Triodanis biflora is a species of flowering plant in the Campanulaceae family, or bellflower family, and is in the genus Triodanis. It is native to the Americas and known commonly as "Small Venus' Looking-Glass". This common name was given because of its similarity to the European plant, Legousia speculum that has seeds with a shiny appearance resembling looking glass.[1] This flower is a small, solitary wildflower with purple bell-shaped petals and an alternate leaf arrangement. It can be found in a diverse range of habitats, especially in disturbed areas across the U.S. and into South America.[2] An important life history strategy of this flower is that it is an annual flower, which means it germinates, flowers, reproduces, and dies all within one growing season. Specifically, it flowers from May to June which impacts its phenology for other processes, pollination type, and reproductive strategies.[6]
Triodanis biflora in a natural habitat, visual of the stem and leaf arrangement, and chasmogamous flower example at the tip of the stem
Habitat
Triodanis biflora can occupy a broad subset of environments; however, it is mainly found in southern regions. Within the United States, the most common regions include Eastern Virginia, Kentucky, Kansas, Arkansas, Oregon, the south central peninsula of Florida.[2] While these are common locations to find this species, it can be found in many other states in the U.S. For example, there have been sightings of Triodanis biflora at higher elevations in the Sonoran Desert in Arizona.[3] This flower can also be found in some parts of Mexico and South America due to their preference for southern geographies and warmer climates.
Being a wildflower, Triodanis biflora can survive in disturbed lands, or areas that have been extensively manipulated by humans, and exist in a wide variety of environmental niches within these geographical locations. Examples of someniches it will occupy are prairies, roadsides, gravel pits, open woods, grassy slopes, and it can even be found growing through the cracks in sidewalks.[4] As a result, this species prefers poor, dry, and sandy soil because it eliminates competition with most other plants that cannot survive in those conditions. Although Triodanis biflora can inhabit disturbed lands, it is not limited to these environments, and can be found in forests, brush, riparian zones, grasslands, and sand.[1] Ultimately, this flower has the ability to survive in a wide range of environments, which be a result of an evolutionary advantage, but is typically local to southern regions and warmer climates.
Anatomy
Close up of dehiscence pore on fruit capsule (located near the apex)
Triodanis biflora is generally a small flower, only reaching heights between 12-30 inches.[1] It is very closely related to other flowers in the Triodanis family, especially Triodanis perfoliata. However, this species differs from Triodanis perfoliata because the pore that releases its seeds from the fruit is located much higher, on the apex, of the capsule compared to the pore being in the middle of the capsule on Triodanis perfoliata.[5] This oval to round shaped pore is referred to as the dehiscence pore and it is a defining characteristic of the species due to its alternative placement. The capsule of Triodanis biflora is an elliptic shaped fruit that encases reddish-brown, smooth seeds that are an ellipsoidal to orbicular shape. Each fruit is similarly shaped and sized once the inferior positioned ovary, or female reproductive organ below the sepals and petals, has matured.[6]
Inflorescence
Close up example of a solitary, cosexual chasmogamous flower
There are usually 1-3 flowers per node; however, Triodanis biflora is known for typically having a solitary flower at the tip of the stem.[6] This species displays a cleistogamous floral system which means each individual is composed of cleistogamous flowers that do not open. The cleistogamous flowers are typical of the lower flowers and rely on self-pollination. Alternatively within this floral system, some individuals can have chasmogamous flowers, or flowers that do open. However, this is only seen in one or two inflorescences and is more common for the upper, solitary flower of Triodanis biflora.[5] Cleistogamous flowers are typically created first, as suggested by their lower placement on the stem, chasmogamous flowers will then be produced, and sometimes cleistogamous flowers will be created again afterwards.[8]
This type of floral system is consistent with its life history strategy of being an annual plant because cleistogamous flowers do not require outcrossing for reproduction, which can be a riskier form of reproduction when they only have one growing season. Similar to its dehiscence pore, the low number of chasmogamous flowers are a defining characteristic that separates Triodanis biflora from other species in its family. However, variation does exist across this species, and some South American populations in Paraguay, Brazil, and Argentina have been reported to have more than two chasmogamous flowers.[5]
Example of cleistogamous flower on Triodanis biflora
Each flower has 5 petals, giving it radial symmetry, that can be a blue-violet, pink-purple, or lavender color. The upper, open flowers are more showy and each petal is an elliptical shape that is fused together with the other petals at the base and connected to a short tube. Within each individual flower are 5 stamens that have translucent filaments, yellow anthers, 3 stigmas, and a many-ovuled ovary with axile placentation. Pollen is presented to pollinators on a stylar brush, which is a term used to described pollen collecting hairs on the style, after the anthers release their pollen before the stigma is receptive.[8] The styles are 5mm long and have a 3-lobed tip. These can be basal or apical, where the basal styles are generally white while the apical styles are purple and can be covered in a few small hairs. The sepals form a cup around the flower and have narrow triangular lobes that make a sharp tip at the end.[6]
Leaves
Full length stem with cleistogamous flowers and solitary chasmogamous at the tip of the stemLeaves of Triodanis biflora; alternate pattern
The leaves throughout most of the stem are in an alternate pattern and are directly attached to the stem without a stalk, known as a sessile leaf. However, there can occasionally be a pair of larger, opposite leaves near the top of the stem. The leaf sizes can range from 5–20mm long and 2–10mm wide, where the leaf size gradually decreases moving up the stem. Each leaf itself is cordate, or heart-shaped, and has a wavy margin. They can be considered to have a simple leaf type which can be classified further as having an entire or serrate leaf margin. These descriptions mean they can have detailed toothing on the exterior of the leaf or a smooth, curved outline. Similar to the other shaped features on this flower, the leaves are elliptic to narrowly ovate that are pointed at the tip. Small hairs can be found along the undersides of the leaves that are usually inconspicuous and along the veins. The stems and leaves contain a milky sap that may be suggestive of a defense mechanism against herbivores.[6]
Pollination
Due to its cleistogamous floral system, Triodanis biflora does not have very extensive pollinator relationships. Only the solitary, or occasionally multiple open flowers, at the tip of the stem will require pollination to facilitate outcrossing. The other cleistogamous flowers reproduce without any external pollination because the pollen from the stamens will reach the stigma of the female reproductive structure in the same flower. The flowers on this species are generally cosexual, for both the cleistogamous and chasmogamous flowers, which allows selfing to occur in the cleistogamous flowers. Similarly, because the chasmogamous flowers are cosexual, it also gives them an opportunity to reproduce through selfing instead of outcrossing, which is beneficial as an annual plant.[7] The stems of the plant were not found to elicit any sort of aromatic compounds, reducing the probability that Triodanis biflora uses its stems as a pollination attraction or defense mechanism.[1]
When pollination does occur, the main vectors include various bees and other small insects. Specifically, these may be plasterer bees, little carpenter bees, bumblebees, flies, small butterflies, and skippers.[1] Generally, if the upper, chasmogamous flowers display a showy and vibrant color, it is in the effort to attract pollinators and facilitate outcrossing. Since this flower is not limited to only outcrossing however, the form and specialized relation with pollinators is not as important compared to other species of flowers or other species in its family. It was experimentally studied that Triodanis biflora received visits from a generalized subset of small bees, that could be eusocial or solitary, and generalist flies such as the Syphrid fly. As a result, this flower is not considered to have a specialized group of pollinators.[8]
Genetics
Hybridization
Triodanis biflora is a member of the Campanulaceae family and the genus Triodanis, where the other relatives of this species include Triodanis perfoliata, Triodanis holzingeri, Triodanis lamprosperma, Triodanis leptocarpa, and Triodanis texana.[9] Across all of these species in this genus, hybridization between T. biflora and T. perfoliata is the most common and there are many similarities between the two flowers. The hybrids created between the cross of these two species are fertile and viable, allowing natural hybrids to exist in the environment.[11] It is also because of this reason that scholars sometimes categorize T. biflora as a variation or even the same species as T. perfoliata.[11]
Phylogeny
Triodanis biflora and Triodanis perfoliata are hypothesized to have diverged from their most recent common ancestor nearly 1.9 million years ago. Between these two species, T. perfoliata is thought to have diverged from T. biflora 3.3 million years ago. 2.8% of the alleles in T. perfoliata can also be traced back as being inherited by T. biflora which implies there is strong gene flow between the two species. [10]
Gene expression and phenology
A key feature of Triodanis biflora is that the genes for seed type express a smooth, lustrous seed coat. This defining characteristic has evolved across the Triodanis genus, where this type of seed coat developed from the ancestral trait of matriculate, or rough, seed coats.[9] Due to populations of T. biflora existing in more southern regions, this phenotype has expanded from being traditionally in Northern locations to now being found in the south.[5] In addition to being predominately found in southern regions, T. biflora has different phenology patterns compared to other Triodanis species. For instance, the flowering time for T. biflora is typically earlier than its relatives because of the local abiotic and biotic influences of their environment.[9] The increasing temperatures from climate change may also be strongly impact their flowering time as they may have to adapt more rapidly to the changing environmental conditions. However, the fruiting time has remained relatively constant even with temperatures rising. Research has suggested that cleistogamy floral systems may be more sensitive to environmental factors such as light and resource availability which is important to consider for the future implications of how the environment and genetics influence T. biflora expression.[9]
Tillotson-Chavez, Kate; Lukavsky, Sarah; Weber, Jennifer (2024-03-24). "Assessing Pollinator Assembly And Efficacy Across Species Ranges In The Genus Triodanis (Campanulaceae)". Journal of Pollination Ecology. 36: 47–60. doi:10.26786/1920-7603(2024)767. ISSN 1920-7603.
Berg, Colette S.; Burr, Naomi; Weber, Jennifer J. (2024). "Phenological Advancement in the North American Wildflower Genus Triodanis (Campanulaceae)". BioOne Digital Library. 125 (1001): 1–18. doi:10.3119/22-18.
Simmonds, Taylor; Geralds, BreAnn N.; Cellinese, Nico; Crowl, Andrew A.; Brown, Jason L.; Weber, Jennifer (2024). "Paleo-climates and past introgression explain the spatio-temporal distribution of genetic structure in Triodanis perfoliata". Frontiers of Biogeography. 16 (2). doi:10.21425/F5FBG61656.
Zhan, Alyssa Wu; Linhart, Jolie E.; McConnell, Keegan; Simmonds, Taylor; Weber, Jennifer; Neubig, Kurt M. (2025-06-23). "A simple, PCR-based method for the identification of Triodanis (Campanulaceae) species and their hybrids". Journal of the Botanical Research Institute of Texas. 19 (2): 71–76. doi:10.17348/jbrit.v19.i2.1398. ISSN 2644-1608.
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