or tubular-inflated structure. In Aristolochia, the tubular perianth is differentiated into an inflated basal chamber (utricle), a narrowed cylindrical tube (often S-shaped or bent), and a 1–3-lobed, expanded, uniseriate limb. However, in the Lactoridoideae, the perianth is small and sepaline. Only in Saruma and select Asarum is the corolla whorl conspicuous and well developed.

The Hydnoroideae are hermaphrodites, and pollination is via beetles. Flowers are solitary and arise endogenously from the pilot roots on short stalks that barely emerge above the ground. They are medium to large, malodorous, regular, and either tricyclic or tetracyclic with a short free hypanthium. The perianth is sepaline or petaline with 3–4 (5) parts joined in one whorl with valvate lobes that are often fleshy with retrorse bristles used as temporary traps.  

Androecium of Aristolochiaceae

The androecium has 3–36 members in one or two whorls (always 2 in Lactoridoideae) that are free of the perianth and free of the gynoecium (always in Lactoridoideae) or, more often, united with the gynoecium, forming a gynostemium. All members are usually fertile (may contain staminodes in Lactoridoideae), usually with 5-6(4) stamens in Aristolochioideae, 6 in Lactoridoideae, 12 in Asaroideae, or 3-4(5) in Hydnoroideae.

Members are mostly fertile, but staminodes are sometimes present in Prosopanche and Lactoris.

Stamens usually have short filaments, but they can be well-developed in Asaroideae and are typically absent in Hydnoroideae.

Anthers are usually separate from one another but connate into a massive, sessile, fleshy synandrium in Hydnora and some Prosopanche. Anthers are 2-locular (tetrasporangiate), usually extrorse, basifixed, and dehiscing by longitudinal lateral slits but may also be irregular or transverse slits in Hydnoroideae. Anthers are sometimes appendaged apically with the expanded connective, but they may also be unappendaged.

Gynoecium of Aristolochiaceae

The gynoecium is highly variable, depending on the subfamily.

In the Aristolochioideae, there are 6 (sometimes 4–5) syncarpous carpels with an inferior ovary that is typically 6-locular with axile placentation (or 1-locular with deeply intruded parietal placentas in Thottea). The style column is fused with the stamens into a gynostemium, terminating in (3)6 fleshy, commissural, stigmatic lobes. There are 20-100+ ovules per locule, anatropous, bitegmic, crassinucellate, horizontal to pendulous with a distinct funicle.

In the Asaroideae there are 6 syncarpous carpels with an inferior to half-inferior (superior/half-inferior in Saruma) ovary that is 6-locular with axile placentation or 1-locular with intrusive parietal placentation. There are 6 free or basally connate styles tipped with decurrent or capitate stigmas covered with multicellular papillae. Ovules are numerous, anatropous, and bitegmic.

In the Lactoroideae, there are 3 apocarpous carpels arranged in a single whorl, with a superior ovary. The stigmas are sessile and decurrent along the adaxial suture of each free carpel. Placentation is marginal/ventral; ovules are 4–8 per carpel, anatropous, bitegmic, and crassinucellate.

In the Hydnoroideae, carpels are 3–4(5) and syncarpous with a strictly inferior, unilocular ovary, but the cavity is often heavily occluded by massively expanded, intruding lamellate placentas. Placentation is parietal (in Prosopanche) or apical/pendulous (in Hydnora). The stigma is cushion-like, 3-lobed, and sessile directly on the ovary tip/hypanthium floor. Ovules are extremely numerous (thousands to hundreds of thousands), minute, orthotropous to hemianatropous, and unitegmic.

Fruit of Aristolochiaceae

The fruits, like the gynoecium, are very diverse morphologically.

In the Aristolochioideae, the fruits are mostly dry, 6-valved, septicidally dehiscent capsules opening basipetally (forming a hanging basket in pendulous species) or acropetally; elongated and siliquiform in Thottea; or an indehiscent, elongated berry-like fruit in Aristolochia subg. Pararistolochia. Seeds are numerous with copious endosperm, oily, sometimes weakly ruminate, and may be winged or not.

In the Asaroideae, the fruit is an irregularly bursting, fleshy to leathery subglobose capsule in Asarum (often dehiscing irregularly upon decomposition in leaf litter) or a follicular schizocarp splitting into 6 dry, dehiscent, follicle-like mericarps in Saruma. Seeds unwinged, obovoid to oblong, smooth to rugose, bearing a prominent, fleshy lateral elaiosome/caruncle for ant dispersal. The endosperm is copious, oily, and non-ruminate.

In the Lactoroideae, the fruit is an aggregate of 3 free, beaked, dry follicles that dehisce along their adaxial (ventral) sutures. Seeds are 4–8 per follicle, small, ovoid, ribbed, unwinged, and lacking an aril. The endosperm is copious and oily.

The Hydnoroideae is a large, globose to subglobose, geocarpic (subterranean) baccate fruit with a thick, scaly or leathery/woody pericarp (subterranean and indehiscent or splitting irregularly in Hydnora; circumscissile near the apex in Prosopanche), containing tens of thousands of minute, globose, hard-coated seeds. Seeds possess both a hard outer seed coat and rich perisperm and endosperm.

Global Distribution of Aristolochiaceae

The traditional Aristolochiaceae family is widespread in warm temperate to tropical areas around the world except in Australasia, and they are not present in arctic areas. Thy Hydnoroideae are mostly found in South America and Africa, while the Lactoridoideae is a rare microendemic genus confined to Juan Fernández Island off the coast of Chile.

Distribution of Aristolochiaceae in the Americas

Canadian Aristolochiaceae Genera

Aristolochioideae in Canada

Aristolochia 2 spp. including 1 native to ON and 1 intro (sometimes referred to as Isotrema) to ON and QC.

Asaroideae in Canada

Asarum 2 spp. native to BC, MB, ON, QB, and NB.

USA Aristolochiaceae Genera

Aristolochioideae in the USA

Aristolochia 16 spp., mostly native, some intro to most of the USA except WA, OR, NV, UT, ID, MT, WY, CO, ND, SD, NE, MN, and VT.

Asaroideae in the USA

Asarum (inc. Hexastylis) 17 spp. native in most of the USA except NV, AZ, UT, WY, CO, NM, NE, and TX.

Mexico Aristolochiaceae Genera

Aristolochioideae in Mexico

Aristolochia 47 spp. native throughout all of Mexico, including the Mexican Pacific Islands.

Asaroideae in Mexico

Asarum 1 sp. native to Yuc.

Neotropical Aristolochiaceae Genera

Aristolochioideae in the Neotropics

Aristolochia ~200 spp. native throughout Mexico, CAM, Bermuda, the Bahamas, Cuba, Hispaniola, Jamaica, Puerto Rico, the Leeward & Windward Is., and tropical SAM S to Santiago, C Chile, and N Argentina, and it is intro in the Venezuelan Antilles and Galapagos.

Hydnoroideae in the Neotropics

Prosopanche 7 spp. neoendemic genus native to Costa Rica, Peru, Bolivia, Argentina, Paraguay, NE & S Brazil.

Lactoridoideae in the Neotropics

Lactoris monospecific endemic of Juan Fernandez Island off the coast of Chile. 

Patagonia Aristolochiaceae Genera

Hydnoroideae in Patagonia

Prosopanche 1 sp. native to S. Argentina.

Additional Information and References

• Visit Lyrae’s Dictionary of Botanical Terms to learn the terminology of botanists. Note that if you hover over most of the words in the articles, you can also get definitions from them there.
• Willis, Lyrae (Unpublished). Plant Families of North America. This is where all of the family descriptions come from. Below should be most of my references for this, as well as my own personal observations.
• Canadensys: Acadia University, Université de Montréal Biodiversity Centre, University of Toronto Mississauga, University of British Columbia. http://data.canadensys.net/explorer (accessed 2020 – current)
• Flora of North America. (1993+). https://floranorthamerica.org/Main_Page
• Delta: Watson, L., and Dallwitz, M.J. (1992+). The Families of Flowering Plants: descriptions, illustrations, identification, and information retrieval. Version: 2nd May 2020. delta-intkey.com. Accessed spring through fall of 2020.
• GBIF.org (2020), GBIF Home Page. Available from: https://www.gbif.org
• Naturalista: CONABIO http://www.naturalista.mx Accessed 2020–current.
• Neotropikey: Milliken, W., Klitgård, B., & Baracat, A. eds. (2009 onwards). Neotropikey: Interactive key and information resources for flowering plants of the Neotropics. www.kew.org/neotropikey.com (accessed 2020 – current).
• Patagonia Wildflowers: Wildflower Identification Site. https://patagoniawildflowers.org/ Accessed throughout the fall of 2020.
• POWO (2019). Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet: http://www.plantsoftheworldonline.org/
• Stevens, P. F. (2001+). Angiosperm Phylogeny Website. Version 14, July 2017 [more or less continuously updated since]. http://www.mobot.org/MOBOT/research/APweb/
• USDA, NRCS. 2020. The PLANTS Database (http://plants.usda.gov, 2 June 2020). National Plant Data Team, Greensboro, NC, USA; accessed throughout fall of 2020.
• WFO (2022): World Flora Online. Published on the Internet: http://www.worldfloraonline.org. Accessed Spring 2022 – current

My Current Plant Family Education Fundraiser

I am currently seeking funding to expand my website and SEO capabilities as I keep adding new families, and I am also looking to invest in a new macro lens, as I will soon be adding floral dissections to the families as they become available to me. You can donate to help support native plant education using the GoFundMe link, also at the bottom of the page.

Copyright Information

The information and the photos on this site are free to use for educational purposes, with proper attribution. For other uses, please contact me first.

You can cite this site as follows: Willis, Lyrae (2020+). Lyrae’s Nature Blog – Plant Families of North America. https://lyraenatureblog.com/. Accessed [Enter Date].