Hornworts Descriptive Essay

Bryophytes



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Bryophytes are seedless plants without specialized water-conducting tissues. Bryophytes include mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta Hepatophyta), and hornworts (phylum Anthocerophyta). They are plants that virtually everyone has seen, but many have ignored. The most commonly encountered group is the green mosses that cover rotting logs, anchor to the bark of trees, and grow in the spray of waterfalls, along streams and in bogs. Even though mosses often thrive in wet habitats, many mosses and some liverworts can survive in relatively dry environments such as sandy soils and exposed rock outcrops.

The liverworts can take leafy forms, which are very similar superficially to mosses, but differ in the details of leaf size and arrangement. Other liverwort genera are characterized by a thallus made up of relatively small, flattened, ribbonlike segments of photosynthetic tissue, which have the general appearance of short, branched pieces of rich dark green egg noodles or linguini.

The leafy liverworts and the mosses differ in the appearance of their spore-forming structures. The mosses have thin stalks called seta extending from the ends of leafy branches. Seta bear capsules, which produce spores. The leafy and thalloid liverworts have very small, balloon-shaped spore-producing stages that remain virtually hidden within, and totally dependent upon, the photosynthetic plant tissues. The third major group of bryophytes is the hornworts. They received this common name because their spore producing structures, called sporangia, are generally long, slender, hornlike, and without capsules. More than eighteen thousand different bryophyte species have been identified throughout the world, and there are perhaps ten thousand species of moss, approximately eight thousand liverwort species, and only a little more than one hundred species of hornworts.

Characteristics of Bryophytes

There are several characteristic features of bryophytes. First, the green tissue that makes up most of the plant body is not vascularized; it does not have xylem and phloem cells. This absence of specialized tissues for transporting water and dissolved food throughout the organism limits terrestrial forms to being very short plants, since the only way to move substances through the plant body is by osmosis and diffusion from surface moisture.

Second, bryophytes do not have roots, but have rhizoids, which are relatively simple, sometimes multicellular filaments of thin-walled cells that extend from the photosynthetic tissue into the soil or other substrate . They anchor the plant somewhat and in some cases facilitate water and nutrient uptake.

Liverworts can either resemble mosses or have the general appearance of short, branched pieces of rich, dark green egg noodles.

Sexual Reproduction

The third characteristic of bryophytes is something that one could not guess by just looking at the conspicuous green tissue. Unlike other plants (and indeed most other multicellular organisms), the conspicuous portion of bryophytes is composed of haploid cells, containing only one set of chromosomes .

Sexual reproduction in animals involves the union of an egg and a sperm to form a fertilized egg (zygote). This diploid (2n) cell divides mitotically to produce an embryo, and ultimately a mature adult organism. These adults have specialized cells, which divide meiotically to produce haploid (n) sperm or eggs depending on the sex of the individual. In the plant kingdom, this cycle of fertilization and meiosis involves an alternation of generations between the haploid gamete -producing stage (gametophyte) and the diploid organism (sporophyte).

Vascular plants, including flowering plants, conifers, and many, such as ferns, that do not produce seeds, have life cycles with the diploid sporophyte being the predominant generation. In the bryophytes, it is the haploid gametophyte that produces the leaves and thali and therefore predominates. This change from predominant gametophyte to sporophyte was a major evolutionary advancement, which along with the development of vascular tissue facilitated the ultimate success of plants in a diversity of terrestrial habitats.

In order to accomplish sexual reproduction, bryophyte gametophytes produce eggs (n) in the archegonium, a vase-shaped structure that is the female reproductive organ. The sperm (n) are produced in antheridia, which may occur on the same gametophyte, but are often located on separate male plants. Water is generally required for them to swim to the eggs for fertilization. The resulting zygote (2n) develops into the sporophyte (2n). The sporophytes remain attached to and dependent on the female gametophyte. These parasitic sporophytes produce spores (n) by meiosis that then divide mitotically to produce the obvious multicellular gametophyte.

Bibliography

Conard, Henry Shoemaker, et al. How to Know the Mosses and Liverworts, 2nd ed. New York: McGraw-Hill, 1980.

Malcolm, Bill, and Nancy Malcolm. Mosses and Bryophytes: An Illustrated Glossary. Portland, OR: Timber Press/Micro-Optics Press, 2000.

Shaw, A. Jonathan, and Bernard Goffinet, eds. Bryophyte Biology. New York: Cambridge University Press, 2000.

Also read article about Bryophytes from Wikipedia

Brain Buffon, Count (Georges-Louis Leclerc)

The name Hornwort also refers to aquatic plants of the genus Ceratophyllum, in the family Ceratophyllaceae

Hornworts are a group of non-vascular plants constituting the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. As in mosses and liverworts, the flattened, green plant body of a hornwort is the gametophyte plant.

Hornworts may be found worldwide, 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.

The total number of species is still uncertain. While there are more than 300 published species names, the actual number could be as low as 100-150 species.[2]

Description[edit]

The plant body of a hornwort is a haploidgametophyte 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. 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 photosyntheticcyanobacteria, 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.

The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. The sporophyte of a hornwort is unusual in that it 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 their sporophyte as mosses do. The exceptions are the generaNotothylas and Megaceros, which do not have stomata. The sporophyte of most hornworts are also photosynthetic, which is not the case with liverworts.[3]

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; they twist and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 µm 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[edit]

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.[4] The tip of the germ tube divides to form an octant (solid geometry) of cells, and the first rhizoid grows as an extension of the original germ cell.[clarification needed] 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.[4] In either case, the protonema is a transitory stage in the life of a hornwort.

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.

Evolutionary history[edit]

This section needs expansion. You can help by adding to it.(June 2008)

While the fossil record of crown group hornworts only begins in the upper Cretaceous, the lower Devonian Horneophyton may represent a stem group to the clade, as it possesses a sporangium with central columella not attached at the roof.[5] However, the same form of columella is also characteristic of basal moss groups, such as the Sphagnopsida and Andreaeopsida, and has been interpreted as a character common to all early land plants with stomata.[6]

Classification[edit]

Hornworts were traditionally considered a class within the division Bryophyta (bryophytes). However, it now appears that this former division is paraphyletic, so the hornworts are now given their own division, Anthocerotophyta (sometimes misspelled Anthocerophyta). The division Bryophyta is now restricted to include only mosses.

Traditionally, there is a single class of hornworts, called Anthocerotopsida, or older Anthocerotae. More recently, a second class Leiosporocertotopsida has been segregated for the singularly unusual species Leiosporoceros dussii. All other hornworts remain in the class Anthocerotopsida. These two classes are divided further into five orders, each containing a single family.

Among land plants, hornworts appear to be one of the oldest surviving lineages; cladistic analysis implies that the group originated prior to the Devonian, around the same time as the mosses and liverworts. 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.

Structural features that have been used in the classification of hornworts include: the anatomy of chloroplasts and their numbers within cells, the presence of a pyrenoid, the numbers of antheridia within androecia, and the arrangement of jacket cells of the antheridia.[7]

Phylogeny[edit]

Recent studies of molecular, ultrastructural, and morphological data have yielded a new classification of hornworts.[8]

See also[edit]

References[edit]

  • Grolle, Riclef (1983). "Nomina generica Hepaticarum; references, types and synonymies". Acta Botanica Fennica. 121: 1–62. 
  • Hasegawa, J. (1994). "New classification of Anthocerotae". Journal of the Hattori Botanical Laboratory. 76: 21–34. 
  • Renzaglia, Karen S. (1978). "A comparative morphology and developmental anatomy of the Anthocerotophyta". Journal of the Hattori Botanical Laboratory. 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. 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[edit]

Life cycle of a typical hornwort Phaeoceros. Click on the image to enlarge.
The hornwort Dendroceros crispus growing on the bark of a tree.
  1. ^Stotler, Raymond E.; Barbara J. Candall-Stotler (1977). "A checklist of the liverworts and hornworts of North America". The Bryologist. American Bryological and Lichenological Society. 80 (3): 405–428. doi:10.2307/3242017. JSTOR 3242017. 
  2. ^What is a hornwort? - Australian National Botanic Gardens
  3. ^Mosses, Liverworts, and Hornworts
  4. ^ abChopra, R. N.; Kumra, P. K. (1988). Biology of Bryophytes. New York: John Wiley & Sons. ISBN 0-470-21359-0. 
  5. ^Qiu, Y.L.; Li, L.; Wang, B.; Chen, Z.; Knoop, V.; Groth-malonek, M.; Dombrovska, O.; Lee, J.; Kent, L.; Rest, J.; et al. (2006). "The deepest divergences in land plants inferred from phylogenomic evidence". Proceedings of the National Academy of Sciences. 103 (42): 15511–6. doi:10.1073/pnas.0603335103. PMC 1622854. PMID 17030812. 
  6. ^Kenrick, Paul; Peter R. Crane (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. pp. 55–56. ISBN 1-56098-730-8. 
  7. ^D. Christine Cargill, Karen S. Renzaglia, Juan Carlos Villarreal, and R. Joel Duff (2005), "Generic concepts within hornworts: Historical review, contemporary insights and future directions", Australian Systematic Botany, 18: 7–16, doi:10.1071/sb04012 
  8. ^ abDuff, R. Joel; Juan Carlos Villarreal; D. Christine Cargill; Karen S. Renzaglia (2007). "Progress and challenges toward a phylogeny and classification of the hornworts". The Bryologist. 110 (2): 214–243. doi:10.1639/0007-2745(2007)110[214:PACTDA]2.0.CO;2. 
  9. ^Villareal, J. C.; Cargill, D. C.; Hagborg, A.; Söderström, L.; Renzaglia, K. S. (2010). "A synthesis of hornwort diversity: Patterns, causes and future work"(pdf). Phytotaxa. 9: 150–166. 

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