| Conocephalum | |
|---|---|
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| Conocephalum conicum | |
Scientific classification | |
| Kingdom: | Plantae |
| Division: | Marchantiophyta |
| Class: | Marchantiopsida |
| Order: | Marchantiales |
| Family: | Conocephalaceae Müll.Frib. ex Grolle |
| Genus: | Conocephalum Hill, 1773, corr. Wiggers 1780, >nom. et orth. cons. |
Conocephalum is agenus of complexthalloidliverworts in theorderMarchantiales and is the only extant genus in thefamilyConocephalaceae.[1][2] Some species ofConocephalum are assigned to theConocephalum conicum complex, which includes severalcryptic species.[1]Conocephalum species are large liverworts with distinct patterns on the upperthallus, giving the appearance of snakeskin.[3] The speciesConocephalum conicum is named for its cone-shaped reproductive structures, calledarchegoniophores.[4] Common names include snakeskin liverwort, great scented liverwort[5][6] and cat-tongue liverwort.[7]
Species ofConocephalum are relatively common[5] and widely distributed throughout North America, Europe and East Asia.[8]Conocephalum often occurs in moist and shaded habitats[1][5][9][10] and are also found in open woodlands, sandy banks, wet rocks and cliffs and moist soils.[4] Species ofConocephalum are also often associated withcalcareous substrates.[9]
Conocephalum has a relatively large thallus[4][5] with irregular branching.[4] Plants grow by overlapping lobes, often creating large mats.[5] Regarding reproduction, species ofConocephalum aredioicous.[11] Species ofConocephalum produce different terpenes and aromatic compounds.[12][13] Considerable variation in species have been identified based on chemical composition[14] and different species have been identified based on their unique compounds. A unique sesquiterpene alcohol known as conocephalenol was identified and extracted formC. conicum.[12][13]
Some species ofConocephalum are placed in theConocephalum conicum complex, which includes severalcryptic species.[1] Consequently, it has been challenging to identify the exact number of species in this genus.[15] Cryptic species refers to a species which demonstrates a genetic difference but lacks morphological differences. Within liverworts, cryptic species are suggested to be related to both geographical disjunction and to reproductive biology in combination with isolation and habitat differentiation.[14]
Molecular research has indicated thatConocephalum comprises a complex of six cryptic species (A, C, F, J, L and S).[14][16] In 2005C. conicum cryptic species S was described as a separate species,C. salebrosum.[1][9][14]Conocephalum salebrosum has a wider distribution and is present in North America, in contrast toC. conicum.[1][3][9] More recent examinations of theConocephalum conicum complex in Japan and Taiwan have identified three new species withinConocephalum, C. orientalis, C. purpureorubum andC. toyotae, which were formerly described asC. conicum J, F, and R respectively.[2]
Species ofConocephalum are distributed throughout North America, Europe and East Asia.[8][11][15]Conocephalum salebrosum displays the widest distribution and is found throughout North America,[5][9][15] Europe and Asia.[17] In North America,C. salebrosum occurs throughout Canada and parts of the United States and has also been reported from Russia.[9] In contrast toC. salebrosum, C. conicum is found throughout Europe[11][15] and has been recorded in Norway, Finland, Great Britain, Ireland, Belgium, France, Germany, Czech Republic, Austria, Hungary, Romania, Italy, Portugal, Spain, Croatia, Bulgaria, Greece, Ukraine, Poland and Russia.[18]
The speciesC. supradecompositum is more restricted in its distribution and is mainly found in China and Japan.[19][20] Regarding the most recently described species ofConocephalum,C. purpureorubum has been observed in Japan, China, Taiwan and South Korea andC. orientalis has been found in Japan and Taiwan.[2][11]
Species ofConocephalum often occur in moist and shaded habitats.[1][5][9][10]Conocephalum species also grow in specialized micro-habitats near both running and standing water.[17]Conocephalum conicum is often found in open woodlands, sandy banks, wet rocks and cliffs and moist soils.[4] BothC. conicum andC. salebrosum are strongly associated withcalcareous substrates.[9] It has also been suggested thatC. salebrosum is likely more tolerant of desiccation thanC. conicum.[9][10][11][17][21]
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Conocephalum conicum andC. salebrosum share some similarities in morphological characteristics, in addition to having their own unique traits which help distinguish the two species.[21]
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The vegetative structure ofConocephalum is a thallus which has the appearance of a flattened body of plant tissue.[5][22] The thallus is irregularly branched[4] and relatively large, reaching lengths of roughly 20-24 cm.[4][5] In contrast toC. conicum andC. salebrosum, the thallus ofC. supradecompositum is relatively small, measuring 2-3 cm long.[20] The thallus grows by developing lobes[22] which wither away as the plant matures.[23] Plants ofC. salebrosum often grow by overlapping lobes, sometimes creating large mats.[5]
Species ofConocephalum have a thallus that is either dull in appearance, such asC. salebrosum, or distinctly shiny, such asC. conicum.[1][11] The upper surface of the thallus has characteristic hexagonal outlines formed by shallow grooves around each air chamber.[4] Photosynthetic tissue and chloroplasts are located within the air chambers.[24] In the middle of each air chamber is a white-ringed pore.[4][23] The upper walls of large air chambers are often visible on the surface of the thallus.[23] The air chamber pore remains open, in contrast to the stomata ofvascular plants where the pores can open and close.[5]
The underside of the thallus has bothrhizoids and scales.[4] The scales are purple in colour and are arranged along the middle of the underside of the thallus.[4] Rhizoids are also present on the underside of the thallus. There are two types of rhizoids, both long smooth rhizoids and short pegged rhizoids.[4][22][25] The short rhizoids are thought to play a role in absorbing water and nutrients. In contrast, the longer rhizoids help anchor the thallus to the underlying substrate. The rhizoids are single-celled, in contrast to the multicellular rhizoids found inmosses.[22]
Liverworts cells often containcomplex oil bodies.[14][22][25][26] The oil bodies are intracellularorganelles bounded by a single membrane.[24][27] The oil bodies have been known to contain a variety of uniquephytochemicals, such asterpenes andflavonoids.[26] The function of oil bodies is still poorly understood.[24][25][27] It has been suggested that oil bodies might function as a deterrent toherbivory or could protect from cold temperatures or harmfulultraviolet radiation.[24][25]
Thesporophyte consists of an unbranched stalk called a seta, which bears a terminal spore capsule called asporangium.[25] The sporangia ofConocephalum are borne beneath stalkedgametophytic structures calledarchegoniophores.[4] In contrast to mosses, the sporophyte matures before the seta elongates.[25][28] Unlike mosses, liverwort sporophytes lack stomata, acolumella andperistome teeth.[24][25]
The life cycles of liverworts involvesalternatinghaploid gametophyte anddiploid sporophyte generations. Thegametophyte generation is more dominant, while the sporophyte generation is relatively short-lived. The gametophyte produces haploidgametes,egg andsperm, which fuse to form a diploidzygote. The zygote then develops into a sporophyte which ultimately produces haploidspores throughmeiosis. The sporophyte requires nutrients supplied by the gametophyte to sustain growth and development.
The life cycle ofMarchantia liverworts also applies toConocephalum, with the exception thatConocephalum lacks a stalkedantheridiophore and instead has small flat antheridial heads on the surface of the thallus.[25]
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Liverworts reproduce through bothsexual andasexual reproduction.[5] In natural populations, the high genetic variation observed suggests that sexual reproduction might dominate. Species ofConocephalum aredioicous,[11] meaning that the male and female reproductive structures are produced on separate plants.[29]
InConocephalum the male and female reproductive parts are embedded in receptacles on separate plants. On male plants, the receptacle is slightly raised, lacking a stalk, and often circular or oval shaped.Antheridia are embedded in the receptacle[5] and at maturity the sperm is released into the air.[5][25][30] In contrast, on female plants the receptacles are dome-shaped, with several drooping lobes at the end of an erect stalk. The receptacles are often described as a tiny umbrella, with thearchegonia beneath.[5]
Gametophytes produce eggs and sperm in the archegonia and antheridia, respectively.[22]Fertilization occurs when the sperm reach the egg within the archegonia of a female plant.[23] Once fertilization occurs, theovule within an archegonium develops into a sporophyte.[22] Mature sporangia on the underside of the receptacle resemble black capsules. These capsules split open to release both spores andelaters,[23] which are dispersed mainly by wind.[22][23] The elaters function to propel spores during dispersal.[28]
Conocephalum elaters are unique and display a wide range of variability in shape, size and number. Often the abundance of elaters within a capsule are 2-3 times more abundant than spores.[31] Elaters form from an initial mother cell which develops into a diploid cell with spiral thickenings. In contrast, spores develop from an initial diploid mother cell that ultimately forms haploid spores by meiosis.[24]
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The production ofgemmae is a common method of asexual reproduction in liverworts.[5][28][32] Gemmae are small packets of tissue consisting of haploid cells that are genetically identical with those of the parent plant. They are dispersed by rainfall and ultimately grow into new individuals.[32] InC. conicum, gemmae are located on the lower layers of the thallus and are released as the thallus degrades.[4] In contrast,C. salebrosum does not produce gemmae.
Vegetative reproduction can occur when a piece of the thallus breaks off and is transported away from the parent plant.[5] The individuals resulting from vegetative reproduction are genetically identical to the parent plant and therefore clonal colonies often exist as either all male or all female.[28]
Conocephalum species areperennial, meaning that they can overwinter and produce new growth in the spring. These new buds are covered and protected by small scales.[11]
Many liverworts produce different terpenes and aromatic compounds.[12][13]Terpenoids and aromatic compounds are often accumulated within the oil bodies of many liverworts, includingConocephalum. WithinConocephalum, considerable variation in species have been identified based on chemical composition and these compounds been used to identify different cryptic species.[14]
Three different groups ofConocephalum were identified baed on their unique primaryvolatile compounds. For example, the compoundcubebol, asesquiterpenealcohol, is characteristic ofC. salebrosum.[14] It has also been noted thatC. supradecompositum has a distinct chemical composition compared toC. conicum as well, mainly that theMonoterpenoid content inC. supradecompositum is much less than observed inC. conicum.[20]
A unique sesquiterpene alcohol known as conocephalenol was identified and extracted fromC. conicum.[12][13] Conocephalenol has a unique chemical skeleton that is characteristic of a sesquiterpenes present inred algae.[33]
Conocephalum can form associations withfungi that are similar to themycorrhizal associations observedvascular plants. Molecular analyses demonstrated thatConocephalum contained fungalendophytes from the group of fungi known as theGlomeromycota.[34]
Conocephalum conicum often colonizes bare soils or rocky substrates, where mineral nutrients can often be limiting. The fungal endophyte establishes a complex relationship withC. conicum, which is characterized by the formation ofarbuscules. These fungi form a highly branchedmycelium outside of the plant which thencolonize the outside of the rhizoids and pass into the gametophyte. The fungal infection induces grown of fungalhypha within the host cells ofC. conicum. This association of the fungal hypha with the hostsplastids suggests that photosynthates produced throughphotosynthesis inC. conicum are likely transferred to the fungus. A similar situation regarding this fungal association has also been identified in the thalloid liverwortPellia epiphylla. Although these associations are common in vascular plants, they have rarely been described in non-vascular plants.[35]
Approximately 25 species ofmoths which areendemic to East Asia associate exclusively withConocephalum.[11] The larval stage ofEpimartyria pardella moths feed onC. conicum.[36][37] In addition, the fungal speciesLoreleia marchantiae also feeds onC. conicum.[11]
The fungalpathogen belonging to the genusPythium has often been isolated from infected rhizoids and thallus ofConocephalum.[11]Bryoscyphus conocephali is another fungal pathogen that has been associated withC. conicum.[38]
Conocephalum conicum has been identified as being tolerant ofheavy metals and has therefore been suggested to have a possible role as abioindicator forpollution.Conocephalum conicum takes upions from both the soil and the atmosphere. Therefore, heavy metals contamination ofC. conicum is related not only to air pollution, but environmental contamination from different sources.[39] Recent research has also examinedC. conicum as a bioindicator forcadmium pollution. Cadmium a toxicmetal and considered the third highest contaminant, aftermercury andlead.Conocephalum conicum was shown to respond to cadmium stress by changing its biological activity. These biological changes could be used asbiomarkers for cadmium pollution.[40]
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Conocephalum conicum has been suggested to have a possible role in the management offood borne disease caused by species ofAspergillus fungi.Aspergillus produces highly potenttoxins,carcinogens, referred to asaflatoxins. Aflatoxins are harmful both plants and animals.Aspergillus can cause disease in many importantcrops, which can ultimately cause disease in humans.Conocephalum has been shown to have a variety of bioactive compounds which promote anti-fungal property againstAspergillus.[26]
Throughout North America, China and India, liverworts such asConocephalum have been used forethnomedical purposes.Conocephalum is known to be important to Bhotia, Raji, Tharus and Boxas tribes inPithoragarh district of Kumaon Himalaya.Conocephalum conicum is used to treat burns, and the extract ofC. conicum is also used in treatinggallstones.Conocephalum has also demonstratedantidote activity againstvenomoussnake bites. The role ofConocephalum regarding modern medicine has yet to be investigated.[26]
Liverworts often contain highlypungent compounds.[41]Conocephalum species often exude anodour that is characteristic ofturpentine. The odour is thought to be related to the presence of monoterpenoids. The compound conocephalenol is widely used in thecosmetic industry for itsodourant properties.[33]
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