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Anthocerophyta

Anthocerophyta

:This is an article about the non-vascular plants. The name Hornwort is also often applied to the aquatic plant Ceratophyllum demersum in the family Ceratophyllaceae Anthocerotaceae
- Anthoceros
- Folioceros
- Leiosporoceros
- Phaeoceros
- Sphaerosporoceros Dendrocerotaceae
- Dendroceros
- Megaceros
- Notoceros Notothyladaceae
- Notothylas Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant. Hornworts may be found world-wide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees.

Description

The plant body of a hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Each cell of the thallus usually contains just one chloroplast per cell. In most species, this chloroplast is fused with other organelles to form a large pyrenoid that both manufactures and stores food. This particular feature is very unusual in land plants, but is common among algae. Many hornworts develop internal mucilage-filled cavities when groups of cells break down. These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants. stomata The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. Hornworts sporophytes are unusual in that the sporophyte grows from a meristem near its base, instead of from its tip the way other plants do. Unlike liverworts, most hornworts have true stomata on the sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros, which do not have stomata. When the sporophyte is mature, it has a multicellular outer layer, a central rod-like columella running up the center, and a layer of tissue in between that produces spores and pseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. They have helical thickenings that change shape in response to drying out, and thereby twist in and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 um in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines.

Life cycle

The life of a hornwort starts from a haploid spore. In most species, there is a single cell inside the spore, and a slender extension of this cell called the germ tube germinates from the proximal side of the spore. The tip of the germ tube divides to form an octant of cells, and the first rhizoid grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid protonema. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates. In either case, the protonema is a transitory stage in the life of a hornwort. photosynthetic From the protonema grows the adult gametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant. When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoicous, with both sex organs on the same plant, but some plants (even within the same species) are dioicous, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells. The biflagellate sperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form a zygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Further divisions produce three basic regions of the sporophyte. At the bottom of the sporophyte (closest to the interior of the gametophyte), is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the foot), is a meristem that will continue to divide and produce new cells for the third region. This third region is the capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores. These are released from the capsule when it splits lengthwise from the tip.

Classification of Hornworts

Hornworts were traditionally considered a class within the Division Bryophyta (bryophytes). However, it now appears that this group is paraphyletic, so the hornworts tend to be given their own division, called Anthocerotophyta. The Bryophyta is now restricted to include only mosses. There is a single class of hornworts, called Anthocerotopsida, or traditionally Anthocerotae. This class includes a single order of hornworts (Anthocerotales) in this classification scheme. In some other classification schemes, a second order Notothyladales (containing only the genus Notothylas) is recognized because of the unique and unusual features present in that group. Among land plants, hornworts appear to be one of the oldest surviving groups. There are only about 100 species known, but new species are still being discovered. The number and names of genera are a current matter of investigation, and several competing classification schemes have been published since 1988. genera

Families and genera

Anthocerotaceae
- Anthoceros
- Folioceros
- Leiosporoceros
- Phaeoceros
- Sphaerosporoceros Dendrocerotaceae
- Dendroceros
- Megaceros
- Notoceros Notothyladaceae
- Notothylas

References

- Chopra, R. N. & Kumra, P. K. (1988). Biology of Bryophytes. New York: John Wiley & Sons. ISBN 0-470-21359-0.
- Grolle, Riclef (1983). "Nomina generica Hepaticarum; references, types and synonymies". Acta Botanica Fennica 121, 1-62.
- Hasegawa, J. (1994). "New classification of Anthocerotae". J. Hattori Bot. Lab 76: 21-34.
- Renzaglia, Karen S. (1978). "A comparative morphology and developmental anatomy of the Anthocerotophyta". J. Hattori Bot. Lab 44: 31-90.
- Renzaglia, Karen S. & Vaughn, Kevin C. (2000). Anatomy, development, and classification of hornworts. In A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology, pp. 1-20. Cambridge: Cambridge University Press. ISBN 0-521-66097-1.
- Schofield, W. B. (1985). Introduction to Bryology. New York: Macmillan.
- Schuster, Rudolf M. (1992). The Hepaticae and Anthocerotae of North America, East of the Hundredth Meridian, Volume VI. Chicago: Field Museum of Natural History.
- Smith, Gilbert M. (1938). Cryptogamic Botany, Volume II: Bryophytes and Pteridophytes. New York: McGraw-Hill Book Company.
- Watson, E. V. (1971). The Structure and Life of Bryophytes (3rd ed.). London: Hutchinson University Library. ISBN 0-09-109301-5.

External links

- [http://www3.uakron.edu/biology/hornworts/hornworts.html Hornwort Web Portal]
- [http://koning.ecsu.ctstateu.edu/Plant_Biology/hornwort.html Hornwort biology information]
- [http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html Anthocerotophyta description and fossil history at UCMP]
- [http://www.natureserve.org/explorer/speciesIndex/Class_Anthocerotopsida_106589_1.htm Hornwort species in the United States and Canada]
- [http://www.peripatus.gen.nz/Taxa/Bryophyta/NZAnthocerotae.html New Zealand Anthocerotae] ---- Category: Plants Category: Bryophytes Category: cryptogams

Ceratophyllaceae

C. demersum
C. submersum Ceratophyllum is a cosmopolitan genus of flowering plants, commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions. They are usually called hornworts, although this name is also used for unrelated plants of the division Anthocerotophyta. Ceratophyllum grows completely submerged, usually though not always floating on the surface. They do not tolerate drought. At intervals along nodes of the stem they produce rings of bright green leaves, which are narrow and often branched. The forked leaves feel brittle and stiff to the touch. The plants have no roots at all, but sometimes they develop modified leaves with a rootlike appearance, which anchor the plant to the bottom. The flowers are small and don't attract the attention, with the male and female flowers on the same plant. Because of their appearance and their high oxygen production, they are often used in freshwater aquaria. Hornwort plants float in great numbers just under the surface. They offer excellent protection to fish-spawn, but also to snails, infected with bilharzia. By screening the lighting, they obstruct algal growth. Hornwort, also known as coontail, is a very easy to grow plant that is tolerant of most water conditions and temperatures. It makes an excellent, tough to kill beginner's plant. Ceratophyllum is unique enough to warrant its own family, the Ceratophyllaceae, and in newer systems its own order, the Ceratophyllales. In APG it has a place of its own, just basal to the eudicots. The division into species is not completely settled. There are two main species:
- Ceratophyllum demersum - Common Hornwort or Rigid Hornwort
- Ceratophyllum submersum - Tropical Hornwort More than 30 other species have been described, but many of them are probably mere variants of these.
eudicots eudicots Category:Magnoliopsida Category:Aquatic plants

Anthoceros

see text. Anthoceros is a genus of hornworts in the family Anthocerotaceae. The genus is global in its distribution. Its name means 'flower horn', and refers to the characteristic horn-shaped sporophytes that all hornworts produce. The dark color of the spores is the easiest way to distinguish Anthoceros from the related genus Phaeoceros, which produces spores that are yellow. The genus is distinguished by having spores that are dark brown to black, a relatively frilly thallus when compared to Phaeoceros, and larger and more internal cavities than Phaeoceros.

References

- Proskauer, J. (1951). "Studies on Anthocerotales. III". Bull. Torrey Bot. Club 78: 331-349.

External links

---- Category: Bryophytes Category: cryptogams

Folioceros

Folioceros appendiculatus
Folioceros assamicus
Folioceros dixitianus
Folioceros fuciformis
Folioceros glandulosus
Folioceros indicus
Folioceros kashyapii
Folioceros mamillisporus
Folioceros mangaloreus
Folioceros paliformis
Folioceros physocladus
Folioceros satpurensis
Folioceros spinisporus
Folioceros udarii
Folioceros vesiculosus Folioceros is a genus of hornworts in the family Anthocerotaceae. The genus is common locally in the tropical and subtropical regions of Asia, growing on moist rocks, in fallow fields, and near waterfalls. It has a yellow-green gametophyte thallus that is crispy and translucent, with short branchings that are almost pinnate. Plants are usually less than a centimeter wide and 3 centimeters long. They may be monoicous or dioicous. The genus Folioceros was formally diagnosed by the botanist D. C. Bharadwaj (1971) and based on the type species F. assamicus. Some features that he cited as distinguishing the genus were:
- Pseudoelaters less than 7 um wide and more than 300 um long.
- Spore ornamentation that is spinose or baculate, rather than reticulate.
- Thallus with large cavities formed by splitting of the internal tissue. The classification system of Hässel de Menendez (1988) places Folioceros in its own family Foliocerotaceae and order Foliocerotales. This classification is based on a cladistic morphological analysis, but has not been generally accepted or supported by additional research in the literature. For the present, Folioceros is usually placed in the Anthocerotaceae.

References

- Asthana, A. K. & Srivastava, S. C. (1991). Indian Hornworts (A Taxonomic Study). J. Cramer: Bryophytorum Bibliotheca, Band 42. ISBN 3-443-62014-0.
- Bharadwaj, D. C. (1971). On Folioceros, A New Genus of Anthocerotales. Geophytology 1 (1): 6-15.
- Hässel de Menendez, G. G. (1988). A proposal for a new classification of the genera within the Anthocerotophyta. J. Hattori Bot. Lab. 64: 71-86.
- Zhu, R. L. & So, M. L. (1996). Mosses and Liverworts of Hong Kong, volume 2. Hong Kong: Heavenly People Depot. ISBN 962-7350-80-X.

External links

[http://bryophytes.plant.siu.edu/folipic.html Images of Folioceros] ---- Category: Bryophytes Category: cryptogams

Phaeoceros

Phaeoceros bulbiculosus (Brot.) Prosk.
Phaeoceros dichotomus
Phaeoceros evanidus
Phaeoceros hallii (Austin) Prosk.
Phaeoceros laevis (L.) Prosk.
Phaeoceros laevis subsp. carolinianus (Michx.) Prosk.
Phaeoceros mohrii (Austin) Hässel
Phaeoceros novazealandicus
Phaeoceros pearsonii (M. Howe) Prosk.
Phaeoceros striatisporus J. Haseg.
Phaeoceros tjipanasanus Phaeoceros is a genus of hornworts in the family Anthocerotaceae. The genus is global in its distribution. Its name means 'yellow horn', and refers to the characteristic yellow spores that the plants produce in the horn-shaped sporophyte. The yellow color of the spores is the easiest way to distinguish Phaeoceros from the related genus Anthoceros, which produces spores that are dark brown to black. Anthoceros The genus Phaeoceros was first recognized in 1951 by Johannes Max Proskauer. The type specimen is Phaeoceros laevis. The genus is distinguished by having yellow spores, different chloroplast structure, relatively less frilliness of the thallus when compared to Anthoceros, and a relative lack of internal cavities in Phaeoceros.

References

- Proskauer, J. (1951). "Studies on Anthocerotales. III". Bull. Torrey Bot. Club 78: 331-349.

External links

- [http://vis-pc.plantbio.ohiou.edu/moss/Tomescu.htm Description and photos] ---- Category: Bryophytes Category: cryptogams

Megaceros (botany)

Megaceros aenigmaticus Schust.
Megaceros alatifrons Steph.
Megaceros denticulatus
Megaceros endiviaefolius (Mont.) Steph.
Megaceros flagellaris (Mitt.) Steph.
Megaceros fuegiensis Steph.
Megaceros gracilis
Megaceros guatemalensis Steph.
Megaceros novae-zelandiae Steph.
Megaceros pallens (Steph.) Steph.
Megaceros pellucidus (Colenso) E.A. Hodgs.
Megaceros salakensis D. Campb.
Megaceros tjibodensis D. Campb.
Megaceros vincentianus (Lehm. & Lindenb.) Campb. Megaceros is a genus of hornworts in the family Dendrocerotaceae. The genus is found in east Asia, Australia,in eastern North America and in tropical America. Its name means 'big horn', and refers both to the exceptionally large size of the gametophyte thallus and to the large horn-shaped, sporophyte that the plants produce. Many species have a branching thallus that is more than two centimeters wide. The gametophytes are monoicous. The genus Megaceros is unusual among hornworts in that the sporophyte does not have stomata, and the spores are green because they contain chloroplasts, as does the related genus Dendroceros. The thallus cells often contain more than one chloroplast, as opposed to other hornwoet genera. The elaters are helical. The genus Megaceros was first recognized in 1907 by D. Campbell.

References

- Hicks, Marie L. (1992). Guide to the Liverworts of North Carolina. Durham: Duke University Press.
- Prihar, N. S. (1961). An Introduction to Embryophyta, Volume I. Allahbad: Central Book Depot.

External links

- [http://www.anbg.gov.au/cpbr/summer-scholarship/2003-projects/vella-megaceros.html Megaceros in Australia]
- [http://www.rsnz.org/publish/nzjb/1995/73.php Name changes in Megaceros] ---- Category: Bryophytes Category: cryptogams

Notothyladaceae

Notothylas anaporata
Notothylas breutellii
Notothylas chaudhurii
Notothylas dissecta
Notothylas flabellata
Notothylas himalayensis
Notothylas indica
Notothylas japonicus
Notothylas javanicus
Notothylas khasiana
Notothylas levieri
Notothylas orbicularis
Notothylas pandei
Notothylas pfleidereri Notothylas is a genus of hornworts in the family Notothyladaceae. The genus is found globally, but is usually overlooked. It is the smallest of all the hornworts, with a yellow-green gametophyte thallus that is seldom more than a centimeter in diameter, and usually much smaller. The genus Notothylas is also unusual among hornworts in that the sporophyte is bullet-shaped and does not grow very large (less than two millimeters). The sporophytes grow outwards rather than upwards, and like Megaceros, there are no stomata on the surface of the sporophyte. The elater cells do not grow helical thickenings. A number of classification systems place Notothylas in its own order Notothyladales (frequently misspelled Notothylales in the literature). This classification is based on the assumption that the unique physical characteristics of the genus reflect an early divergence from other hornworts. However, this assumption has not yet been rigorously tested or supported by either phylogenetic analysis or fossil evidence. For the present, Notothylas is usually placed in the Anthocerotales with all other species of hornworts.

References

- Asthana, A. K. & Srivastava, S. C. (1991). Indian Hornworts (A Taxonomic Study). J. Cramer: Bryophytorum Bibliotheca, Band 42. ISBN 3-443-62014-0.
- Hässel de Menendez, G. G. (1988). A proposal for a new classification of the genera within the Anthocerotophyta. J. Hattori Bot. Lab. 64: 71-86.
- Hicks, Marie L. (1992). Guide to the Liverworts of North Carolina. Durham, NC: Duke University Press. ISBN 0-8223-1175-5.
- Prihar, N. S. (1961). An Introduction to Embryophyta, Volume I. Allahbad: Central Book Depot. ---- Category: Bryophytes Category: cryptogams

Bryophyte

Bryophytes are embryophyte plants ('land plants') that are nevertheless non-vascular: they have tissues and enclosed reproductive systems, but they lack vascular tissue that circulates liquids. They neither flower nor produce seeds, reproducing via spores.

Bryophyte classification

spore There are three groups, the Marchantiophyta (liverworts), Anthocerotophyta (hornworts), and Bryophyta (mosses). Modern studies generally show one of two patterns. In one of these patterns, the liverworts were the first to diverge, followed by the hornworts, while the mosses are the closest living relatives of the vascular plants. In the other pattern, the hornworts were the first to diverge, followed by the vascular plants, while the mosses are the closest living relatives of the liverworts. Originally the three groups were brought together as the three classes of division Bryophyta. However, since the three groups of bryophytes form a paraphyletic group, they now are placed in three separate divisions.

Bryophyte sexuality

These plants are generally gametophyte-oriented; that is, the normal plant is the haploid gametophyte, with the only diploid structure being the sporangium in season. As a result, bryophyte sexuality is very different from that of other plants. There are two basic categories of sexuality in bryophytes:
- dioicous - These plants produce only antheridia (male organs) or archegonia (female organ) on a single plant body.
- monoicous - These plants produce both antheridia and archegonia on the same plant body. Some bryophyte species may be either monoicous or dioicous depending on environmental conditions. Other species grow exclusively with one type of sexuality. Notice that these terms are not the same as monoecious and dioecious, which refer to whether or not a sporophyte plant bears one or both kinds of gametophyte. Those terms apply only to seed plants.

References

- Chopra, R. N. & Kumra, P. K. (1988). Biology of Bryophytes. New York: John Wiley & Sons. ISBN 0-470-21359-0.
- Crum, Howard (2001). Structural Diversity of Bryophytes. Ann Arbor: University of Michigan Herbarium. ISBN 0-9620733-4-2.
- Goffinet, Bernard. (2000). Origin and phylogenetic relationships of bryophytes. In A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology, pp. 124-149. Cambridge: Cambridge University Press. ISBN 0-521-66097-1.
- Oostendorp, Cora (1987). The Bryophytes of the Palaeozoic and the Mesozoic. Bryophytorum Bibliotheca, Band 34. Berlin & Stuttgart: J. Cramer. ISBN 3-443-62006-X.
- Prihar, N. S. (1961). An Introduction to Embryophyta: Volume I, Bryophyta (4th ed.). Allahabad: Central Book Depot.
- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
- Schofield, W. B. (1985). Introduction to Bryology. New York: Macmillan. ISBN 0-02-949660-8.
- Watson, E. V. (1971). The Structure and Life of Bryophytes (3rd ed.). London: Hutchinson University Library. ISBN 0-09-109301-5. sort11 Bryophyta Category: Bryophytes Category: cryptogams ja:センタイ類 simple:Bryophyta

Sporophyte

A sporophyte is the diploid structure or phase of life of a sexually reproducing plant. Each living cell of the sporophyte contains two complete sets of chromosomes. The sporophyte is the dominant life form in ferns, gymnosperms, and angiosperms (flowering plants). In plants that undergo alternation of generations, the sporophyte produces haploid spores that develop into a gametophyte. Through mitosis, the gametophyte produces a zygote that becomes the sporophyte. In some plants, the sporophyte is initially parasitic on the gametophyte for a time. Category:Plant morphology Category:Botany

Haploid

Ploidy indicates the number of copies of the basic number of chromosomes. The number of basic sets of chromosomes in an organism is called the monoploid number (x). The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), though sex cells (sperm and oocytes) are haploid. In contrast, tetraploidy (four sets of chromosomes), a type of polyploidy, is not uncommon in healthy plant species. Euploidy, or the euploid number, is a species' normal number of chromosomes per cell. For example, the euploid number of chromosomes in a human cell is 46.

Haploid

Haploid (meaning simple in Greek) cells bear one copy of each chromosome. Most fungi, and a few algae exist as haploid organisms, male bees, wasps and ants are also haploid. For organisms that only ever have one set of chromosomes, the term monoploid can be used interchangeably with haploid. Plants and other algae switch between a haploid and a diploid or polyploid state, with one of the stages emphasized over the other. This is called alternation of generations. Most diploid organisms produce haploid sex cells that can combine to form a diploid zygote, for example animals are primarily diploid but produce haploid gametes. During meiosis, germ cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid germ cells (sperm and ovum).

Diploid

Diploid cells (meaning double in Greek) have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. Most somatic cells (body cells) of higher organisms are diploid.

Haplodiploidy

A haplodiploid species is one in which one of the sexes has haploid cells and the other has diploid cells. Most commonly, the male is haploid and the female is diploid. In such species, the male develops from unfertilized eggs, a process called arrhenotokous parthenogenesis or simply arrhenotoky, while the female develops from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg. Haplodiploidy is found in many species of insects from the order Hymenoptera, particularly ants, bees, and wasps. It increases the significance of kin selection, which may explain the eusociality of these sorts of insects. In some Hymenopteran species, worker insects are also able to produce diploid (and therefore female) fertile offspring, which develop as normal queens. The second set of chromosomes comes not from sperm, but from one of the three polar bodies during anaphase II of meiosis. This process is called thelytokous parthenogenesis or simply thelytoky.

Haploidisation

Haploidisation (from the Greek απλοποίηση = simplification) is the process of creating a haploid cell from a diploid cell. This is a laboratory procedure that forces a normal cell to spit out half of its chromosome content, leaving just one set. In mammals this renders this cell equal to sperm or egg. This was one of the procedures used by Japanese researchers to produce Kaguya the fatherless mouse.

Aneuploidy

Aneuploidy is when a cell contains an abnormal or non-integer ploidy number. This may lead to problems in cell development. Most forms of aneuploidy in humans are lethal, but trisomy (three copies) of the sex chromosome (the cause of Klinefelter's syndrome and others) and of chromosome 21 (the cause of Down syndrome) are relatively common. Many forms of cancer have incorrect ploidy numbers, due to the accumulation of mutations which increase chromosome missegregation.

Polyploidy

Polyploidy is the state where all cells have multiple pairs of chromosomes beyond the basic set. These may be from the same species or from closely related species. In the later case these are known as allopolyploids (also known as amphidiploids or allotetraploids), that are formed from the hybridisation of two separate species followed by their subsequent chromosome doubling. A good examples is the so called Brassica triangle where three different parent species have hybridized in each pair combination to form three different allopolyploid species. Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms many somatic cells are polyploid due to a process called endoreduplication where duplication of the genome occurs without mitosis (cell division).

Variable or Indefinite Ploidy

Depending on growth conditions, prokaryotes such as bacteria may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because such organisms tend to multiply continuously.

References

Category:Classical genetics

Rosette (botany)

In botany, a rosette indicates a circular arrangement of the leaves, with all the leaves at a single height. Often, perennial plants whose regular foliage dies down to the ground in winter retain a rosette of leaves at the soil's surface. Category: plant morphology

Centimeter

To help compare different orders of magnitude this page lists lengths between 10^-2 m and 10^-1 m (1 cm and 10 cm).
- Distances shorter than 1 cm
- 1.0 cm is equal to
- 10 millimetres
- .39 inches
- edge of square of area 1 cm²
- edge of cube of volume 1 ml
- 1.5 cm — length of an average mosquito.
- 2.54 cm — 1 inch
- 3.1 cm — 1 attoparsec (10^-18 parsecs)
- 35 mm — width of film commonly used in motion pictures and still photography
- 42.7 mm — diameter of a golfball
- Distances longer than 10 cm

Chloroplast

Chloroplasts are organelles found in plant cells and eukaryotic algae which conduct photosynthesis. Chloroplasts capture light energy from the sun to produce the free energy stored in ATP and NADPH through a process called photosynthesis.

Origins

Chloroplasts are one of the forms a plastid may take, and are generally considered to have originated as endosymbiotic cyanobacteria. In this respect they are similar to mitochondria, but are found only in plants and protista. Both organelles are surrounded by a double membrane with an intermembrane space; both have their own DNA and are involved in energy metabolism; and both have reticulations, or many infoldings, filling their inner spaces. In green plants chloroplasts are surrounded by two lipid-bilayer membranes. The inner membrane is now thought to correspond to the outer membrane of the ancestral cyanobacterium. The chloroplast genome is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities. Many of the missing genes are encoded in the nuclear genome of the plant, algae or protist. It is interesting to note that in some algae (such as the heterokonts and other protists such as Euglenozoa and Cercozoa), chloroplasts seem to have arisen through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts have themselves been engulfed by still other eukaryotes, forming tertiary endosymbionts.

Structure

Cercozoa The fluid within the chloroplast is called the stroma, corresponding to the cytoplasm of the bacterium, and contains tiny circular DNA and ribosomes, though most of their proteins are encoded by genes contained in the cell nucleus, with the protein products trafficked to the chloroplast. Within the stroma are stacks of thylakoids, the sub-organelles where photosynthesis actually takes place. A stack of thylakoids is called a granum. A thylakoid looks like a flattened disk, and inside is an empty area called the thylakoid space or lumen. The photosynthesis reaction takes place on the membrane of the thylakoid, and, as is also the case with mitochondria, involves the coupling of cross-membrane fluxes with biosynthesis.

Biochemistry

The photosynthetic proteins in the membrane bind chlorophyll, which is present with various accessory pigments. These give chloroplasts their green color. During autumn, the removal of chlorophyll from plant leaves exposes red and yellow pigments (such as xanthophyll) which were previously masked. Algal chloroplasts may be golden, brown, or red and show variation in the number of membranes and the presence of thylakoids. Pigments undergo electronic excitations driven by the absorption of sunlight — red and blue for chlorophyll. The green we see is the color not absorbed. The energy released by the electronically-excited pigments as they return to their ground state is the basis for the energy captured by photosynthesis to produce ATP and NADPH and the ultimate formation of sugars. Energy of the absorbed photons not used to produce chemical energy is eventually given off to the surroundings. Thus, chloroplasts are small heat engines operating between the hot light from the sun and the lower ambient molecular temperature. (Photovoltaic cells do likewise.)

References

External links

- [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chloroplasts.html Chloroplasts] and [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LightReactions.html Photosynthesis: The Role of Light] from [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/ Kimball's Biology Pages]
- [http://reference.allrefer.com/encyclopedia/C/chloropl.html Chloroplast, Botany]
- [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=8041699 Use of chloroplast DNA in studying plant phylogeny and evolution] Category:Organelles Category:Photosynthesis ko:엽록체 ja:葉緑体

Species

In biology, a species is the basic unit of biodiversity. In scientific classification, a species is assigned a two-part name in Latin. The genus is listed first (and capitalized), followed by a specific epithet. For example, humans belong to the genus Homo, and are in the species Homo sapiens. The name of the species is the whole binomial not just the second term (the specific epithet). The binomial, and most other purely formal aspects of the biological codes of nomenclature, were formalized by Carolus Linnaeus in the 1700's and as a result are called the "Linnaean system". At that time, species were thought to represent independent acts of creation by God, and were therefore considered objectively real and immutable. Since the advent of the theory of evolution, the conception of species has undergone vast changes in biology, however no consensus on the definition of the word has yet been reached. The most commonly cited definition of "species" was first coined by Ernst Mayr. By this definition, called the biological species concept or isolation species concept, species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups". However, many other species concepts are also used (see other definitions of species below). The scientific name of a species is properly typeset in italics. When an unknown species is being referred to this may be done by using the abbreviation "sp." in the singular or "spp." in the plural in the place of the second part of the scientific name. Note that the word "specie" is not the singular of "species". It refers to coined money.

Definitions of species

The definition of a species given above as taken from Mayr, is somewhat idealistic. Since it assumes sexual reproduction, it leaves the term undefined for a large class of organisms that reproduce asexually. Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Further, there is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist: ; Typological species : A group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties. The clusters of variations or phenotypes within specimens (ie: longer and shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, we now know that different phenotypes do not always constitute different species (e.g.: a 4-winged Drosophila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies. ; Morphological species : A population or group of populations that differs morphologically from other populations. For example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. This species concept is much criticised because more recent genetic data reveals that genetically distinct populations may look very similar and, contrarily, large morphological differences sometimes exist between very closely-related populations. Nonetheless, most species known have been described solely from morphology. ; Biological / Isolation species : A set of actually or potentially interbreeding populations. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations and is thus impractical in instances of allopatric (geographically isolated) populations. The results of breeding experiments done in artificial conditions may or may not reflect what would happen if the same organisms encountered each other in the wild, making it difficult to gauge whether or not the results of such experiments are meaningful in reference to natural populations. ; Mate-recognition species : A group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually. Unlike the isolation species concept, it focuses specifically on pre-mating reproductive isolation. ; Phylogenetic / Evolutionary / Darwinian species : A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. ; Microspecies : Species that reproduce without meiosis or mitosis so that each generation is genetically identical to the previous generation. See also apomixis. In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement. Given the complexity of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition. For most vertebrates, this is the biological species concept, and to a lesser extent (or for different purposes) the phylogenetic species concept. Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete.

Importance in biological classification

The idea of species has a long history. It is one of the most important levels of classification, for several reasons:
- It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another.
- It is the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms.
- It is the only taxonomic level which has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them. After thousands of years of use, the concept remains central to biology and a host of related fields, and yet also remains at times ill-defined and controversial.

Implications of assignment of species status

The naming of a particular species should be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be confirmed or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. When two named species are discovered to be of the same species, the older species name is usually retained, and the newer species name dropped, a process called synonymization, or convivially, as lumping. Dividing a taxon into multiple, often new, taxons is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms (see lumpers and splitters). Traditionally, researchers relied on observations of anatomical differences, and on observations of whether different populations were able to interbreed successfully, to distinguish species; both anatomy and breeding behavior are still important to assigning species status. As a result of the revolutionary (and still ongoing) advance in microbiological research techniques, including DNA analysis, in the last few decades, a great deal of additional knowledge about the differences and similarities between species has become available. Many populations which were formerly regarded as separate species are now considered to be a single taxon, and many formerly grouped populations have been split. Any taxonomic level (species, genus, family, etc.) can be synonymized or split, and at higher taxonomic levels, these revisions have been still more profound. From a taxonomical point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology only the subspecies is used, while in botany the variety, subvariety, and form are used as well.

The isolation species concept in more detail

In general, for large, complex, organisms that reproduce sexually (such as mammals and birds), one of several variations on the isolation or biological species concept is employed. Often, the distinction between different species, even quite closely related ones, is simple. Horses (Equus caballus) and donkeys (Equus asinus) are easily told apart even without study or training, and yet are so closely related that they can interbreed after a fashion. Because the result, a mule or hinny, is not usually fertile, they are clearly separate species. But many cases are more difficult to decide. This is where the isolation species concept diverges from the evolutionary species concept. Both agree that a species is a lineage that maintains its integrity over time, that is diagnosably different to other lineages (else we could not recognise it), is reproductively isolated (else the lineage would merge into others, given the chance to do so), and has a working intra-species recognition system (without which it could not continue). In practice, both also agree that a species must have its own independent evolutionary history—otherwise the characteristics just mentioned would not apply. The species concepts differ in that the evolutionary species concept does not make predictions about the future of the population: it simply records that which is already known. In contrast, the isolation species concept refuses to assign the rank of species to populations that, in the best judgement of the researcher, would recombine with other populations if given the chance to do so.

The isolation question

There are, essentially, two questions to resolve. First, is the proposed species consistently and reliably distinguishable from other species? Secondly, is it likely to remain so in the future? To take the second question first, there are several broad geographic possibilities.
- The proposed species are sympatric—they occupy the same habitat. Observation of many species over the years has failed to establish even a single instance of two diagnostically different populations that exist in sympatry and have then merged to form one united population. Without reproductive isolation, population differences cannot develop, and given reproductive isolation, gene flow between the populations cannot merge the differences. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than pure-bred individuals of either species.
- The proposed species are allopatric—they occupy different geographical areas. Obviously, it is not possible to observe reproductive isolation in allopatric groups directly. Often it is not possible to achieve certainty by experimental means either: even if the two proposed species interbreed in captivity, this does not demonstrate that they would freely interbreed in the wild, nor does it always provide much information about the evolutionary fitness of hybrid individuals. A certain amount can be inferred from other experimental methods: for example, do the members of population A respond appropriately to playback of the recorded mating calls of population B? Sometimes, experiments can provide firm answers. For example, there are seven pairs of apparently almost identical marine snapping shrimp (Altheus) populations on either side of the Isthmus of Panama, which did not exist until about 3 million years ago. Until then, it is assumed, they were members of the same seven species. But when males and females from opposite sides of the isthmus are placed together, they fight instead of mating. Even if the isthmus were to sink under the waves again, the populations would remain genetically isolated: therefore they are now different species. In many cases, however, neither observation nor experiment can produce certain answers, and the determination of species rank must be made on a 'best guess' basis from a general knowledge of other related organisms.
- The proposed species are parapatric—they have breeding ranges that abut but do not overlap. This is fairly rare, particularly in temperate regions. The dividing line is often a sudden change in habitat (an ecotone) like the edge of a forest or the snow line on a mountain, but can sometimes be remarkably trivial. The parapatry itself indicates that the two populations occupy such similar ecological roles that they cannot coexist in the same area. Because they do not crossbreed, it is safe to assume that there is a mechanism, often behavioral, that is preventing gene flow between the populations, and that therefore they should be classified as separate species.
- There is a hybrid zone where the two populations mix. Typically, the hybrid zone will include representatives of one or both of the 'pure' populations, plus first-generation and back-crossing hybrids. The strength of the barrier to genetic transmission between the two pure groups can be assessed by the width of the hybrid zone relative to the typical dispersal distance of the organisms in question. The dispersal distance of oaks, for example, is the distance that a bird or squirrel can be expected to carry an acorn; the dispersal distance of Numbats is about 15 kilometres, as this is as far as young Numbats will normally travel in search of vacant territory to occupy after leaving the nest. The narrower the hybrid zone relative to the dispersal distance, the less gene flow there is between the population groups, and the more likely it is that they will continue on separate evolutionary paths. Nevertheless, it can be very difficult to predict the future course of a hybrid zone; the decision to define the two hybridizing populations as either the same species or as separate species is difficult and potentially controversial.
- The variation in the population is clinal; at either extreme of the population's geographic distribution, typical individuals are clearly different, but the transition between them is seamless and gradual. For example, the Koalas of northern Australia are clearly smaller and lighter in colour than those of the south, but there is no particular dividing line: the further south an individual Koala is found, the larger and darker it is likely to be; Koalas in intermediate regions are intermediate in weight and colour. In contrast, over the same geographic range, black-backed (northern) and white-backed (southern) Australian Magpies do not blend from one type to another: northern populations have black backs, southern populations white backs, and there is an extensive hybrid zone where both 'pure' types are common, as are crossbreeds. The variation in Koalas is clinal (a smooth transition from north to south, with populations in any given small area having a uniform appearance), but the variation in magpies is not clinal. In both cases, there is some uncertainty regarding correct classification, but the consensus view is that species rank is not justified in either. The gene flow between northern and southern magpie populations is judged to be sufficiently restricted to justify terming them subspecies (not full species); but the seamless way that local Koala populations blend one into another shows that there is substantial gene flow between north and south. As a result, experts tend to reject even subspecies rank in this case.

The difference question

Obviously, when defining a species, the geographic circumstances become meaningful only if the populations groups in question are clearly different: if they are not consistently and reliably distinguishable from one another, then we have no grounds for believing that they might be different species. The key question in this context, is "how different is different?" and the answer is usually "it all depends". In theory, it would be possible to recognise even the tiniest of differences as sufficient to delineate a separate species, provided only that the difference is clear and consistent (and that other criteria are met). There is no universal rule to state the smallest allowable difference between two species, but in general, very trivial differences are ignored on the twin grounds of simple practicality, and genetic similarity: if two population groups are so close that the distinction between them rests on an obscure and microscopic difference in morphology, or a single base substitution in a DNA sequence, then a demonstration of restricted gene flow between the populations will probably be difficult in any case. More typically, one or other of the following requirements must be met:
- It is possible to reliably measure a quantitative difference between the two groups that does not overlap. A population has, for example, thicker fur, rougher bark, longer ears, or larger seeds than another population, and although this characteristic may vary within each population, the two do not grade into one another, and given a reasonably large sample size, there is a definite discontinuity between them. Note that this applies to populations, not individual organisms, and that a small number of exceptional individuals within a population may 'break the rule' without invalidating it. The less a quantitative difference varies within a population and the more it varies between populations, the better the case for making a distinction. Nevertheless, borderline situations can only be resolved by making a 'best-guess' judgement.
- It is possible to distinguish a qualitative difference between the populations; a feature that does not vary continuously but is either entirely present or entirely absent. This might be a distinctively shaped seed pod, an extra primary feather, a particular courting behaviour, or a clearly different DNA sequence. Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. This is often the case with plants in particular. In eucalypts, for example, Corymbia ficifolia cannot be reliably distinguished from its close relative Corymbia calophylla by any single measure (and sometimes individual trees cannot be definitely assigned to either species), but populations of Corymbia can be clearly told apart by comparing the colour of flowers, bark, and buds, number of flowers for a given size of tree, and the shape of the leaves and fruit. When using a combination of characteristics to distinguish between populations, it is necessary to use a reasonably small number of factors (if more than a handful are needed, the genetic difference between the populations is likely to be insignificant and is unlikely to endure into the future), and to choose factors that are functionally independent (height and weight, for example, should usually be considered as one factor, not two).

Historical development of the species concept

In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. No other relationships beyond that group were implied. Aristotle used the words genus and species to mean generic and specific categories. Aristotle and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. To the modern mind, many of the schemes delineated are whimsical at best, such as those that determined consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants). In the 18th century Carolus Linnaeus classified organisms according to differences in the form of reproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, it made no claims about the relationship between similar species. At the time, it was still widely believed that there is no organic connection between species, no matter how similar they appear; every species was individually created by God, a view today called creationism. This approach also suggested a type of idealism: the notion that each species exists as an "ideal form". Although there are always differences (although sometimes minute) between individual organisms, Linnaeus considered such variation problematic. He strove to identify individual organisms that were exemplary of the species, and considered other non-exemplary organisms to be deviant and imperfect. By the 19th century most naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. As such, the new emphasis was on determining how a species could change over time. Jean-Baptiste Lamarck suggested that an organism could pass on an acquired trait to its offspring, i.e., the giraffe's long neck was attributed to generations of giraffes stretching to reach the leaves of higher treetops (this well-known and simplistic example, however, does not do justice to the breadth and subtlety of Lamarck's ideas). Lamarck's most important insight may have been that species can be extraordinarily fluid; his 1809 Zoological Philosophy contained one of the first logical refutations of creationism. With the acceptance of the work of Charles Darwin in the 1860s, Lamarck's view of evolution was quickly eclipsed. It was not until the late 20th century that his work began to be reexamined, and took its place as a fundamental stepping stone to the modern theory of adaptive mutation. Lamarck's long-discarded ideas of the goal-oriented evolution of species, also known the teleological process, have also received renewed attention, particularly by proponents of artificial selection. Charles Darwin and Alfred Wallace provided what scientists now consider the most powerful and compelling theory of evolution. Basically, Darwin argued that it is populations that evolve, not individuals. His argument relies on a radical shift in perspective from Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species. Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive will be eliminated. It should be emphasized that whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. In this model, the length of a giraffe's neck would be explained by positing that proto-giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals. In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangenesis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable.) Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory are genes. The theory of the evolution of species through natural selection has two important implications for discussions of species -- consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable. The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendant species. Although the current scientific understanding of species suggests there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring, then they are in different species. This definition captures a number of intuitive species boundaries, but nonetheless has some problems, however. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this notion of species. In recent years we have witnessed the drastic reduction in the size of breeding populations and the geographical range of many physically large mammals. In earlier times it was assumed that every species existed in at least a few thousand living individuals, except very rare relic, isolated groups. In the present, many well know mammal & bird species are so stressed by habitat loss, and other effects of the modern world, that only a very few breeding males may contribute the genetic material to a small number of breeding females. In these highly stressed conditions, the likelihood of change is very much greater. Mammals may become smaller, have darker fur, more stripes, more cautious behavior, even over time learn to co-exist with the human world. Very likely, evolution is radically accelerated, and we are only beginning to notice it. Species in transition before our eyes. It is possible that this severe stress is essential to the creation of new species, and may have been a prime factor throughout biological history, from other population reducing influences. Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). However, most if not all taxonomists would strongly disagree. For example, in many amphibians, most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes which are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool. The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century and coming into full swing with Charles Sibley's ground-breaking DNA-DNA hybridisation studies in the 1970s. The results of the technique caused revolutionary changes in the higher taxonomic categories (such as phyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow, yielding clear differences only after considerable periods of reproductive separation. Instances of hybridization can result in misleading molecular data, the Pomarine Skua - Great Skua phenomenon being a famous example. Turtles have been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. The hybridization technique is however no longer considered a good technique and more reliable computational techniques for sequence comparison are now used for. Molecular taxonomy does not directly measure the evolutionary processes, but rather the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation, crossover and selection are not uniform (see also molecular clock). DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself.

External links

- http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
- [http://www.sciencedaily.com/releases/2003/12/031231082553.htm 2003-12-31, ScienceDaily: Working On The 'Porsche Of Its Time': New Model For Species Determination Offered] Quote: "...two species of dinosaur that are members of the same genera varied from each other by just 2.2 percent. Translation of the percentage into an actual number results in an average of just three skeletal differences out of the total 338 bones in the body. Amazingly, 58 percent of these differences occurred in the skull alone. "This is a lot less variation than I'd expected", said Novak..."
- [http://www.sciencedaily.com/releases/2003/08/030808081854.htm 2003-08-08, ScienceDaily: Cross-species Mating May Be Evolutionarily Important And Lead To Rapid Change, Say Indiana University Researchers] Quote: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change..."
- [http://www.sciencedaily.com/releases/2004/01/040109064407.htm 2004-01-09 ScienceDaily: Mayo Researchers Observe Genetic Fusion Of Human, Animal Cells; May Help Explain Origin Of AIDS] Quote: "...The researchers have discovered conditions in which pig cells and human cells can fuse together in the body to yield hybrid cells that contain genetic material from both species... "What we found was completely unexpected", says Jeffrey Platt, M.D."
- [http://www.sciencedaily.com/releases/2000/09/000913211733.htm 2000-09-18, ScienceDaily: Scientists Unravel Ancient Evolutionary History Of Photosynthesis] Quote: "...gene-swapping was common among ancient bacteria early in evolution..."
- [http://plato.stanford.edu/entries/species/ Stanford Encyclopedia of Philosophy entry]
- [http://www.barcodinglife.org/ Barcoding of species] rank22 rank22 ms:Spesies ja:種 (生物) th:สปีชีส์

Pyrenoid

In cell biology, pyrenoids are centers of carbon dioxide fixation. They are not membrane-bound organelles, but specialized areas in algal plastids and contain high amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). RubisCO takes carbon dioxide and adds it to the sugar ribulose-1,5-bisphosphate. It needs six molecules of carbon dioxide and six molecules of ribulose-1,5-bisphosphate = six cycles of the Calvin cycle to make one new molecule of glucose. In some organisms, the concentration of RubisCO in the pyrenoid is so high that the contents of the organelle assume a crystalline appearance. Complex pyrenoids are highly differentiated areas of chloroplast surrounded by a thick starch sheath. This is assumed to aid the concentration of dissolved carbon dioxide by preventing diffusion away from the site of fixation while simultaneously reducing the level of oxygen in the pyrenoid which can inhibit the carbon fixation reaction catalysed by RubisCO. Pyrenoids are not found in higher plants and it is thought that the slower rate of diffusion of carbon dioxide in water compared to air (1:1000) necessitates their use by small aquatic organisms. Category:Cell biology Category:molecular biology Category:biochemistry

Pyrenoid in Botany

Differentiated region of the chloroplast that may be the center of starch formation and depositions, or may be the site of certain photosynthetic enzymes.

Alga

:This article is about an organism. See algae programming language for a programming language in computing. computing The algae (singular alga) consist of several different groups of living organisms that capture light energy through photosynthesis, converting inorganic substances into simple sugars with the captured energy. Algae have been traditionally regarded as simple plants, and some are closely related to the higher plants. Others appear to represent different protist groups, alongside other organisms that are traditionally considered more animal-like (protozoa). Thus algae do not represent a single evolutionary direction or line, but a level of organization that may have developed several times in the early history of life on earth. Algae range from single-celled organisms to multi-cellular organisms, some with fairly complex differentiated form and (if marine) called seaweeds. All lack leaves, roots, flowers, and other organ structures that characterize higher plants. They are distinguished from other protozoa in that they are photoautotrophic, although this is not a hard and fast distinction as some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have reduced or lost their photosynthetic apparatus. All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike the non-cyanobacterial photosynthetic bacteria. Algae are usually found in damp places or bodies of water and thus are common in terrestrial as well as aquatic environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptions to live on land. Algae can endure dryness and other conditions in symbiosis with a fungus as lichen. The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column—called phytoplankton—provide the food base for most marine food chains. In very high densities (so-called algal blooms) they may discolor the water and outcompete or poison other life forms. The seaweeds grow mostly in shallow marine waters. Some are used as human food or are harvested for useful substances such as agar or fertilizer. The study of algae is called phycology or algology.

Relationships among algal groups

Prokaryotic algae

Traditionally the cyanobacteria have been included among the algae, referred to as the cyanophytes or Blue-green Algae, though some recent treatises on algae specifically exclude them. Cyanobacteria are some of the oldest organisms to appear in the fossil record, dating back about 3.8 billion years (Precambrian). Ancient cyanobacteria likely produced much of the oxygen in the Earth's atmosphere. Cyanobacteria can be unicellular, colonial, or filamentous. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm, rather than in specialized organelles. Some filamentous blue-green algae have specialized cells, termed heterocysts, in which nitrogen fixation occurs.

Eukaryotic algae

All other algae are eukaryotes and conduct photosynthesis within membrane-bound structures (organelles) called chloroplasts. Chloroplasts contain DNA and are similar in structure to cyanobacteria, presumably representing reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, possibly reflecting different endosymbiotic events. There are three groups that have primary chloroplasts:
- Green algae, together with higher plants
- Red algae
- Glaucophytes In these groups the chloroplast is surrounded by two membranes, both now thought to come from the chloroplast. The chloroplasts of red algae have a more or less typical cyanobacterial pigmentation, while the green algae and higher plants have chloroplasts with chlorophyll a and b, the latter found in some cyanobacteria but not most. There is reasonably solid evidence that these three groups originated from a common pigmented ancestor; i.e., chloroplasts developed in a single endosymbiotic event. Red and green algae have an alternation of generations life cycle. This is the same life cycle as the mosses, suggesting that the mosses evolved from the green algae. Two other groups have green chloroplasts containing chlorophyll b:
- euglenids and
- chlorarachniophytes. These are surrounded by three and four membranes, respectively, and were probably retained from an ingested green alga. Those of the chlorarchniophytes contain a small nucleomorph, which is the remnant of the alga's nucleus. It has been suggested that the euglenid chloroplasts only have three membranes because they were acquired through myzocytosis rather than phagocytosis. The remaining algae all have chloroplasts containing chlorophylls a and c. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with the red algae suggest a relationship there. These groups include:
- Heterokonts (e.g., golden algae, diatoms, brown algae)
- Haptophytes (e.g., coccolithophores)
- Cryptomonads
- Dinoflagellates In the first three of these groups (Chromista), the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and it now appears that they share a common pigmented ancestor. The typical dinoflagellate chloroplast has three membranes, but there is considerable diversity in chloroplasts among the group, some members presumably having acquired theirs from other sources. The Apicomplexa, a group of closely related parasites, also have plastids though not actual chloroplasts, which appear to have a common origin with those of the dinoflagellates. Note many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost them entirely.

Forms of algae

Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and non-motile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the life cycle of a species, are:
- Colonial - small, regular groups of motile cells
- Capsoid - individual non-motile cells embedded in mucilage
- Coccoid - individual non-motile cells with cell walls
- Palmelloid - non-motile cells embedded in mucilage
- Filamentous - a string of non-motile cells connected together, sometimes branching
- Parenchymatous - cells forming a thallus with partial differentiation of tissues In three lines even higher levels of organization have been reached, leading to organisms with full tissue differentiation. These are the brown algae—some of which may reach 70 m in length (kelps)—the red algae, and the green algae. The most complex forms are found among the green algae (see Charales), in a lineage that eventually led to the higher land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other algal groups.

Algae and symbioses

Algae frequently form part of a symbiosis with other organisms. In these symbioses, the algae photosynthesise and supply photosynthates to their host. The host organism is then capable of deriving some or all of its energy requirements from the alga. Examples include:
- lichens - a fungus is the host, usually with a green alga or a cyanobacterium as the symbiont. Both fungi and algae found in lichens are capable of living independantly.
- corals - several algae form symbioses (zooxanthellae) with corals. Notable amongst these is the dinoflagellate Symbiodinium, found in many hard corals. The loss of Symbiodinium, or other zooxanthellae, from the host leads to coral bleaching.

External links

[http://www.rbgsyd.nsw.gov.au/information_about_plants/botanical_info/australian_freshwater_algae2 Australian freshwater algae] - Sydney Botanic Gardens [http://www.algae.info/ Learn about Algae & Algal Blooms] - Rural Chemical Industries (Aust.) Pty Ltd. [http://www.whoi.edu/redtide/ Harmful Algal Blooms - "Red tide"] - National Office for Marine Biotoxins and Harmful Algal Blooms, USA. [http://www.nmnh.si.edu/botany/projects/algae/Alg-Menu.htm Algae Section, National Museum of Natural History] - Smithsonian Institution
- Important publications in phycology Category: Botany ja:藻類

Photosynthetic

Photosynthesis is an important biochemical process in which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. It is also responsible for producing the oxygen that makes up a large portion of the Earth's atmosphere. Organisms that produce energy through photosynthesis are called photoautotrophs.

Plant photosynthesis

Plants are photoautotrophs, which means they are able to synthesize food directly from inorganic compounds using light energy, instead of eating other organisms or relying on material derived from them. This is distinct from chemoautotrophs that do not depend on light energy, but use energy from inorganic compounds. The energy for photosynthesis ultimately comes from absorbed photons and involves a reducing agent, which is water in the case of plants, releasing oxygen as a waste product. The light energy is converted to chemical energy, in the form of ATP and NADPH, using the light-dependent reactions and is then available for carbon fixation. Most notably plants use the chemical energy to fix carbon dioxide into carbohydrates and other organic compounds through light-independent reactions. The overall equation for photosynthesis in green plants is: :n CO₂ + 2n H₂O + light energy → (CH₂O)_n + n O₂ + n H₂O Where n is defined according to the structure of the resulting carbohydrate. However, hexose sugars and starch are the primary products, so the following generalised equation is often used to represent photosynthesis: :6 CO₂ + 12 H₂O + light energy → C₆H₁₂O₆ + 6 O₂ + 6 H₂O More specifically, photosynthetic reactions usually produces an intermediate product, which is then converted to the final hexose carbohydrate products. These carbohydrate products are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis or as a fuel in cellular respiration. The latter not only occurs in plants, but also in animals when the energy from plants get passed through a food chain. In general outline, cellular respiration is the opposite of photosynthesis: glucose and other compounds are oxidised to produce carbon dioxide, water, and chemical energy. However, both processes actually take place through a different sequence of reactions and in different cellular compartments. food chain Plants capture light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. The function of chlorophyll is often supported by other accessory pigments such as carotenoids and xanthophylls. Both chlorophyll and accessory pigments are contained in organelles (compartments within the cell) called chloroplasts. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, contain about half a million chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant, waxy cuticle, that protects the leaf from excessive evaporation of water as well as decreasing the absorption of ultraviolet or blue light to reduce heating. The transparent, colourless epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

Photosynthesis in algae and bacteria

Algae range from multicellular forms like kelp to microscopic, single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. Light is absorbed by chlorophyll, although various accessory pigments to give them a wide variety of colours, located inside chloroplasts. All algae produce oxygen, and many are autotrophic. However, some are heterotrophic, relying on materials produced by other organisms. Photosynthetic bacteria do not have chloroplasts. Instead, photosynthesis takes place directly within the cell. The cyanobacteria contain chlorophyll and oxygen, in the same way that chloroplasts do, in fact chloroplasts are now considered to have evolved from cyanobacteria by endosymbiosis. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen.

Molecular production

Light-dependent reaction

bacteriochlorophyll bacteriochlorophyll The products of the light dependent reactions are ATP from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

Z scheme

In plants, the light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to sythesize ATP and NADPH. The photons are captured in the antenna complexes of photosystem I and II by chlorophyll and accessory pigments (see diagram at right). When a chorophyll a molecule at a photosystems reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule through a process called Photoinduced charge separation. These electrons are shuttled through an electron transport chain that initially functions to generate a chemiosmotic potential across the membrane, the so called Z-scheme shown in the diagram. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation while NADPH is a product of the terminal redox reaction in the Z-scheme.

Water photolysis

The NADPH is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in photosystem I are replaced from the electron transport chain by plastocyanin. However, since photosystem II includes the first steps of the Z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. This role is played by water during a reaction known as photolysis and results in water being split to give electrons, oxygen and hydrogen ions. Photosystem II is the only known biological enzyme that carries out this oxidation of water. Initially, the hydrogen ions from photolysis contribute to the chemiosmotic potential but eventually they combine with the hydrogen carrier molecule NADP⁺ to form NADPH. Oxygen is a waste product of photosynthesis but it has a vital role for all organisms that use it for cellular respiration.

Oxygen and photosynthesis

With respect to oxygen and photosynthesis, there are two important concepts.
- Plant and algal cells also use oxygen for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.
- Oxygen is a product of the photolysis reaction not the fixation of carbon dioxide during the light-independent reactions. Consequently, the source of oxygen during photosynthesis is water, NOT carbon dioxide.

Bacterial variations

The second concept was first proposed by Cornelis Bernadus van Neil in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced. Others, such as the halophiles (an Archeae) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping

Anthocerophyta

Description

Life cycle

Classification of Hornworts

Families and genera

See also

References

External links

Ceratophyllaceae

Anthoceros

References

External links

Folioceros

References

External links

Phaeoceros

References

External links

Megaceros (botany)

References

External links

Notothyladaceae

References

Bryophyte

Bryophyte classification

Bryophyte sexuality

See also

References

Sporophyte

Haploid

Haploid

Diploid

Haplodiploidy

Haploidisation

Aneuploidy

Polyploidy

Variable or Indefinite Ploidy

References

Rosette (botany)

Centimeter

See also

Chloroplast

Origins

Structure

Biochemistry

See also

References

External links

Species

Definitions of species

Importance in biological classification

Implications of assignment of species status

The isolation species concept in more detail

The isolation question

The difference question

Historical development of the species concept

See also

External links

Pyrenoid

Pyrenoid in Botany

Alga

Relationships among algal groups

Prokaryotic algae

Eukaryotic algae

Forms of algae

Algae and symbioses

See Also

External links

Photosynthetic

Plant photosynthesis

Photosynthesis in algae and bacteria

Molecular production

Light-dependent reaction

Z scheme

Water photolysis

Oxygen and photosynthesis

Bacterial variations