Introduction to the Aristolochiaceae Family

The Aristolochiaceae are a family with bizarre-looking flowers, and each genus has its own unique morphology. Since I always love the bizarre, the first time I saw an Asarum and an Aristolochia, I was instantly in love with their unique beauty. APG IV recently combined two very small families into it otherwise, it would 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, which will be described in detail as one (differences between these two subfamilies can still be found in the Taxonomy section below.

Flowers of Aristolochiaceae

Flowers of Traditional Aristolochiaceae

Plants are hermaphrodite. Pollination is entomophilous by Diptera, and the mechanism is conspicuously specialized via an elaborate system for trapping flies within the perianth tube using articulated hairs that end up withering to release the flies so they can visit another flower. Flowers can be solitary or aggregated in cymes, racemes, or spikes, and they are often axillary but sometimes terminal. Flowers are very unusual in shape and can be small, medium, or large. They are often malodorous and smelling of carrion or may be odorless. They are actinomorphic to zygomorphic and tricyclic to pentacyclic. The perianth has 3 or 6 parts that are joined in one whorl or two (then isomerous), and they either have a distinct calyx and corolla or are petaline. Only in Saruma is the corolla whorl conspicuous and well developed. The calyx has three parts in one whorl: connate, entire or blunt-lobed, and campanulate or tubular, with the tube often S-shaped. The calyx is unequal but not bilabiate, bilabiate, or regular, persistent or not, and valvate or valvate–induplicate. Corolla, when present, has three parts in one whorl but otherwise is usually reduced or absent.

Flowers of Hydnoroideae

Plants 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, often with the lower parts remaining beneath the soil. They are medium to large in size, malodorous, regular, and either tricyclic or tetracyclic. A short free hypanthium is present. The perianth is sepaline, petaline, or of tepals. It has 3–4(5) parts joined in one whorl. The lobes are valvate, fleshy, often bare retrorse bristles, and are white, red, pink, or brown in color. 

Flowers of Lactoridoideae

Plants are polygamodioecious with axillary flowers that are solitary or aggregated in cymes. Flowers are small, 3 merous, and tetracyclic. There is no free hypanthium or hypogynous disk. The perianth has three parts in one whorl and is sepaline.

Androecium of Aristolochiaceae

Androecium of Traditional Aristolochiaceae

The androecium has 6–36 members in one or two whorls that are free of the perianth but may be free of or united with the gynoecium or coherent via the gynostemium and one adelphous. Often, there are six stamens but may also be 4 or 12(13–36), and they are all exclusively fertile. Stamens are isomerous with the perianth to polystemonous and are either filantherous or have sessile anthers. Anthers are cohering or separate from one another, basifixed or adnate, non-versatile, and dehiscing via longitudinal slits. They are extrorse or both extrorse and introrse only in Heterotropa. Anthers are tetrasporangiate and appendaged apically with the expanded connective that assumes stigmatic functions associated with the gynostemium, or they are unappendaged.

Androecium of Hydnoroideae

The androecium has 3–4(5) or 6–8(9–10) members that are free of the perianth but inserted on the hypanthium. They are one whorled, coherent, and united into a thick sinuose annular in Hydnora or an ovoid synandrium. The androecium is either made exclusively of fertile stamens or, in Prosopanche, includes staminodes. In Prosopanche 3–4(5), small fleshy staminodes alternate with the stamens. There are 3–4(5) stamens that are oppositisepalous and isomerous with the perianth. They either have sessile anthers in Hydnora or are filantherous in Prosopanche, where the very short filaments arise from the hypanthium, and the anthers are connate to form a dome or cap with a small central opening. Anthers may be separate from one another or coherent in Prosopanche. They are dehiscing via longitudinal slits or transversely, are extrorse, and many locular where each one has numerous pollen sacs.

Androecium of Lactoridoideae

There are six androecial members that are free of the perianth and one another and are two whorled. The androecium is either exclusively all fertile stamens or sometimes includes staminodes. When present, three non-petaloid staminodes are internal to the fertile stamens. There are 6(3) stamens that are diplostemonous, narrowly laminar, and short. Anthers are adnate, non-versatile, and dehiscing via longitudinal slits. They are extrorse, bilocular, and almost as long as the blade. Anthers are tetrasporangiate and shortly appendaged apically. 

Gynoecium of Aristolochiaceae

Gynoecium of Traditional Aristolochiaceae

The gynoecium has 4–6 carpels, and the pistil has 1 or 4–6 cells. It is synstylovarious, syncarpous, or in Hexastylis, it is synovarious. The gynoecium is usually inferior but sometimes may be partly inferior. The ovary is either 4–6 or 1 locular (where the septa are sometimes incompletely intruded). An epigynous disk is either present or absent. There are 1 or 4–6 free or partially joined styles that are apical. Placentation when unilocular is parietal and when plurilocular it is axile. There are 50-100 ovules in a single cavity if unilocular and 20-50 when plurilocular. Ovules possess a funicle, are pendulous or horizontal, and are anatropous.

Gynoecium of Hydnoroideae

The gynoecium is 3(4) carpelled and is either partly or completely buried in the soil. The pistil is one-celled. The gynoecium is synstylovarious to syncarpous and inferior. The ovary is one locular but becomes occluded by ingrowth of the accrescent placentas. There are 1 or 3 sessile and commissural (Prosopanche) stigmas that are mostly 3-lobed in Hydnora. Placentation is parietal (Prosopanche) or apical (Hydnora). There are 50-200 undifferentiated ovules in the single cavity that are orthotropous and tenuinucellate.

Gynoecium of Lactoridoideae

The gynoecium is three-carpelled and isomerous with the perianth. The pistil is basally three-celled. It is superior and semicarpous with the carpels in a single whorl and more or less basally connate. The carpel is shortly stylate with a decurrent stigma and 6(4–8) ovuled. Placentation is marginal, and the placenta is intruded. The ovary is three locular basally. Ovules have long funicles, are biseriate, anatropous, bitegmic, and weakly crassinucellate.

Fruit of Aristolochiaceae

Fruit of Traditional Aristolochiaceae

The fruit is usually non-fleshy, but sometimes it has a fleshy endocarp. Usually, they are dehiscent septicidal, valvular (usually basally but only rarely at the top), or an irregularly splitting capsule. Rarely are they an indehiscent berry or a nut, or in Saruma, they area schizocarp with 4-6 mericarps of follicles. Seeds are endospermic, ruminate or not, and are oily. The Embryo is rudimentary to weakly differentiated at the time of seed release.

Fruit of Hydnoroideae

The fruit is fleshy inside but with a more or less woody pericarp. It is a dehiscent circumscissile capsule (Prosopanche) or an indehiscent capsule or berry. Fruits contain 500–2000 tiny seeds that are endospermic.

Fruit of Lactoridoideae

The fruit is a non-fleshy aggregate with the fruiting carpels coalescing into a secondary syncarp, a dehiscent follicle. Seeds are endospermic and oily.

Habit & Leaf Form of Aristolochiaceae

Habit & Leaf Form of Traditional Aristolochiaceae

Shrubs, lianas, or herbs, but mostly woody vines bearing essential oils. Plants are green and photosynthesize. They are always perennial and do not have a conspicuous aggregation of leaves. They are usually climbing stem twiners, but sometimes they are self-supporting. Plants are mesophytic. Leaves are alternate, spiral, flat, and either herbaceous or herbaceous and membranous. They are petiolate, sheathing to non-sheathing, simple, aromatic, and may be gland-dotted pellucid punctate. Lamina is usually entire or sometimes dissected palmatifid or trilobed and is often cordate. They are either palmately or pinnately veined and are cross-venulate. Leaves are exstipulate, but sometimes the first 1–2 leaves of suppressed axillary branches simulate stipules.  

Habit & Leaf Form of Hydnoroideae

Ectoparasitic herbs with a very peculiar vegetative form that lacks leaves and is more or less fungoid in habit. The vegetative component consists of a coarse, rhizome-like pilot root with many slender, unbranched haustorial roots that parasitize the roots of host plants). Plants are succulent and not green. 

Habit & Leaf Form of Lactoridoideae

Shrubs bearing essential oils with small, simple, alternate, distichous, petiolate, and gland-dotted leaves. The lamina is entire, obovate, emarginate, and pinnately veined. 

Uses of the Aristolochiaceae Family 

A few Aristolochia and Asarum spp are routinely cultivated as ornamentals. Aristolochia and some Asarum contain a toxin known to be carcinogenic to humans and rates, so they are never ingested in any way. However, Pipevine Swallowtail butterflies are immune to the toxin and lay their eggs on Aristolochia spp, and their caterpillars eat the leaves when they hatch. The fruits of Hydnora, on the other hand, have edible, fragrant pulp, which attracts animals, including porcupines, monkeys, jackals, rhinoceros, and armadillos, and are sometimes eaten by humans.

Morphology of Aristolochiaceae in North America

Species of Aristolochiaceae I have Covered So Far

Aristolochioideae Subfamily

Aristolochia taliscana – Mexican Birthwort

This is a lush medium green vine with cordate leaves with a deep cleft and a rounded to emarginate tip. Flowers are solitary and axillary with an S-shaped petaline calyx in burgundy, purple, or brown, usually with yellow markings. The calyx 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 sagittate green leaves with various purple markings. Calyx tube is straight or S-shaped and the limb has maroon spots and no elaborate hairs. Native to southwestern USA, northwestern Mexico.

Asaroideae Subfamily

Note that the genus Hexastylis was recently combined in with the Asarum based on phylogenetic research, though not all sources have accepted this. Since it is still not widely accepted, and their floral morphology is different, I have kept it separate for now.

Asarum caudatum – Western WIld Ginger

This has classic cordate leaves with very long petioles from the ground up. Flowers are solitary and burgundy or greenish-yellow, with a 3 part calyx with very long acute lobes tapering to a fine point. Native to the Pacific Northwest from BC, Canada south to California, east to Idaho, Montana, USA.

Hexastylis arifolia Little Brown Jug

This low herbaceous perennial has triangular-sagittate, ovate–sagittate, somewhat cordate, to deltoid leaves that are often variegated. Flowers are urceolate (jug-shaped) in brown, purple, or reddish with 3 lobes that may or may not be flared. Native to the southeastern USA.

Hexastylis shuttleworthii – Shuttleworth Ginger

This one has orbicular to cordate & usually variegated leaves. Its flowers are similar to H. arifolia but much larger. Its a narrow endemic of the southern Appalachians & the surrounding area, eastern USA.

Taxonomy of Aristolochiaceae

The Aristolochiaceae family has 500-700 spp in 8 genera of the Piperales Order in the Magnoliids Clade of Basal Angiosperms. This clade is not part of either the monocots or the dicots. APG IV has combined the former Hydnoraceae and Lactoridaceae because their exclusion would make the Aristolochiaceae paraphyletic.

1. Aristolochioideae – Plants are mostly lianas or occasionally shrubs or herbs. Inflorescences are usually axillary, and the flowers or at least the floral buds, are zygomorphic. The gynoecium has 4-6(2-3) carpels that are constricted apically. Stigmas are wet or dry and are commissural in Aristolochia. Fruits are dehiscent and open laterally, acropetally or basipetally septicidal, or rarely adaxially if a schizocarp or berry. Seeds are winged.
2. Asaroideae – Plants are rhizomatous perennial herbs. Flowers are solitary and terminal, the gynoecium is inferior to half inferior, and the stigma has multicellular papillae. The fruit of the Asarum is an irregularly dehiscent capsule and a follicular schizocarp in Saruma. 
3. Hydnoroideae – see descriptions above for a complete description.
4. Lactoris or Lactoridoideae – see descriptions above for a complete description.

Genera:

Aristolochioideae: Aristolochia (~400 – 565 inc Isotrema, Euglypha, Pararistolochia), Thottea (43 inc Apama).

Asaroideae: Asarum (126 inc Heterotropa, Hexastylis), Saruma (1).

Hydnoroideae: Hydnora (6), Prosopanche (4).

Lactoridoideae: Lactoris monospecific Lactoris fernandeziana.

Key Differences From Similar Families

The unique flowers of most members of the Aristolochiaceae make them hard to confuse with other families. The subfamilies Aristolochioideae and Asaroideae are so unique they are rarely confused, and the Lactoroideae is a rare microendemic from Juan Fernandez Island. The Hydnoroideae, on the other hand, despite its uniqueness, do have some superficially similar-looking families. However, the other families all contain features lacking in Hydnoraceae. They include the Apodanthaceae with its very small unisexual flowers, the Balanophoraceae with its fungus-like inflorescence with numerous small flowers, the Cytinaceae with its spicate inflorescence, the Mitrastemonaceae with its whorl of opposite decussate leaves and a superior ovary, and the Orobanchaceae with its zygomorphic flowers on racemes or spikes.   

Distribution of Aristolochiaceae

The Aristolochiaceae family is widespread in warm temperate to tropical areas around the world except in Australasia, and they are not present in arctic areas. The Lactoridoideae is a rare microendemic genus confined to Juan Fernandez Island off the coast of Chile.

Distribution of Aristolochiaceae in the Americas

Canadian Genera Include:

Aristolochioideae: Aristolochia 1 of 525 cosmopolitan spp native ON, intro QC; Isotrema 1 sp intro ON. Asaroideae: Asarum 2 of 126 N temperate spp native BC, MB, ON, QB, NB.

USA Genera Include:

Aristolochioideae: Aristolochia 16 of 525 cosmopolitan spp intro and native to most of the USA exc WA, OR, NV, UT, ID, MT, WY, CO, ND, SD, NE, MN, VT. Asaroideae: Asarum (inc Hexastylis) 17 of 126 N temperate spp native in most of the USA exc NV, AZ, UT, WY, CO, NM, NE, TX.

Mexico Genera Include:

Aristolochioideae:Aristolochia 47 of 525 cosmopolitan spp native throughout all of Mexico inc Mexican Pacific Is. Asaroideae:Asarum (inc Hexastylis) 1 of 126 N temperate spp native Yuc.

Neotropical Genera Include:

Aristolochioideae: Aristolochia ~200 of 525 cosmopolitan spp native throughout Mexico, CAM, Bermuda, Bahamas, Cuba, Hispaniola, Jamaica, Puerto Rico, Leeward & Windward Is, tropical SAM S to Santiago C Chile, N Argentina, intro Venezuelan Antilles, Galapagos, inc 15 spp formerly known as Isotrema native in CAM and Euglypha a former monospecific genus endemic to Bolivia, C+SE Brazil, Paraguay, NE Argentina. Hydnoroideae: Prosopanche 4 of 4 CAM + SAM endemic spp native Costa Rica, Peru, Bolivia, Argentina, Paraguay, NE+S Brazil inc 2 narrow endemics of Costa Rica (1), Bahia Brazil (1). Lactoridoideae: Lactoris 1 sp endemic of Juan Fernandez Island off the coast of Chile. 

Patagonia Genera Include:

Hydnoroideae: Prosopanche 1 of 4 CAM + SAM endemic spp native 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 (2022).  Plant Families of North America.  Not yet published. This is where all of the family descriptions come from. Below should be most of my references for this.
• Canadensys: Acadia University, Université de Montréal Biodiversity Centre, University of Toronto Mississauga, University of British Columbia. http://data.canadensys.net/explorer (accessed 2020 – current)
• Delta: Watson, L., and Dallwitz, M.J. 1992 onwards. 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 onwards). Angiosperm Phylogeny Website. Version 14, July 2017 [more or less continuously updated since]. http://www.mobot.org/MOBOT/research/APweb/
• The Plant List (2013). Version 1.1. Published on the Internet; http://www.theplantlist.org/ (accessed 2020 to 2021). No longer updated. Use WFO below.
• USDA, NRCS. 2020. The PLANTS Database (http://plants.usda.gov, 2 June 2020). National Plant Data Team, Greensboro, NC 27401-4901 USA; accessed throughout fall of 2020.
• Wikipedia: The free encyclopedia. (2004, July 22). FL: Wikimedia Foundation, Inc. Retrieved throughout 2019-current, from https://www.wikipedia.org
• WFO (2022): World Flora Online. Published on the Internet; http://www.worldfloraonline.org. Accessed Spring 2022 – current