Conocephalum
Conocephalum is a genus of complex thalloid liverworts in the order Marchantiales and is the only extant genus in the family Conocephalaceae. Some species of Conocephalum are assigned to the Conocephalum conicum complex, which includes several cryptic species. Conocephalum species are large liverworts with distinct patterns on the upper thallus, giving the appearance of snakeskin. The species Conocephalum conicum is named for its cone-shaped reproductive structures, called archegoniophores. Common names include snakeskin liverwort, great scented liverwort and cat-tongue liverwort.
Species of Conocephalum are relatively common and widely distributed throughout North America, Europe and East Asia. Conocephalum often occurs in moist and shaded habitats and are also found in open woodlands, sandy banks, wet rocks and cliffs and moist soils. Species of Conocephalum are also often associated with calcareous substrates.
Conocephalum has a relatively large thallus with irregular branching. Plants grow by overlapping lobes, often creating large mats. Regarding reproduction, species of Conocephalum are dioicous. Species of Conocephalum produce different terpenes and aromatic compounds. Considerable variation in species have been identified based on chemical composition and different species have been identified based on their unique compounds. A unique sesquiterpene alcohol known as conocephalenol was identified and extracted form ''C. conicum.''
Classification and taxonomy
Some species of Conocephalum are placed in the Conocephalum conicum complex, which includes several cryptic species. Consequently, it has been challenging to identify the exact number of species in this genus. 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.Molecular research has indicated that Conocephalum comprises a complex of six cryptic species. In 2005 C. conicum cryptic species S was described as a separate species, C. salebrosum. Conocephalum salebrosum has a wider distribution and is present in North America, in contrast to C. conicum. More recent examinations of the Conocephalum conicum complex in Japan and Taiwan have identified three new species within Conocephalum, C. orientalis, C. purpureorubum and C. toyotae, which were formerly described as C. conicum J, F, and R respectively.
Species
- Conocephalum conicum complex – includes several cryptic species:
- * Conocephalum conicum
- * Conocephalum salebrosum
- * Conocephalum orientalis
- * Conocephalum purpureorubum
- * Conocephalum toyotae
- ''Conocephalum supradecompositum''
Distribution
The species C. supradecompositum is more restricted in its distribution and is mainly found in China and Japan. Regarding the most recently described species of Conocephalum, C. purpureorubum has been observed in Japan, China, Taiwan and South Korea and C. orientalis has been found in Japan and Taiwan.
Habitat
Species of Conocephalum often occur in moist and shaded habitats. Conocephalum species also grow in specialized micro-habitats near both running and standing water. Conocephalum conicum is often found in open woodlands, sandy banks, wet rocks and cliffs and moist soils. Both C. conicum and C. salebrosum are strongly associated with calcareous substrates. It has also been suggested that C. salebrosum is likely more tolerant of desiccation than C. conicum.Morphology
Conocephalum conicum and C. salebrosum share some similarities in morphological characteristics, in addition to having their own unique traits which help distinguish the two species.Gametophyte
The vegetative structure of Conocephalum is a thallus which has the appearance of a flattened body of plant tissue. The thallus is irregularly branched and relatively large, reaching lengths of roughly 20-24 cm. In contrast to C. conicum and C. salebrosum, the thallus of C. supradecompositum is relatively small, measuring 2-3 cm long. The thallus grows by developing lobes which wither away as the plant matures. Plants of C. salebrosum often grow by overlapping lobes, sometimes creating large mats.Species of Conocephalum have a thallus that is either dull in appearance, such as C. salebrosum, or distinctly shiny, such as C. conicum. The upper surface of the thallus has characteristic hexagonal outlines formed by shallow grooves around each air chamber. Photosynthetic tissue and chloroplasts are located within the air chambers. In the middle of each air chamber is a white-ringed pore. The upper walls of large air chambers are often visible on the surface of the thallus. The air chamber pore remains open, in contrast to the stomata of vascular plants where the pores can open and close.
The underside of the thallus has both rhizoids and scales. The scales are purple in colour and are arranged along the middle of the underside of the thallus. Rhizoids are also present on the underside of the thallus. There are two types of rhizoids, both long smooth rhizoids and short pegged rhizoids. 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 in mosses.
Complex oil bodies
Liverworts cells often contain complex oil bodies. The oil bodies are intracellular organelles bounded by a single membrane. The oil bodies have been known to contain a variety of unique phytochemicals, such as terpenes and flavonoids. The function of oil bodies is still poorly understood. It has been suggested that oil bodies might function as a deterrent to herbivory or could protect from cold temperatures or harmful ultraviolet radiation.Sporophyte
The sporophyte consists of an unbranched stalk called a seta, which bears a terminal spore capsule called a sporangium. The sporangia of Conocephalum are borne beneath stalked gametophytic structures called archegoniophores. In contrast to mosses, the sporophyte matures before the seta elongates. Unlike mosses, liverwort sporophytes lack stomata, a columella and peristome teeth.Life cycle
The life cycles of liverworts involves alternating haploid gametophyte and diploid sporophyte generations. The gametophyte generation is more dominant, while the sporophyte generation is relatively short-lived. The gametophyte produces haploid gametes, egg and sperm, which fuse to form a diploid zygote. The zygote then develops into a sporophyte which ultimately produces haploid spores through meiosis. The sporophyte requires nutrients supplied by the gametophyte to sustain growth and development.The life cycle of Marchantia liverworts also applies to Conocephalum, with the exception that Conocephalum lacks a stalked antheridiophore and instead has small flat antheridial heads on the surface of the thallus.
Reproduction
Liverworts reproduce through both sexual and asexual reproduction. In natural populations, the high genetic variation observed suggests that sexual reproduction might dominate. Species of Conocephalum are dioicous, meaning that the male and female reproductive structures are produced on separate plants.Sexual reproduction
In Conocephalum 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 and at maturity the sperm is released into the air. 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 the archegonia beneath.Gametophytes produce eggs and sperm in the archegonia and antheridia, respectively. Fertilization occurs when the sperm reach the egg within the archegonia of a female plant. Once fertilization occurs, the ovule within an archegonium develops into a sporophyte. Mature sporangia on the underside of the receptacle resemble black capsules. These capsules split open to release both spores and elaters, which are dispersed mainly by wind. The elaters function to propel spores during dispersal.
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. 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.
Asexual reproduction
The production of gemmae is a common method of asexual reproduction in liverworts. 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. In C. conicum, gemmae are located on the lower layers of the thallus and are released as the thallus degrades. 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. 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.
Conocephalum species are perennial, meaning that they can overwinter and produce new growth in the spring. These new buds are covered and protected by small scales.