Hornworts – Morphology, Life Cycle, Importance, Examples

What are Hornworts?

• Hornworts, classified under the division Anthocerotophyta, represent a unique group of non-vascular plants known for their distinctive horn-like sporophytes. The term “hornwort” derives from the sporophyte’s elongated, cylindrical structure, which resembles a horn.
• Hornworts are non-vascular embryophytes that exhibit a gametophyte-dominant life cycle. In this cycle, the gametophyte stage, which is the green, flattened part of the plant, carries a single set of genetic information. This stage typically manifests as a thin, ribbon-like thallus ranging from one to five centimeters in diameter.
• Hornworts are found globally but thrive in damp or humid environments. While some species are common as small weeds in gardens and cultivated fields, larger species such as Dendroceros often grow on tree bark in tropical and subtropical regions. Despite over 300 species names being published, the actual number of hornwort species is estimated to be between 100 and 150.
• The hornwort thallus exhibits several distinctive features. It generally has only one chloroplast per cell—a condition known as monoplastidy. However, in some genera, such as Megaceros, multiple chloroplasts (polyplastidy) are present. The presence of a pyrenoid, a structure that enhances photosynthesis by storing food and facilitating carbon fixation, varies. Pyrenoids are found in about half of the species and have evolved independently multiple times.
• A notable characteristic of hornworts is their interaction with cyanobacteria. The thallus can develop mucilage-filled cavities, which attract and support colonies of cyanobacteria like Nostoc. These symbiotic relationships impart a blue-green color to the hornwort and enhance nitrogen fixation. Some species, however, lack these symbiotic bacteria and do not have the characteristic slime pores.
• The sporophyte, which develops from an archegonium embedded within the gametophyte, grows from a persistent basal meristem, distinguishing it from the apical growth seen in mosses and the intercalary growth in liverworts. Most hornwort species have true stomata on their sporophyte, though exceptions include Folioceros incurvus, Notothylas, and some genera such as Megaceros, Nothoceros, and Dendroceros. Unlike other bryophytes, hornwort sporophytes have a long lifespan and maintain photosynthetic activity.
• The mature sporophyte consists of a multicellular outer layer, a central rod-like columella, and a tissue layer that produces spores and pseudo-elaters. Pseudo-elaters are multi-cellular structures with helical thickenings that aid in spore dispersal through twisting mechanisms when dry. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 μm in diameter, and are characterized by a distinctive Y-shaped ridge on their proximal surface and a spiny distal surface.

Habitat of Hornworts

Hornworts, a group of non-vascular plants, exhibit specific habitat preferences and distribution patterns. Their environmental requirements and widespread presence are outlined as follows:

• Moist and Shady Environments
  Hornworts thrive in moist, shady locations. They are adapted to environments where water availability is high, crucial for their reproductive processes. This moisture is essential for the successful development and dispersal of their gametes.
• Geographical Distribution
  Hornworts are primarily distributed in tropical and warm temperate regions. These areas provide the humid conditions necessary for their growth and reproduction. The widespread occurrence of hornworts in these climates highlights their adaptation to warm, moist environments.
• Varied Substrates
  Hornworts exhibit flexibility in their growth substrates, including:
  • Epiphytic: Some species grow on the bark of trees. These epiphytic hornworts benefit from the moist microhabitats found on tree surfaces.
  • Terrestrial: Other species are found in damp soils, where they contribute to the local flora by colonizing moist ground areas.
  • Lithophytic: A few species can grow on rocks, indicating their ability to adapt to less conventional habitats.
• Species Diversity
  Approximately 300 species of hornworts have been documented. This diversity reflects their adaptability to various moist environments across different regions.

Characteristics Features of Hornworts

Hornworts exhibit several distinct characteristics that highlight their unique biological and ecological adaptations. These features are outlined as follows:

• Gametophyte Structure
  • Thalloid Gametophyte: The adult plant is an independent gametophyte with a thalloid structure. This thallus is dorsiventrally flattened, providing a broad surface area for photosynthesis and nutrient absorption.
  • Attachment: The thallus attaches to the substrate through smooth-walled rhizoids. Unlike other bryophytes, hornworts lack tuberculate rhizoids and ventral scales.
• Internal Organization
  • Uniform Cellular Structure: Internally, the thallus of hornworts does not exhibit differentiation into distinct zones. Instead, it maintains a uniform cellular organization throughout its structure.
• Reproductive Characteristics
  • Symbiotic Relationship: Hornworts form a symbiotic association with cyanobacteria. These bacteria reside in internal cavities of the thallus, aiding in nitrogen fixation and contributing to the hornwort’s nutrient acquisition.
  • Sexual Reproduction: The thallus may be monoecious or dioecious. Sex organs, including antheridia (male) and archegonia (female), are embedded within the dorsal surface of the thallus. These reproductive structures are sunken into the thallus, facilitating internal fertilization.
• Sporophyte Structure
  • Differentiated Components: The sporophyte is composed of three main parts:
    • Foot: Anchors the sporophyte to the gametophyte.
    • Intercalary Meristematic Zone: Responsible for growth and elongation of the sporophyte.
    • Capsule: Contains the spore-producing tissue.
  • Dependency: The sporophyte is partially independent of the gametophyte, although it relies on the gametophyte for nutrients and support.

Morphology of Hornworts

Hornworts display distinctive morphological features that differentiate them from other bryophytes. Their structural components and organization are described as follows:

• Gametophyte Structure
  • Thallus Form: The gametophyte, or plant body, of hornworts is characterized by a lobed thallus. This thallus is generally simple, lacking dichotomous branching. The thallus can either possess a midrib or be without it.
  • Attachment: The thallus adheres to the substrate through simple, unicellular, smooth-walled rhizoids. These rhizoids function in anchoring the plant but do not have the complex structures seen in some other bryophytes, such as tuberculate rhizoids or ventral scales.
• Sex Organs
  • Location: The reproductive structures, including antheridia (male organs) and archegonia (female organs), are embedded within the thallus. This internal placement is a notable feature of hornworts and distinguishes them from some other plant groups.
• Absence of Vascular System
  • Similar to liverworts, hornworts lack a vascular system. This means they do not have specialized tissues for transporting water and nutrients, relying instead on diffusion across their tissues.
• Sporophyte Structure
  • Elongated Sporophyte: The sporophyte, which arises from the gametophyte, is an elongated structure. It attaches directly to the gametophytic thallus. This sporophyte structure is unique for its persistent and photosynthetic nature.

Classification of Hornworts

Hornworts are classified within the division Anthocerophyta. Their classification is organized into two primary classes, each with distinct orders. This system helps in understanding their diversity and relationships within the bryophytes.

• Division Anthocerophyta
  This is the overarching classification for hornworts, distinguishing them from other bryophytes.
• Class Leiosporocerotopsida
  • Order Leiosporocerotales: This is the sole order within the class Leiosporocerotopsida. It represents a specific group of hornworts with distinct morphological and reproductive traits.
• Class Anthocerotopsida
  • Order Anthocerotales: This order includes a variety of hornwort species, characterized by their unique sporophyte structures and gametophyte features.
  • Order Notothyladales: This order encompasses species with distinctive characteristics that differentiate them from those in the Anthocerotales.

Reproduction in Hornworts

Asexual Reproduction

Hornworts exhibit several methods of asexual reproduction, allowing them to propagate effectively in their habitats. The primary mechanisms include:

1. Fragmentation
   • Process: Fragmentation occurs when cells in the basal region of the thallus die and disorganize. As the decay progresses to the branching area, the thallus lobes become separated.
   • Outcome: Each separated lobe can develop into a new individual. This method of reproduction is observed in species such as Anthoceros.
2. Gemmae Formation
   • Process: Gemmae are small vegetative propagules that form on short stalks or along the margins of the thallus.
   • Outcome: These gemmae detach from the parent thallus and give rise to new thalli. This type of reproduction has been reported in species like A. glandulosus and A. formosae.
3. Tubers
   • Process: In response to drought, some species of Anthoceros develop marginal thickenings known as tubers.
   • Outcome: When environmental conditions improve, these tubers can produce new individuals. Tubers are found in species such as A. himalayensis and A. tuberosus.
4. Persistent Growing Apices
   • Process: During unfavorable conditions, most of the plant dries up, leaving only some cells at the growing points of the thallus lobes.
   • Outcome: These persistent apices can resume growth when conditions become favorable again. This type of reproduction is seen in species like A. pearsoni and A. fusiformis.
5. Apospory
   • Process: Apospory involves the production of a gametophyte thallus directly from vegetative cells of the sporogonium.
   • Outcome: This process enables the formation of new gametophytes without the typical alternation of generations. Some species of Anthoceros exhibit this reproductive strategy.

Sexual Reproduction

Sexual reproduction in hornworts involves the formation of specialized structures known as antheridia and archegonia, which facilitate gamete production and fertilization.

1. Antheridia
   • Location and Structure: Antheridia are located on the upper surface of the thallus, either singly or in groups. Each antheridium is an ovoid, pouch-like structure mounted on a long, slender, multicellular stalk.
   • Function: The antheridium consists of a jacket layer, or antheridial wall, enclosing a mass of androcytes. These androcytes mature into male gametes, or sperm. Upon maturity, the antheridia release biflagellate sperm into the surrounding water.
   • Mechanism: The sperm are capable of swimming towards the archegonia for fertilization. This release of sperm is triggered when the antheridia reach full maturity.
2. Archegonia
   • Location and Structure: Archegonia are embedded within the thallus on its upper surface. Each archegonium is composed of several parts: neck canal cells, ventral canal cells, and an egg. Notably, the archegonium lacks a sterile jacket, except for the rosette cells located at the distal end, which are referred to as cover cells.
   • Function: The archegonia serve as the site for fertilization. The egg, enclosed within the archegonium, receives the sperm from the antheridia. Fertilization leads to the development of the sporophyte.
3. Monoecious and Dioecious Species
   • Monoecious Species: In monoecious species, both antheridia and archegonia are present on the same thallus, allowing for self-fertilization or cross-fertilization within the same individual.
   • Dioecious Species: In dioecious species, separate individuals produce either antheridia or archegonia, necessitating cross-fertilization between different plants.
4. Photoperiod Sensitivity
   • Development Influence: According to Bell and Woodcock (1968), the development of sex organs in some hornwort genera, such as Anthoceros, can be influenced by photoperiod. This indicates that light conditions play a role in the timing and formation of sexual reproductive structures.

Hornwort development

1. Gametophyte Development

Hornwort gametophytes exhibit a structured development involving multiple apical cells. Key features include:

• Apical Cell Structure: Hornwort gametophytes possess wedge-shaped apical cells with four cutting faces. These cells divide and give rise to the flattened, orbicular thallus.
  • Dorsal Derivatives: Form the gametangia and upper thallus.
  • Ventral Derivatives: Develop into rhizoids, Nostoc cavities, and the lower thallus region.
  • Lateral Derivatives: Contribute to tissue that merges adjacent notches, forming a rosette pattern.
• Comparison with Other Bryophytes:
  • Liverworts (e.g., Marchantia polymorpha): Exhibit a thallus with fewer apical notches, leading to a strap-shaped gametophyte with dichotomous branching.
  • Mosses (e.g., Physcomitrium patens): Develop filamentous protonemata from spores, which then form leafy gametophores.
• Genetic Regulation:
  • CLE Genes: Found in all land plants, including hornworts. These genes, such as CLE1 and CLE2 in Marchantia polymorpha, regulate stem cell maintenance in the gametophyte.
  • RSL and LRL Genes: Control rhizoid development and are crucial for maintaining stem cell activity.

2. Rhizoids

Rhizoids in hornworts function similarly to root hairs in tracheophytes, facilitating nutrient absorption and anchorage. Key aspects include:

• Structure: Hornwort rhizoids are typically unbranched, except in some species where they branch at the tip.
• Genetic Regulation:
  • RSL and LRL Genes: These genes regulate rhizoid development across various plant groups. In hornworts, a single RSL homologue and two LRL homologues likely control rhizoid formation.

3. Embryo Development

Hornwort embryo development differs notably from other bryophytes:

• Zygote Division: The first division of the zygote occurs parallel to the archegonium’s longitudinal axis, contrasting with the transversal division seen in other bryophytes.
• Embryonic Tiers:
  • First Tier: Produces the foot of the sporophyte.
  • Second Tier: Forms the basal meristem.
  • Third Tier: Develops into the tip of the sporophyte capsule.
• Genetic Factors:
  • LFY Genes: In Physcomitrella patens, these genes are crucial for the first zygote division. Hornworts like Anthoceros agrestis also possess LFY homologues, though their role remains to be fully elucidated.
  • WOX Genes: Regulate stem cell maintenance and embryo development. Hornworts have WOX13 clade members that contribute to sporophyte development and may have diverse roles across plant stages.

4. Sporophyte Development

Hornwort sporophytes are distinctive in their developmental approach:

• Growth: Initiates from a basal meristem, producing sporophytic tissue and eventually spores and pseudoelaters.
• Structure:
  • Columella: Central tissue aiding in spore dispersal.
  • Archesporium: Differentiates into sporogenous tissue and pseudoelaters.
  • Spore Development: Involves meiosis and the formation of a three-layered spore wall.
• Nutrient Transfer:
  • Involucre: Surrounds the sporophyte, connecting it to the gametophyte through the foot, which has specialized cells for nutrient transfer.

5. Stomata Development in Hornworts

Stomata are crucial for gas exchange in plants, and their development in hornworts offers unique insights into plant evolution and adaptation. Here’s a detailed overview of stomatal development in hornworts:

• Basic Structure and Function
  • Definition: Stomata are specialized structures on plant surfaces that facilitate gas exchange. They consist of two guard cells that surround a pore.
  • Function: The primary role of stomata is to regulate the exchange of gases, particularly oxygen and carbon dioxide, between the plant and its environment.
• Stomatal Development in Hornworts
  • Initial Formation: In hornworts, the development of stomata begins during the early stages of sporophyte development. Hornworts have a unique stomatal arrangement compared to other bryophytes.
  • Single Guard Cells: Unlike other land plants, hornworts typically have a single guard cell per stoma. This is a distinct feature that sets them apart from tracheophytes and other bryophytes, which generally possess paired guard cells.
• Genetic Control and Mechanisms
  • Key Genes: Research into stomatal development in hornworts has identified several key genes involved in this process, including those associated with cell differentiation and patterning.
  • Gene Expression: The expression patterns of these genes suggest that stomatal development in hornworts is regulated by a combination of genetic and environmental factors. Genes involved in guard cell formation and differentiation play a critical role.
• Comparison with Other Bryophytes
  • Mosses and Liverworts: In contrast to hornworts, mosses and liverworts generally have more complex stomatal structures. For example, mosses often have more than one guard cell per stoma.
  • Evolutionary Significance: The simpler stomatal structure in hornworts provides insights into the evolutionary origins of stomata. It suggests that stomatal complexity increased over time as plants adapted to different environmental conditions.
• Functional Implications
  • Gas Exchange Efficiency: The unique stomatal arrangement in hornworts affects their gas exchange efficiency. Single guard cells may limit the rate of gas exchange compared to the paired guard cells found in other plants.
  • Adaptation to Environment: This stomatal feature likely reflects adaptations to specific environmental conditions where hornworts thrive, such as low-moisture habitats.
• Future Research Directions
  • Genetic Studies: Further studies on the genetic control of stomatal development in hornworts could provide more insights into the evolutionary processes that shaped stomatal structures in land plants.
  • Comparative Analysis: Comparing stomatal development across different plant groups will enhance our understanding of plant evolution and adaptation mechanisms.

Hornwort chloroplast

The hornwort chloroplast exhibits several unique features that distinguish it from those in other land plants. These differences are key to understanding both its functional role in the plant and its evolutionary significance.

Monoplastidy

• Single or Few Chloroplasts: Hornworts typically have one or very few chloroplasts per cell. This monoplastidic condition contrasts with the multiple chloroplasts found in most other land plants.
  • Efficiency and Adaptation: The presence of fewer, larger chloroplasts may enhance photosynthetic efficiency. A single large chloroplast maximizes surface area relative to volume, improving light absorption and carbon fixation.
  • Damage Control: Fewer chloroplasts could also allow for better damage control. If one chloroplast is compromised, others can compensate, minimizing the impact on photosynthesis.
• Polyplastidy Evolution: The transition from monoplastidy to polyplastidy in land plants is complex and not fully understood.
  • Peptidoglycan Layer Loss: The loss of the peptidoglycan layer in chloroplasts is hypothesized to play a role in this transition, although it is not the sole factor, as seen in other plant groups.
• Plastid Division: In hornworts, chloroplast division is closely tied to nuclear division during both mitosis and meiosis.
  • Molecular Mechanisms: Key genes involved in plastid division include FtsZ1, FtsZ2, and ARC6. The absence of certain genes like FtsZ2 in some hornwort species may relate to their monoplastidic nature.

The Pyrenoid

• Unique Structure: Hornworts are unique among land plants for having pyrenoids, which are absent in most other land plant groups.
  • Composition: The pyrenoid is a proteinaceous structure within the chloroplast, primarily composed of Rubisco, the enzyme crucial for carbon fixation.
  • Function in Carbon Concentration: In aquatic environments, the pyrenoid aids in concentrating CO2, enhancing Rubisco efficiency. This mechanism involves a complex carbon-concentrating mechanism (CCM), including proteins like CAH2 and LCI1.
• Grana and Channel Thylakoids: Hornwort chloroplasts contain grana, stacked thylakoids enriched in photosystem II (PSII), and channel thylakoids that connect grana.
  • Structural Differences: Grana in hornworts lack the highly curved end membranes typical of other land plants. Channel thylakoids contribute to space separation within the chloroplast stroma and are enriched in photosystem I (PSI).
• Variability: There is notable anatomical variability among hornwort species. For example, some species have star-shaped plastids with large pyrenoids, while others lack pyrenoids altogether.

RNA Editing

• High RNA Editing Levels: Hornworts exhibit one of the highest levels of chloroplast RNA editing among land plants.
  • Types of Editing: RNA editing involves converting cytidines (C) to uridines (U) and vice versa. This editing alters mRNA sequences, affecting protein synthesis and function.
  • Genetic Evidence: The hornwort Anthoceros agrestis shows extensive C-to-U and U-to-C editing sites, with over 1100 C-to-U and 1300 U-to-C sites.
• PPR Proteins: The hornwort genome contains numerous genes for PPR (pentatricopeptide repeat) proteins, which are crucial for RNA processing in chloroplasts.
  • Role in Editing: PPR proteins are involved in regulating RNA editing, with some specific to the unique editing mechanisms in hornworts.

Symbiosis of Hornwort

Hornworts exhibit two notable types of symbiotic relationships: with cyanobacteria and with arbuscular mycorrhizal fungi (AMF).

1. Cyanobacteria Symbiosis

• Role of Cyanobacteria: Cyanobacteria are prokaryotes capable of photosynthesis and nitrogen fixation. They provide usable nitrogen to their host plants. This symbiosis is ancient, with evidence from Aglaophyton major fossils indicating that such relationships have existed for at least 400 million years.
• Occurrence in Hornworts: In hornworts, cyanobacterial endosymbiosis is widespread. Cyanobacteria, primarily from the genus Nostoc, establish endophytic associations with hornworts. This relationship is relatively rare among land plants but is common in hornworts and a few other plant groups like liverworts and cycads.
• Research Findings: Studies on Anthoceros punctatus and Nostoc punctiforme have identified approximately 40 candidate genes involved in this symbiosis. These include receptor kinases, transcription factors, and transporters, which may be critical for maintaining the symbiotic relationship.

2. Mucilage Cleft

• Structure and Function: Mucilage clefts are specialized two-celled epidermal structures found in hornwort gametophytes. They provide an entry point for Nostoc cyanobacteria and lead to a mucilage-filled cavity. Unlike stomata in sporophytes, mucilage clefts lack specialized guard cells and do not have cell wall ledges.
• Symbiotic Process: When Nostoc cyanobacteria enter the mucilage cleft, the surrounding epidermal cells expand and eventually close the opening. This results in the formation of a colony where cyanobacterial and hornwort cells intermingle. This structure and process are also observed in fossil records, such as Aglaophyton major.
• Genetic Insights: The A. agrestis genome contains an EPF-like gene from the EPFL4-6 clade, expressed in gametophytes. This gene may play a role in mucilage cleft formation, potentially influencing pore development and mucilage production.

3. Arbuscular Mycorrhizal Fungi (AMF)

• Importance of AMF: AMF symbiosis is crucial for plant colonization of terrestrial environments. These fungi enhance nutrient uptake, particularly phosphorus, and are prevalent across most land plants.
• Hornwort Associations: In hornworts like Anthoceros agrestis, the fungal endophytes belong to Mucoromycotina and/or Glomeromycota. These fungi facilitate nutrient acquisition and support the plant’s growth.
• Genetic Basis: Key genes regulating AMF symbiosis in angiosperms have orthologues in the hornwort genome. This suggests that the mechanisms controlling AMF interactions are conserved across different plant groups, including hornworts.

Symbiosis of Hornwort

Economic Importance of Hornworts

Hornworts, belonging to the class Anthocerotopsida, offer several economic benefits. Their contributions span various ecological and practical applications. Here is an overview:

1. Soil Formation and Stabilization

• Rock Breakdown: Hornworts contribute to soil formation through the breakdown of rocks. Their rhizoids anchor into substrates, facilitating rock weathering.
• Erosion Prevention: By stabilizing soil, hornworts help prevent soil erosion. Their presence in ecosystems enhances soil structure and reduces surface runoff.

2. Nutrient Cycling

• Organic Matter Decomposition: Hornworts play a crucial role in nutrient cycling. They decompose organic matter, returning essential nutrients to the soil.
• Nutrient Availability: The decomposition process makes nutrients available for uptake by other plants, promoting ecosystem productivity.

3. Bioindicators

• Environmental Sensitivity: Hornworts are sensitive to changes in their environment. They respond to pollutants and alterations in habitat conditions.
• Ecosystem Monitoring: Due to their sensitivity, hornworts serve as effective bioindicators. They help monitor ecosystem health and detect environmental pollution.

4. Phytochemicals and Medicines

• Bioactive Compounds: Certain hornwort species contain unique bioactive compounds with potential medicinal properties.
• Medicinal Uses: Species such as Anthoceros are utilized for their medicinal properties, demonstrating the pharmacological potential of hornworts.

5. Genetic Research

• Simple Structure: Hornworts have a relatively simple structure, making them valuable for genetic research.
• Developmental Studies: Their straightforward lifecycle aids in understanding plant development and evolutionary processes.

6. Bioremediation

• Contaminant Removal: Hornworts can remove or neutralize contaminants in soil and water through bioremediation.
• Aquarium Use: In aquariums, hornworts are used for both decorative purposes and to enhance water quality by absorbing pollutants.

Examples of Hornworts

1. Anthoceros agrestis
   • Description: Known for its distinctive horn-like sporophyte structure. Often studied for its symbiotic relationships and life cycle.
2. Anthoceros punctatus
   • Description: Recognizable for its thallus colonized by cyanobacteria. It has been a model organism in research on cyanobacterial symbiosis.
3. Phaeoceros carolinianus
   • Description: Noted for its interaction with arbuscular mycorrhizal fungi and its mucilage clefts.
4. Dendroceros crispus
   • Description: This hornwort has a distinctive wavy thallus and is found in tropical and subtropical regions.
5. Megaceros aenigmaticus
   • Description: Features large, horn-like sporophytes and is often used in studies of hornwort morphology.
6. Notothylas javanica
   • Description: Known for its unique reproductive structures and adaptation to specific ecological niches.

References

• Frangedakis, E., Shimamura, M., Villarreal, J.C., Li, F.-W., Tomaselli, M., Waller, M., Sakakibara, K., Renzaglia, K.S. and Szövényi, P. (2021), The hornworts: morphology, evolution and development. New Phytol, 229: 735-754. https://doi.org/10.1111/nph.16874
• Villarreal, J. C., & Renner, S. S. (2014). A review of molecular-clock calibrations and substitution rates in liverworts, mosses, and hornworts, and a timeframe for a taxonomically cleaned-up genus Nothoceros. Molecular Phylogenetics and Evolution, 78, 25-35.
• Ligrone, R., Duckett, J. G., & Renzaglia, K. S. (2012). Major transitions in the evolution of early land plants: a bryological perspective. Annals of Botany, 109(5), 851-871.
• Szövényi, P., Frangedakis, E., Ricca, M., Quandt, D., Wicke, S., & Langdale, J. A. (2015). Establishment of Anthoceros agrestis as a model species for studying the biology of hornworts. BMC Plant Biology, 15(1), 98.
• Li, F. W., Nishiyama, T., Waller, M., Frangedakis, E., Keller, J., Li, Z., … & Szövényi, P. (2020). Anthoceros genomes illuminate the origin of land plants and the unique biology of hornworts. Nature Plants, 6(3), 259-272.
• https://plants.usda.gov/home/classification/7206