Orobanchaceae
Broomrape Family
Orobanchaceae, the broomrape family, is a large and remarkably diverse family within the order Lamiales, comprising about 99 genera and over 2,000 species. Its defining characteristic is parasitism; it includes the only angiosperm family where all members are parasitic on other plants. This ranges from hemiparasites (green plants like paintbrushes, louseworts, and false foxgloves, formerly often in Scrophulariaceae) which photosynthesize but draw water and nutrients from host roots, to holoparasites (lacking chlorophyll, like broomrapes and squawroot) which are entirely dependent on their hosts. Flowers are typically zygomorphic and bilabiate, and the fruit is usually a capsule containing numerous tiny seeds.
Overview
The modern circumscription of Orobanchaceae is much broader than traditionally recognized, now including numerous genera of hemiparasitic herbs previously placed in Scrophulariaceae (Figwort family). This reclassification, driven by molecular phylogenetic evidence, unites all root parasites (hemi- and holo-) from that lineage into a single large family. Holoparasitic members, like the traditional broomrapes (Orobanche) and cancer-roots (Conopholis), completely lack chlorophyll and appear as fleshy, yellowish, brownish, or purplish flowering stalks emerging from the ground near their host plants. Hemiparasitic members, such as Indian paintbrushes (Castilleja), louseworts (Pedicularis), and false foxgloves (Agalinis), possess green leaves and photosynthesize but supplement their resources by tapping into the roots of neighboring plants via specialized structures called haustoria.
The family is cosmopolitan, found worldwide except Antarctica, but is most diverse in temperate regions of the Northern Hemisphere and parts of Africa and Madagascar. While some species are grown ornamentally (especially Castilleja), the family contains no major food crops and is arguably more significant ecologically and economically for its negative impacts. Several holoparasitic species, notably witchweeds (Striga) in Africa and Asia and certain broomrapes (Orobanche, Phelipanche) in Europe, Africa, and the Middle East, are devastating agricultural pests that attack major crops like grains, legumes, and vegetables.
Oklahoma (current location as of March 27, 2025) hosts a variety of Orobanchaceae members, including several species of colorful Indian Paintbrushes (Castilleja), purple-flowered False Foxgloves (Agalinis), the intricate Louseworts (Pedicularis canadensis), and native holoparasites like Louisiana Broomrape (Orobanche ludoviciana) parasitizing members of the Asteraceae family, and Squawroot (Conopholis americana) parasitizing oaks.
Quick Facts
- Scientific Name: Orobanchaceae Vent.
- Common Name: Broomrape family
- Number of Genera: Approximately 99
- Number of Species: Approximately 2,060
- Distribution: Cosmopolitan, especially temperate Northern Hemisphere, Africa, Madagascar.
- Evolutionary Group: Eudicots - Asterids (Lamiids)
- Order: Lamiales
Key Characteristics
Growth Form and Habit
Primarily annual or perennial herbs, rarely shrubs. All members are parasitic on the roots of other plants. This ranges from hemiparasites (green, photosynthetic plants with functional leaves) to holoparasites (non-photosynthetic plants lacking chlorophyll, appearing yellowish, brownish, reddish, or purplish, with highly reduced vegetative structures). Connection to the host is via specialized root structures called haustoria.
Leaves
Highly variable depending on parasitic strategy. In hemiparasites, leaves are usually well-developed, green, photosynthetic, arranged alternately or oppositely, and can be simple and entire to deeply pinnately lobed or dissected (fern-like in some Pedicularis). In holoparasites, leaves are reduced to small, alternate, non-functional scales lacking chlorophyll. Stipules are absent.
Inflorescence
Inflorescences are typically indeterminate, most commonly terminal spikes or racemes, sometimes branched into panicles or thyrses. Flowers are sometimes solitary. Flowers are usually subtended by bracts, which can be leaf-like or scale-like; in Castilleja, the bracts (and often upper leaves/calyces) are brightly colored and more conspicuous than the actual corollas.
Flowers
Flowers are typically bisexual and strongly zygomorphic (bilaterally symmetric), though sometimes nearly actinomorphic. Key features include:
- Calyx: Composed of usually 5 fused sepals (synsepalous), but often modified; the tube may be split, resulting in apparently 2 or 4 lobes, or distinctly 2-lipped. Persistent in fruit.
- Corolla: Composed of 5 fused petals (sympetalous), typically forming a 2-lipped (bilabiate) tube. The upper lip (galea) is usually 2-lobed or hood-like (sometimes enclosing the anthers); the lower lip is 3-lobed and often serves as a landing platform for pollinators. Corolla colors are diverse (white, yellow, pink, red, purple, brownish).
- Androecium: Stamens typically number 4, attached to the corolla tube (epipetalous), and are usually didynamous (arranged in two pairs of unequal length - two long, two short). The fifth (posterior) stamen, expected in a 5-merous flower, is usually absent or represented by a small staminode. Anthers often stick together in pairs, sometimes hairy or awned, opening via longitudinal slits.
- Gynoecium: Ovary is superior, composed of 2 fused carpels. It typically forms a single chamber (unilocular) or is incompletely divided into two chambers. Ovules are usually numerous, attached to two (often large or intruding) parietal placentas, or sometimes appearing axile if the placentas meet centrally. A single terminal style is present, topped by a stigma that is usually 2-lobed or capitate.
Fruits and Seeds
The fruit is characteristically a dehiscent capsule, usually splitting along 2 or 4 valves (loculicidal or septicidal dehiscence). It contains numerous (often hundreds or thousands) seeds which are typically very small to dust-like. This large number of tiny seeds increases the probability that one will land near a suitable host root and receive the chemical cues needed for germination. Seeds contain endosperm.
Chemical Characteristics
Hemiparasitic members often contain iridoid glycosides (also found in related families like Plantaginaceae and Scrophulariaceae s.s.). Holoparasites lack chlorophyll but may contain other pigments like carotenoids or anthocyanins (depending on the group); orobanchin is a glycoside associated with browning reactions. Mannitol and various alkaloids are also found in the family.
Field Identification
Identifying Orobanchaceae hinges on recognizing evidence of parasitism (either green plants attached to hosts, or non-green plants) combined with characteristic Lamiales floral features.
Primary Identification Features
- Parasitic Habit: Evidence of attachment to host roots (haustoria, often underground and hard to see for hemiparasites) or complete lack of chlorophyll (holoparasites).
- Herbaceous Habit: Most members are non-woody herbs.
- Zygomorphic, Bilabiate Flowers: Flowers typically two-lipped, bilaterally symmetric.
- Sympetalous Corolla: Petals fused into a tube, at least at the base.
- 4 Didynamous Stamens: Usually 4 stamens in two pairs of different lengths, attached to the corolla tube.
- Superior Ovary: Ovary positioned above other floral parts.
- Capsule Fruit with Tiny Seeds: Fruit a dry capsule containing numerous, very small seeds.
Secondary Identification Features
- Leaf Type: Well-developed green leaves (alternate or opposite) in hemiparasites vs. reduced, non-green scales in holoparasites.
- Brightly Colored Bracts: Inflorescence bracts more colorful than corolla (Castilleja).
- Galeate Upper Lip: Upper corolla lip forming a distinct hood (Pedicularis).
- Calyx Structure: Number and shape of calyx lobes (e.g., 2 or 4 lobes common).
- Inflorescence Type: Spike or raceme common.
Seasonal Identification Tips
- Spring/Summer/Fall: Flowering time varies greatly. Hemiparasites often flower for extended periods. Holoparasites typically emerge and flower when their specific hosts are actively growing, often mid-summer to fall.
- Year-round: Evidence of parasitism (for holoparasites, the dead stalks may persist; for hemiparasites, identification relies on other features outside flowering).
- (Current Time Context: Late March in Oklahoma - Early spring hemiparasites like Pedicularis canadensis might show vegetative growth; Castilleja species will flower later in spring. Holoparasites typically emerge later).
Common Confusion Points
- Lamiaceae (Mint Family): Also in Lamiales with zygomorphic flowers and superior ovary. Key differences: Usually square stems, opposite leaves, ovary deeply 4-lobed, fruit 4 nutlets, non-parasitic.
- Scrophulariaceae (Figwort Family sensu stricto): Closely related, similar flowers. Key differences: Non-parasitic; subtle differences in ovary/seed structure. Many former 'scrophs' are now in Orobanchaceae or Plantaginaceae.
- Plantaginaceae (Plantain Family): Includes many former 'scrophs' (Penstemon, Veronica). Similar zygomorphic flowers, superior ovary, capsule fruit. Key difference: Non-parasitic.
- Monotropaceae (Indian Pipe family - now included in Ericaceae): Holoparasitic/mycoheterotrophic herbs lacking chlorophyll, but flowers usually actinomorphic or slightly zygomorphic, different structure (e.g., parts free or only slightly fused, distinct placentation), belong to Ericales order.
Look for the combination: Evidence of parasitism + Zygomorphic/bilabiate flowers + 4 didynamous stamens + Superior ovary + Capsule with tiny seeds.
Field Guide Quick Reference
Look For:
- Parasitic habit (hemi- or holo-)
- Herbaceous (mostly)
- Flowers zygomorphic, bilabiate
- Corolla sympetalous
- Stamens 4, didynamous, epipetalous
- Ovary superior
- Fruit a capsule with numerous tiny seeds
Key Variations:
- Hemiparasite (green) vs. Holoparasite (no chlorophyll)
- Leaves developed vs. scale-like
- Bracts showy vs. inconspicuous
- Calyx lobing (2, 4, 5)
- Corolla shape/color
Notable Examples
The Broomrape family includes colorful wildflowers, inconspicuous parasites, and major agricultural pests.
Castilleja coccinea
Indian Paintbrush / Scarlet Paintbrush
A biennial or perennial hemiparasitic herb native to eastern and central North America, including Oklahoma. Forms a basal rosette of leaves. Flowering stems bear alternate, often lobed leaves and terminal spikes where the actual flowers (greenish-yellow corollas) are largely hidden by brightly colored (typically scarlet red, sometimes yellow) bracts and calyx lobes, creating the "painted" look. Parasitizes various prairie grasses and forbs.
Orobanche ludoviciana
Louisiana Broomrape / Prairie Broomrape
A holoparasitic perennial herb native to much of North America, including Oklahoma. Lacks chlorophyll, appearing as stout, fleshy, yellowish to purplish stems emerging from the ground. Leaves reduced to scales. Flowers are numerous, purplish or yellowish, bilabiate, arranged in a dense spike. Primarily parasitizes members of the Asteraceae family (sunflowers, asters, etc.).
Pedicularis canadensis
Canadian Lousewort / Wood Betony
A perennial hemiparasitic herb native to eastern and central North American woodlands and prairies (found in OK). Features a basal rosette and stems with alternate, pinnately dissected, fern-like leaves. Flowers borne in a dense terminal spike; corolla is strongly bilabiate, typically yellowish or reddish, with a prominent arched or beaked upper lip (galea). Parasitizes various hosts.
Agalinis fasciculata
Beach False Foxglove / Fascicled Purple Gerardia
An annual hemiparasitic herb native to the eastern and central US, including Oklahoma, often in sandy soils or open woods. Has very narrow, linear leaves, often clustered (fascicled) in the axils. Produces showy, pink to purple, bell-shaped to funnelform, slightly zygomorphic flowers in racemes during late summer and fall. Like other Agalinis, it parasitizes grasses and other herbs.
Conopholis americana
Squawroot / Bear Corn
A perennial holoparasitic herb native to eastern North American forests, primarily parasitizing oak trees (Quercus). Emerges from the ground near host roots as thick, fleshy, yellowish-brown, cone-like flowering stalks covered in scale-like leaves and yellowish, bilabiate flowers. Resembles a pine cone or ear of corn.
Striga asiatica
Witchweed
An annual holoparasitic herb native to Africa and Asia, infamous as a devastating agricultural pest of cereal crops like corn, sorghum, millet, and rice. Small plants with scale-like leaves (though sometimes greenish), producing numerous small, often reddish or yellowish flowers. Seeds are dust-like and can remain dormant in soil for decades until stimulated by host root exudates. Subject to strict quarantine and eradication efforts where introduced (e.g., parts of US).
Phylogeny and Classification
Orobanchaceae is a large family within the core Lamiales order, part of the Lamiid clade of Asterids. Molecular phylogenetics dramatically reshaped the family by demonstrating that the holoparasitic Orobanchaceae (sensu stricto) was nested within a larger clade of hemiparasitic plants traditionally placed in Scrophulariaceae. To maintain monophyletic families, Orobanchaceae was expanded to include all these parasitic lineages originating from a common ancestor within Lamiales.
The family's placement within Lamiales aligns it with other major families characterized by zygomorphic flowers and superior ovaries, such as Lamiaceae, Plantaginaceae, Verbenaceae, and Bignoniaceae. The evolution of parasitism is the defining feature of Orobanchaceae. It is believed that hemiparasitism evolved first, with subsequent multiple independent transitions to holoparasitism within different lineages of the family. This provides a powerful model system for studying the evolution of parasitic adaptations, including gene loss, host interaction, and morphological reduction.
Position in Plant Phylogeny
- Kingdom: Plantae
- Clade: Angiosperms (Flowering plants)
- Clade: Eudicots
- Clade: Asterids
- Clade: Lamiids
- Order: Lamiales
- Family: Orobanchaceae
Evolutionary Significance
Orobanchaceae is paramount for understanding the evolution of plant parasitism. Its broad circumscription, uniting photosynthetic hemiparasites with non-photosynthetic holoparasites derived from them, provides a framework for studying the gradual or rapid transitions involved. This includes investigating the genetic changes associated with loss of photosynthesis, development of haustoria, adaptation to specific hosts, and modifications in life history and morphology (like the production of huge numbers of tiny seeds). The family represents a major evolutionary radiation driven by the unique ecological niche of parasitism on other plants.
